Chapter 17 Research on the Yb³⁺ Ion Activated Cubic Molybdates and Molybdato-Tungstates for Optical Transparent Ceramics

Abstract

As widely known, un-doped and rare earth-doped molybdates and tungstates are promising group which have attracted great attention in wide branches of optical material application not only as laser hosts but also as phosphors and scintillators. Most of laser host materials have been obtained as single crystals by Czochralski method. However, the materials in the form of transparent ceramics have comparable or even better important properties than single crystals.

Nowadays, a challenge is to obtain a high quality optical material based on new polycrystalline ceramics applied for laser sources, scintillators and phosphors and to improve manufacturing methods. Nevertheless, only few compositions of cubic transparent ceramics are actually well-developed, these include rare earth (RE³⁺)-doped garnets (Nd³⁺/Yb³⁺-doped Y₃Al₅O₁₂/Lu₃Al₅O_12, Ce³⁺ − doped Y₃Al₅O₁₂, fluorides (Yb³⁺-doped CaF₂), RE³⁺-doped sesquioxides (Nd³⁺/Yb³⁺-doped Lu₂O₃, Sc₂O₃, Y₂O₃) and also perovskite type BMT (Ba(MgZrTa)O₃) and un-doped spinel (MgAl₂O₄).

This is why our attention is focused on fabrication of new rare-earth doped molybdates or tungstates in the form of polycrystalline ceramics, unknown until now in the literature. The manufacture of ceramics takes less time, even only few days in contrast to 4–6 weeks to grow crystals by using the Czochralski method. In comparison with single crystals it is possible to receive samples highly activated by rare earth ions and of large size ceramics in much cheaper way without using expensive iridium or rhenium crucibles. Indeed, two conditions must be fulfilled to obtain transparent ceramics: the compounds should crystallize in the cubic system and the size of the crystallites must be in the order of tens of nanometers.

We present and discuss the structural (XRD and SEM analysis) and spectroscopic properties of three types of materials representing the family of Yb³⁺-doped molybdates and molybdato-tungstates synthesized by the high-temperature solid-state reaction for future new optical ceramics crystallizing in the cubic system: La₂Mo₂O₉ /La₂MoWO₉ /Y₆MoO₁₂. Yb³⁺ rare earth ions has been selected since they can be substitute with trivalent La³⁺ and Y³⁺ cations and then can play the role of a structural probe and, in addition, can be used as laser ions in these materials.

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17.1 Introduction

The rapid development of technology stimulates interest in substances that as a result of excitation can efficiently emit light-phosphor materials but also new high power efficient laser materials. Among inorganic matrices very well known, extensively studied and efficient as phosphors, scintillators and laser materials are tungstates and molybdates because of their thermal and chemical stability. In the literature, the most investigated groups are CaWO₄ and MgWO₄ as phosphors, as well as ZnWO₄, CdWO₄, and PbWO₄ as scintillators [1,2,3,4,5,6,7,8]. Additionally, Nd³⁺-doped CaWO₄ was the first continuously operating crystal laser reported in 1961. Well known laser materials are MRE(WO₄)₂ (M = alkali metal, RE = Y, Gd, Lu) doped with optically active trivalent rare-earth ions such as Nd, Dy, Ho, Er, Tm, or Yb. Both Nd³⁺- and Yb³⁺-doped KGd (WO₄)₂ and KY (WO₄)₂ crystals became very important laser materials for near-infrared region [9,10,11,12]. Also, phosphors like Eu-doped MgWO₄, [13] Eu-doped ZnWO₄, [14] or Ce-doped MWO₄ (M = Ca, Sr, Ba) phosphors [15] have been recently published. Moreover, like for the tungstate family, un-doped and rare earth-doped MMoO₄ (M = Ca, Sr, Ba, Pb, and Cd) also form a wide class of materials used in different fields such as scintillators, phosphors, photoconductive and photocatalytic materials, as can be seen in these mentioned references as well as therein [16,17,18,19,20,21,22,23,24].

Our scientific program involves with trivalent rare earth ions doped CdMoO₄ scheelite-type cadmium molybdate. As an example, materials with the chemical formula of Cd_1 − 3xEu_2x□_xMoO₄ (cationic vacancy is denoted by □) was investigated as a strong, pure, red-emitting phosphor for white light emitting diodes (WLEDs) by taking advantage of the Eu³⁺ spectroscopic probe ion to analyze in detail the structural properties as a continuation of our previous analysis on both, Cd_1 − 3xNd_2x□_xMoO₄ and Cd_1 − 3xYb_2x□_xMoO₄ characterized by very strong emission in the NIR region [25,26,27].

In last years increasing interest of development of the ceramic laser materials has been observed as the most important innovation of laser material fabrication technology. The polycrystalline ceramics possess many advantages in comparison to single-crystals; large size, a great number of varieties of shapes, better mechanical strength, higher content of doping activators, lower temperature of synthesis, less time consuming, cheaper manufacturing process, and ability to engineer profiles and structures, which, moreover, do not require expensive equipment [28,29,30,31].

Indeed, two conditions must be fulfilled to obtain transparent ceramics: the compounds should crystallize in the cubic system and the size of the crystallites must be in the order of tens of nanometers. The above-mentioned advantages of the production of transparent ceramics are the reasons why a tendency to replace single crystals by transparent ceramics is observed. Surprisingly, today available rare earth (RE³⁺) luminescent ions doped cubic optical transparent ceramics used as laser sources or phosphors for lighting are limited to a very small number.

Until now, only few transparent ceramics are known as optical materials with oxides (garnets, sesquioxides, spinels) and fluorides. Especially, we can mention the following references for laser materials:

• Nd³⁺-doped Y₃Al₅O₁₂ garnets [32] In fact, Nd³⁺:YAG ceramics has been fabricated and used to demonstrate an output power at 1.06 μm of 67 [33] and > 100 kW, [34] respectively.

• Yb³⁺-doped Y₃Al₅O₁₂ ceramics [35, 36]

• Yb³⁺-doped Y₂O₃ ceramics made by hot pressing of high submicron purity coprecipitated powder for high power solid-state lasers exploiting hosts with higher thermal conductivity than YAG [31]

• Nd³⁺-doped Lu₂O₃ sesquioxide ceramics fabricated by Hot Isostatic Pressure (HIP) procedure [37]. Laser oscillation has been observed in hot pressed 10% Yb³⁺-doped Lu₂O₃ ceramics [38]

• Nd³⁺-doped Lu₂O₃ sesquioxide ceramics fabricated by the Spark Plasma Sintering (SPS) procedure [39, 40]

• Nd³⁺, Yb³⁺-co-doped SrF₂ laser ceramics [41]

Due to many advantages of rare earth-doped molybdate and tungstate compounds which fit several requests for optical materials, such as good mechanical strength, thermal property and chemical stability, our research was carried out toward new cubic ceramic optical materials different from those already known. The main objective is to succeed the challenge of synthesis of some rare earth ions (Ce³⁺, Nd³⁺, Eu³⁺, Yb³⁺)-doped cubic tungstate and molybdate inorganic materials accompanied by structural and spectroscopic characterizations, within the expected goal of future innovative optical transparent ceramics.

The RE-doped La₂Mo₂O₉ molybdates crystallizing in monoclinic and cubic system were very little studied as the luminescent materials, while there are well-known as fast oxide-ion conductors and have been examined for catalytic applications [42, 43]. Only few articles reported optical properties of La₂Mo₂O₉ activated by the rare earth ions. However, the authors did not consider the complexity of crystal structure of both α-La₂Mo₂O₉ and β-La₂Mo₂O₉ phases and very easily pointed out on the cubic system without any prove, like for example analysis of the pseudo cubic reflection in the XRD patterns, which is an indicator of the cubic phase in La₂Mo₂O₉ molybdate.

We have started with Nd³⁺-doped La₂Mo₂O₉ as described in two previous papers [44, 45] and have demonstrated that Nd³⁺-doped monoclinic structure (α-form) were observed for the concentration of Nd³⁺ ion up to 15 mol%. Pure cubic phase (β-form), necessary to get transparent ceramics, was obtained when the Nd³⁺ content had reached 50 mol%. However, in such a case of phases with high concentration of Nd³⁺ ions, unfavorable and very strong concentration quenching process takes place due to the clustering of Nd³⁺ ions and energy transfer by both down- and up-conversion processes. Continuing research, we have noticed that the partial substitution of Mo⁶⁺ ions by tungsten W⁶⁺ ones (ratio 1:1) can stabilize the cubic phase of mixed Yb³⁺-doped La₂MoWO₉ molybdato-tungstate [46, 47]. We investigated also the influence of Yb³⁺ ions amount on La₂Mo₂O₉ structure, morphology and photoluminescence [48]. Detailed analysis of Yb³⁺-doped La₂Mo₂O₉ shown that in some cases, as for 3 mol% of Yb³⁺, it is cubic and could serve to produce transparent ceramics from the cubic nano-powders. It is worth to note that the Yb³⁺ luminescence in La₂Mo₂O₉ as Near Infra-Red (NIR) emitting optical material has never been reported up to now, so this is also one reason for our interest in this composition as laser material, but also because we can play with the presence of the ²F_7/2 ↔ ²F_5/2 0-phonon line of Yb³⁺ ion used as a structural probe in solids. The complexity of the La₂Mo₂O₉ crystal structure bring some difficulties in the interpretation of the spectroscopic results. Micro-crystalline samples obtained by high-temperature solid-state reaction characterized by higher crystallinity of material, and thus by more intense luminescence, were selected to perform the fundamental investigations of optical properties. Finally, the nano-powder obtained by the combustion method containing 3 mol% of Yb³⁺ ion was used to obtain Yb³⁺-doped La₂Mo₂O₉ in the form of first translucent ceramics [48].

Recently we discovered that RE³⁺-doped Y₆MoO₁₂ molybdates easily crystallize in the cubic form and may be more beneficial relative to the solid solutions investigated by us [49]. Until now novel, yellow, brown and brick-red colored compositions activated by Sm³⁺, Pr³⁺ or Nd³⁺ ions yttrium molybdates have been reported as high near infrared reflecting pigments due to their excellent optical property of reducing the heat build-up effectively [50,51,52]. A large number of rare earth based NIR refractive pigments are proposed as alternatives to traditional transition metal oxides pigments because of their low toxicity. Also a few articles on Eu³⁺-doped Y₆MoO₁₂ and mixed Y₆Mo_1-xW_xO₁₂, Y₆(W,Mo)(O,N)₁₂ or Lu₆Mo(W)O₁₂ phosphors have been reported recently [53,54,55,56,57]. The advantages of this matrices we can find also in the localization of the CT band shifted to the blue region with respect to the other molybdates or tungstates of tetragonal-coordinated W(Mo)O₄ groups. Indeed, the four-fold coordination (MO₄ groups; M = Mo, W) present in the scheelite-type structure shows the charge transfer bands (CTBs) of MO₄ groups located in the UV region, and thus near-UV or blue light cannot be utilized to excite rare earth activators through the host lattices. Compared with MO₄ groups, sixth-fold coordination (MO₆ groups) can efficiently absorb near-UV light by the (O²⁻ → M⁶⁺) CTBs and transfer the excitation energy to the activator [58]. These Eu³⁺-doped compounds have been investigated in the frame of exploring of near-UV/blue light excited color-conversion phosphors in an expectation of obtaining white light with high color rendering index and low color temperature, namely, warm white light. It is worth noting that the Yb³⁺ luminescence in Y₆MoO₁₂ has never been reported before us and this is why we present solid solutions of this material as a very promising NIR emitting optical materials for phosphors and lasers with some hope to obtain them in the nearest future in the form of transparent ceramic due to their cubic structure.

In this review we would like to summarize some results on the Yb³⁺ ion activated cubic molybdates and molybdato-tungstates for optical transparent ceramics which have been presented as a lecture at the Erice’s school. There are presented the detailed structural and spectroscopic studies performed by using complementary techniques as XRD, SEM with EDS, high-resolution low-temperatures absorption and emission with site selective excitation, which together brought very precious information. We are pointing out the advantages and disadvantages of the analyzed host lattices. Finally, we present also some results obtained for the first translucent ceramics.

17.2 Experimental Section

17.2.1 Sample Preparation

17.2.1.1 Synthesis of Yb³⁺-Doped La₂Mo₂O₉

A series of micro-crystalline Yb³⁺-doped La₂Mo₂O₉ molybdates with various concentrations of Yb³⁺ ion (x = 0, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15 and 25 mol%) were successfully synthesized by a high-temperature solid-state reaction. The stoichiometric quantities of high purity commercial powders of La₂O₃, (99.999%, Stanford Materials), Yb₂O₃ (99.995%, Stanford Materials), MoO₃ (99.95%, Alfa Aesar) were used as initial reactants in this synthesis. Both rare-earth oxides were previously calcined at 850 °C in air in two 12-h heating stages to remove adsorbed moisture and carbon dioxide. The starting materials weighed in suitable molar proportion were mixed together and homogenized thoroughly by grinding in an agate mortar. All initial mixtures Yb₂O₃/Y₂O₃/MoO₃ were transferred into corundum crucibles and heated in air in the five stages: 600 °C/6 h, 700°C/6 h, 900°C/12 h, 1000°C/12 h and 1100°C/12 h. After each heating stage, Yb³⁺-doped materials were slowly cooled to room temperature and then reground in an agate mortar with acetone for better reactivity.

The synthesis of Yb³⁺-doped La₂Mo₂O₉ can be described by the following equation:

$$ \left(1-x/2\right)\ {\mathrm{La}}_2{\mathrm{O}}_{3\left(\mathrm{s}\right)}+x/2\ {\mathrm{Yb}}_2{\mathrm{O}}_{3\left(\mathrm{s}\right)}+2\ {\mathrm{Mo}\mathrm{O}}_{3\left(\mathrm{s}\right)}={\mathrm{La}}_{2-x}{\mathrm{Yb}}_x{\mathrm{Mo}}_2{\mathrm{O}}_{9\left(\mathrm{s}\right)} $$

(17.1)

17.2.1.2 Synthesis of Yb³⁺-Doped La₂MoWO₉

The procedure performed for La₂MoWO₉ was very similar. However, the initial mixtures Yb₂O₃/Y₂O₃/MoO₃/ WoO₃ were heated in air only in one stage at 1100 °C during 2 h. The micro-powder was obtained according to the reaction:

$$ \left(1-x/2\right)\ {\mathrm{La}}_2{\mathrm{O}}_{3\left(\mathrm{s}\right)}+x/2\ {\mathrm{Yb}}_2{\mathrm{O}}_{3\left(\mathrm{s}\right)}+{\mathrm{MoO}}_{3\left(\mathrm{s}\right)}+{\mathrm{WO}}_{3\left(\mathrm{s}\right)}={\mathrm{La}}_{2-x}{\mathrm{Yb}}_x{\mathrm{MoWO}}_{9\left(\mathrm{s}\right)} $$

(17.2)

17.2.1.3 Synthesis of Yb³⁺-Doped Y₆MoO₁₂

A series of micro-crystalline molybdates with chemical formula of Y_6-xYb_xMoO₁₂ (later labelled as Yb³⁺-doped Y₆MoO₁₂) with different concentration of optically active ion were synthesized by a high-temperature solid state reaction method. The concentration of the activator was set to 0.1, 1, 3, 5, 10, and 20 mol%, calculated in respect to Y³⁺ substitution. Commercial powders of Y₂O₃ (99.999%, Stanford Materials), Yb₂O₃ (99.995%, Stanford Materials), MoO₃ (99.95%, Alfa Aesar) were used as the starting reactants for synthesis of Yb³⁺-doped Y₆MoO₁₂ solid solutions. Before the synthesis, RE oxides were calcined at 850 °C, with two 12-h heating stages to remove adsorbed moisture and carbon dioxide. Weighed in appropriate molar ratios, the oxides were homogenized by grinding in an agate mortar. Next, as-prepared Yb₂O₃/Y₂O₃/MoO₃ mixtures were heated in corundum crucibles, in air, with 6-h annealing stages, and at temperatures from 550 to 750 °C. In next step, all samples were sintered with 12-h stages and at temperatures from 800–1000 °C. Finally, all ceramic materials were annealed at 1200, 1400, 1500 and 1550 °C for 6-h stages. After each heating stages the samples were cooled slowly down to ambient temperature, and for better reactivity, ground in an agate mortar. Several regrinding and heating sequences at the temperatures mentioned above were necessary to obtain appropriate and high-purity products. The synthesis of the solutions was started at a relatively low temperature (550 °C) due to the low melting point of MoO₃ (ca. 760 °C) and the possible mass loss caused by evaporation of this oxide. The phase composition of the samples was controlled by using powder X-ray diffraction (XRD). The pure and doped RE₆MoO₁₂ phases can arise at ~1500 °C. For this reason, it was absolutely necessary to apply so high temperature of annealing. The synthesis of samples under study can be described by the following equation:

$$ \left(3\hbox{--} 1/2\mathrm{x}\right)\ {\mathrm{Y}}_2{\mathrm{O}}_3+1/2\mathrm{x}\ {\mathrm{Y}\mathrm{b}}_2{\mathrm{O}}_3+{\mathrm{MoO}}_3={\mathrm{Y}}_{6-\mathrm{x}}{\mathrm{Y}\mathrm{b}}_{\mathrm{x}}{\mathrm{MoO}}_{12} $$

(17.3)

17.2.1.4 Fabrication of Yb³⁺-Doped La₂Mo₂O₉ and La₂MoWO₉ Translucent Ceramics

For the preparation of first translucent ceramics the cubic 3 mol% Yb³⁺-doped La₂Mo₂O₉ and 3 mol% Yb³⁺-doped La₂MoWO₉ nano-crystalline powders were used. This powders were prepared by combustion method according to previous paper [47]. Appropriate amounts of nano-powders was transferred to form and pressed under atmospheric pressure of 4 atm. during 5 min. Then, a white tablets was sintered at 1200 °C/6 h in vacuum atmosphere. In result a light yellowish (yellow-orange) (translucent ceramics, presented on the picture in the Sect. 17.4 of this paper, was obtained. Attempts to obtain transparent ceramics were carried out only for 3 mol% Yb³⁺-doped La₂Mo₂O₉ and 3 mol% Yb³⁺-doped La₂MoWO₉.

17.2.2 Apparatus

XRD Phase Analysis

Room temperature (RT) powder XRD method was used to identify the phase purity and the crystal structure of solid solutions. X-ray diffraction patterns were collected over the angular range 2Θ = 10–90° in continuous scanning mode with the scan rate of 0.008° per step on a D8 ADVANCE powder diffractometer with nickel-filtered Cu Kα radiation (λ = 1.5418 Å) and a Vantec detector, as well as on EMPYREAN II diffractometer (PANalytical) using Cu Kα_1,2 radiation (λ = 1.5418 Å) within 2Θ = 10–70° range with a scan rate 0.008° per step and a counting time of 4 s per step. XRD patterns were analyzed by HighScore Plus 4.0 software and lattice parameters were calculated using the least squares refinement procedure by POWDER software [59].

Density Measurements

The density of samples under study was measured on a Ultrapycnometer 1000 Quantachrome Instruments (model Ultrapyc 1200e, USA) using argon (99.999%) as a pycnometric gas.

Scanning Electron Microscopy (SEM)

The particle size and surface morphology of the Yb³⁺-doped micro-powders and translucent ceramics were studied by scanning electron microscopy (SEM) using Hitachi S-3400N equipped with an energy dispersive X-ray spectroscopy EDS detector Thermo Scientific Ultra Dry. The powders were coated with thin gold alloy layer to facilitate conductivity, while the ceramics was analyzed without the gold coating.

Emission and Excitation Measurements

The steady state emission spectra were measured using a SpectraPro 750 monochromator, equipped with a Hamamatsu R928 photomultiplier and a 1200 l/mm grating blazed at 500 nm. A 450 W xenon arc lamp was used as the excitation source. It was coupled with a 275 mm excitation monochromator which used a 1800 l/mm grating blazed at 250 nm. The excitation spectra were corrected for the excitation light intensity while the emission spectra were corrected for the instrument response. The measurements were taken at room temperature (RT). Site selective excitation and time resolved spectroscopy (or Fluorescence Line Narrowing) was performed at 77 K in a nitrogen cryostat (or quartz dewar). A tuneable laser (EKSPLA OPO NT342B, 7 ns, 10 Hz) was used for scanning the excitation wavelength around 465 and 580 nm. The fluorescence was collected with an optical fiber and analysed with the help spectrometer equipped with a 1200 l/mm grating blazed at 500 nm, coupled to intensified charge coupled device (ICCD) camera (Shamrock 303 and iStar from Andor Technology).

Luminescence Decay Measurements

The luminescence decay curves were recorded under pulsed laser excitation (OPO laser, EKSPLA NT342, 10 Hz, 7 ns), the fluorescence intensity around 1.06 μm being detected with a R1767 Hamamatsu photomultiplier through a HRS1 Jobin-Yvon monochromator equipped with a 1 μm blazed grating and coupled to a LECROY LT 342 digital oscilloscope. The luminescence decay curves were recorded at RT and 77 K.

17.3 Results and Discussion

17.3.1 Yb³⁺-Doped Y₂Mo₂O₉ and Yb³⁺-Doped Y₂MoWO₉ Micro-powders

17.3.1.1 Structural Analysis

As we reported in the articles devoted to La₂Mo₂O₉ molybdate activated by Nd³⁺ ions, dilanthanum molybdate with the formula of La₂Mo₂O₉ shows a reversible polymorphic transformation at 555 °C (determined during controlled heating of La₂Mo₂O₉ sample) from low-temperature α-phase (monoclinic structure) to high-temperature β-phase (cubic structure) [44, 45].

The monoclinic structure of La₂Mo₂O₉ phase with unit cell parameters: a = 14.325(3) Å, b = 21.482(4) Å, c = 28.585(6) Å, β=90.40(3)°, V = 8796(3) ų, Z = 48, S.G. P2₁ was characterized based on small clear crystals by Evans et al. [60]. This structure is very complex and does not resemble the typical inorganic oxide due to the presence of 312 crystallographically independent atoms: 48 La, 48 Mo and 216 O. It corresponds to the 2 × 3 × 4 superstructure of the cubic structure of high-temperature La₂Mo₂O₉ phase. Both La³⁺ and Mo⁶⁺ cations have mixing oxygen coordination. The La³⁺ cations are found in irregular geometries containing different coordination numbers, between 6 and 12 oxygen anions, and 30 out of the 48 independent La³⁺ cations possess 9 oxygen anions. In turn, the Mo⁶⁺ cations occur in the three local coordination: tetrahedral, trigonal bipyramidal and octahedral.

The crystallographic data of cubic structure of high-temperature La₂Mo₂O₉ phase based on single crystal were presented by Alekseeva et al. [61]. The X-ray diffraction measurements were performed at 33 K to obtain the following unit-cell parameters: a = 7.1377(2) Å, α=β=γ = 90°, V = 363.64(2) ų, Z = 2, S.G. P2₁3. In contrast to the monoclinic structure, both of La³⁺ and Mo⁶⁺ coordination polyhedra are not be precisely determined because of partially occupied positions by the ions. The analysis of structure showed 15 oxygen anions surround La³⁺ cation and 7 oxygen anions are in the environment of Mo⁶⁺ cation. Moreover the interatomic distances between metal and oxygen vary from 2.3 to 2.93 Å for La-O and from 1.53- to 1.97 Å for Mo-O [62].

Hou et al. have also studied the cubic structure features of high-temperature La₂Mo₂O₉ modification using ab initio MD simulations. In contrast to previous results reported by Alekseeva, there were different data on the geometry optimization calculation. An important conclude of their research is that the all structure consists of mixture of coordinated cation sites. Molybdenum creates MoO₄ tetrahedra and MoO₅ trigonal bipyramids, while the lanthanum cations create much complex polyhedra, because La³⁺ cations are surrounded by seven or eight oxygen anions, forming LaO₇ and LaO₈ polyhedra. However, all Mo cations do not share common oxygens and are isolated by lanthanum polyhedra. In contrast to molybdenum polyhedra, there was noticed that three LaO₈ polyhedra and one LaO₇ polyhedra are connected together by sharing oxygen ions, forming complex network in three dimensions [63].

In case of Yb³⁺-doped La₂Mo₂O₉ solid solutions, is expected that the Yb³⁺ ions occupy the same crystallographic positions as La³⁺ ones. The substitution of La³⁺ ions (1.10 Å for CN = 7 and 1.16 Å for CN = 8) by much smaller Yb³⁺ ones (0.925 Å for CN = 7, and 0.985 Å for CN = 8) and the same oxidation state (+3) does not require of a charge compensation and prevents to the formation of cationic holes.

Figure 17.1 shows the room temperature XRD patterns of the Yb³⁺-doped samples obtained after the final sintering stage with the reference patterns of monoclinic (ICSD #172479) as well as cubic modification (ICSD #420672) of La₂Mo₂O₉ within the 2θ range of 10–70°.

Fig. 17.1

X-ray powder diffraction patterns of monoclinic and cubic Yb³⁺ − doped molybdates and molybdato-tungstates

If we look at XRD patterns from ICSD for the monoclinic (ICSD #174279) and the cubic (ICSD #420672) phases we could have an impression that that the positions of the diffraction lines for both phases are the same. However, the reflections for the monoclinic system have a more complex profiles. As an example in the insert of Fig. 17.1 we present the most informative pseudo-cubic [321] peak at 47Θ48° 2Ɵ angle, which by change of the profile allows to determine the monoclinic (peak split into few components) or cubic (only one component of much broad and symmetrical peak) structure. The content of La₂O₃-MoO₃-Yb₂O₃ initial mixtures, total concentration of Yb³⁺ ions in final doped materials, calculated lattice parameters, calculated and experimental values of density for the identified Yb³⁺-doped La₂Mo₂O₉ and Yb³⁺-doped La₂MoWO₉ materials with detailed analysis of diffractograms were reported recently [46, 48]. It is observed that when the initial Yb³⁺ content is small, i.e. 0.5–2 mol%, the product of reaction between three metal oxides is only La_2-xYb_xMo₂O₉ solid solution with monoclinic structure of La₂Mo₂O₉. We should not forget that Yb³⁺ ion (CN = 7 ionic radius – 0.925 Å, CN = 8 ionic radius – 0.985 Å) has much smaller ionic radius than La³⁺ one (CN = 7 ionic radius – 1.10 Å and CN = 8 ionic radius – 1.16 Å). XRD measurements of the sample obtained by heating the mixture comprising initially 1 mol% of Yb₂O₃ (3 mol% Yb³⁺, x = 0.06) show the presence of one solid phase, i.e. La_2-xYb_xMo₂O₉ solid solution with cubic symmetry. At the concentration range over 4 mol% Yb³⁺, two solid phases are found, i.e. cubic modification of La_2-xYb_xMo₂O₉ and monoclinic Yb₂MoO₆ occurring in the samples on treatment. Starting from the sample with the concentration of 4 mol% Yb³⁺ the lattice parameters are very close to each other and to the a parameter calculated for cubic modification of La_2-xYb_xMo₂O₉ (x = 0.06) [48]. An unexpected mixture of two phases: cubic molybdate of La₂Mo₂O₉ (ICSD #420672) and additional phase of Yb₂MoO₆ (ICSD #99574) is seen for the sample with concentration of Yb³⁺ equal 10 mol% (in Fig. 17.1 green line). The above observations clearly indicate that solubility limit of Yb³⁺ in La₂Mo₂O₉ is not higher than 4 mol%. Thus, in the case of obtained Yb³⁺-doped materials it is observed that a substitution of La³⁺ ions by Yb³⁺ ones does not stabilize a cubic modification of La_2-xYb_xMo₂O₉ within wide concentration range of dopant. It is only seen in the case of 3 mol% (x = 0.06). The experimental results show that for obtaining the Yb³⁺-doped materials with cubic symmetry in all concentration range of activator by using the high-temperature solid-state reaction, the partial substitution of Mo⁶⁺ ions by W⁶⁺ ones is necessary.

17.3.1.2 Morphology and Particle Size by SEM Analysis

Figure 17.2 shows the SEM (Scanning Electron Microscope) micrographs of Yb³⁺-doped La₂Mo₂O₉ molybdates with different content of the active ion. Due to the many stages of sintering at high temperature the grains formed micro-crystallites with a grain size in the range from 5 to even 15 μm. The overview pictures show that the good quality materials formed only for the materials containing small amount of activator. The powders with concentration of Yb³⁺ ions from 0.5 to 3 mol% are composed from homogeneous spherical shape of grains, with slight aggregation and a boundary between the microcrystals of powders clearly seen. Occasionally, on the surface of the grains one can see a few white grains of much smaller sizes of the order of several nanometers, very rich in Yb³⁺ ions, as indicated the EDS analysis. With increasing concentration of Yb³⁺ ions, the single particles are agglomerated into bigger clusters with irregular shape of micro-meter size. For higher concentration of Yb³⁺ ions the agglomerates form the sintered irregular blocks of size even 50–70 μm. However, at high magnification it is seen that on the surface of the big Yb³⁺-doped La₂Mo₂O₉ grains, the second phase in form of cube-shaped crystals is created. The SEM micrographs and the energy dispersive X-ray (EDS) analysis indicate existing of two phases starting from 4 mol% of Yb³⁺ ions, thus morphology of Yb³⁺-doped La₂Mo₂O₉ strongly depends on the amount of Yb³⁺ ions. From the Fig. 17.2 we see that the second phase occurs quite extensively, and the EDS analysis reveals very high content of Yb³⁺ ions in the cube-shaped crystals. The correlation of the results obtained from two applied methods (XRD patterns and SEM micrographs – EDS analysis) allows us supposed that this second Yb³⁺- rich phase correspond to the monoclinic Yb₂MoO₆ phase, as postulated in the previous section (17.3.1.1 Structural analysis). Formation of the second phase is probably due to the small solubility limit of Yb³⁺ ions (3 mol%) in Yb³⁺ − doped La₂Mo₂O₉. The presence of La³⁺ ions in the results from EDS analysis is due to the measurement technique. The cube-shaped phase of Yb₂MoO₆ is located on the surface of Yb³⁺-doped La₂Mo₂O₉, so electrons during analysis penetrate also the La₂Mo₂O₉ phase.

Fig. 17.2

SEM micrographs and EDS elemental analysis of Yb³⁺-doped La₂Mo₂O₉ molybdates

For the cubic Yb³⁺-doped La₂MoWO₉ molybdato-tungstate micro-powders the phenomenon of second Yb₂MoO₆ phase formation was not observed [46].

17.3.1.3 Absorption Spectra

In order to investigate the absorption properties of Yb³⁺-doped La₂Mo₂O_9, the measurements at room temperature and 4.2 K have been performed. As an example, in Fig. 17.3 we present the spectra obtained for 4 mol% of Yb³⁺ion. High-resolution absorption spectra present four broad lines corresponding to ²F_7/2(1) → ²F_5/2 (5,6,7) electronic transitions and to vibronic ones. The half-widths and positions of them practically do not change with decreasing the temperature: 1 → 5 (973.3 nm at RT, 972.5 nm at 4.2 K), 1 → 6 (944 nm at RT, and 946 nm at 4.2 K), 1 → 7 (926 nm at both temperature) and vibronic transitions at 963, 904 nm and 912 nm at RT and 910 nm at 4.2 K. When the temperature decreases, the absorption lines should narrow. In this case only broad well-formed 0-phonon is slightly narrowing. Broad lines still recorded at 4.2 K suggest a large disordering of active ions in the structure. We already proposed the existence of such a disorder in the structure for the similar compositions of La₂Mo₂O₉ dilanthanum dimolybdate doped with Nd³⁺ ions as well as for Yb³⁺-doped mixed La₂MoWO₉ molybdato-tungstates [32, 37]. Here, as we can see from the Table 17.1 [42] the difference between the ionic radii of La³⁺ and Yb³⁺ is bigger than in case of La³⁺ and Nd³⁺ ions, so the disorder in the framework may also be bigger.

Fig. 17.3

Absorption spectra of 4 mol% Yb³⁺-doped La₂Mo₂O₉ molybdates recorded at RT and 4.2 K

Table 17.1 Ionic radii of La³⁺, Nd³⁺ and Yb³⁺ according to CN coordination number [62]

In turn, not presented here, room temperature absorption spectra recorded for molybdates with cubic structure (3–10 mol% Yb³⁺-doped La₂Mo₂O₉) are similar as for monoclinic samples (0.5–2 mol%) and present the broad, well-separated and contain only one component of 0-phonon line (1 → 5). Nevertheless, the calculated value of full width at half maximum (FWHM) is equal 196.5 cm⁻¹ (3 mol% of Yb³⁺) [48]. It may be due to the second, hidden component which suggests the multisite character of Yb³⁺ ions. These results are consistent with studies on Yb³⁺-doped La₂MoWO₉, which demonstrated also one and broad 0-phonon line, most probably related to the existence of two main LaO₈ and LaO₇ polyhedra in the structure of La₂MoWO₉ molybdato-tungstates [46, 47]. In the whole series of Yb³⁺-doped La₂Mo₂O₉ molybdates with the cubic structure the values of the FWHM of 0-phonon line systematically decrease with increasing the concentration of Yb³⁺ ion from 196.5 cm⁻¹ (3 mol%) to 106 cm⁻¹ (10 mol%). The intensity of the lines corresponding to 1 → 7 and 1 → 6 transitions in the 875–960 nm spectral range increases with higher concentration of Yb³⁺ up to 10 mol% of Yb³⁺ ions [48]. For high concentration of the activator the intensity of the line at 925 nm corresponds to the 1 → 7 electronic transition is almost equal to intensity of the 0-phonon line. Moreover, the room temperature absorption spectra revealed the additional two additional absorption bands correspond to transition from second (2) and third (3) Stark sublevel of the ground state ²F_7/2 to the lowest higher sublevel (5) of the excited state ²F_5/2, which are located at 1006 nm (2 → 5) and 1030 nm (3 → 5), respectively.

To see the effect of introduction of W⁶⁺ ion into the La₂Mo₂O₉ matrix on the optical properties we present in Fig. 17.4 the room (Fig. 17.4a) and low (Fig. 17.4b) absorption spectra of 8 mol% Yb³⁺-doped La₂Mo₂O₉ (cubic + additional phase) and 10 mol% Yb³⁺-doped La₂MoWO₉ (cubic phase). Comparison of two similar systems: Yb³⁺-doped La₂Mo₂O₉ molybdates and Yb³⁺-doped mixed La₂MoWO₉ molybdato-tungstates lead to conclusion that at room and low temperatures, the absorption lines for La₂Mo₂O₉ are slightly shifted into higher energies and also that for La₂MoWO₉ molybdato-tungstates the spectra are less structured, so the larger disorder in the structure caused by substitution of one Mo⁶⁺ ion (i.r. – 0.59 Å) by one W⁶⁺ one (i.r. – 0.6 Å) is well seen. A big difference is the spectral shape observed for Yb³⁺-doped La₂Mo₂O₉ and Yb³⁺-doped La₂MoWO₉ especially at 4.2 K is a consequence of the presence of stoichiometric Yb₂MoO₆ second phase in 8 mol% Yb³⁺-doped La₂Mo₂O₉. In both cases, the most intense absorption band is 0-phonon line with one broad distinct component, much broader for La₂MoWO₉. The values of full width at half maximum (FWHM) of 0-phonon line are quite different, around two times bigger for Yb³⁺-doped La₂Mo₂WO₉ (97 cm⁻¹) than in case of Yb³⁺-doped La₂Mo₂O₉ (56 cm⁻¹). Based on the results, it can be concluded that the partial substitution of Mo⁶⁺ ion by W⁶⁺ once in Yb³⁺-doped La₂MoWO₉ leads to larger disorder in the structure, although the ionic radii of the two transition metals is quite similar.

Fig. 17.4

Room (a) and low temperature (b) absorption spectra of 8 mol% Yb³⁺-doped La₂Mo₂O₉ (cubic structure + additional phase, red line) and 10 mol% Yb³⁺-doped La₂MoWO₉ (cubic structure, blue line)

17.3.1.4 Emission Spectra

Presented in Figs. 17.5 and 17.6 site selective emission spectra recorded by using a tunable Ti-Sapphire laser were used to determine the spectroscopic properties of Yb³⁺-doped La₂Mo₂O₉ materials and to compare the differences in photoluminescence properties of molybdates crystallizing in the monoclinic (0–2 mol%) and the cubic system (3–10 mol% Yb³⁺). On the low temperature emission spectra of 0.5 mol% Yb³⁺-doped La₂Mo₂O₉ with monoclinic structure (Fig. 17.5) it is easy to notice the complex structure of emission bands in the near infrared region 970–1090 nm. The most intense is called 0-phonon line and corresponds to the 5 → 1 transition in resonance with the 1 → 5 absorption line. This line is used as a reference for the spectroscopy of Yb³⁺-doped materials. Only one component of the 0-phonon line is expected at low temperature for the same crystallographic sites of Yb³⁺ ions in the structure. In Fig. 17.5 with changing the excitation wavelengths from 890 nm to 925 nm it is observed here that the 0-phonon line splits into three components located at around 975.2 nm, 976.9 nm and 979 nm. These results indicate the different distribution of Yb³⁺ ions in the monoclinic structure of molybdates what is strongly related with crystal structure discussed in Sect. 17.3.1.1: According to Evans the La³⁺ cations are found in irregular geometries containing different coordination numbers, between 6 and 12 oxygen anions, and 30 out of the 48 independent La³⁺ cations possess coordination number 9. In turn, the Mo⁶⁺ cations occur in the three local coordination: tetrahedral, trigonal bipyramidal and octahedral [60]. Let’s remind that in the monoclinic Nd³⁺-doped La₂Mo₂O₉ obtained by us, at least two types of Nd³⁺ sites have been assigned with two coordination numbers of LaO₇ and LaO₈ [44, 45].A reasonable assignment might be that Yb³⁺ dopant reveals another coordination number like for example LaO₉, according to Evans possible to exist in this complex structure [60].

Fig. 17.5

Site selective emission spectra of 0.5 mol% Yb³⁺-doped La₂Mo₂O₉ solid solution, measured at 77 K under different excitation wavelengths of the Ti-sapphire laser

Fig. 17.6

Site selective emission spectra of 10 mol% Yb³⁺-doped La₂Mo₂O₉ molybdate, measured at 77 K under different excitation wavelengths of the Ti-sapphire laser

Despite the 0-phonon line, on the emission spectra of Yb³⁺ ions are expected three other emission bands corresponding to ²F_5/2 (5) → ²F_7/2 (2,3,4) electronic transitions. Usually they show the highest intensities with the electronic transitions than vibronic transitions. In case of emission spectra of monoclinic molybdates three expected intense lines with attributed transitions of Yb³⁺ ions are located at 1005.3 nm (5 → 2), 1026 nm (5 → 3) and 1067 nm (5 → 4) nm, respectively. Additionally, the two narrow components of emission lines at 991.9 and 994.6 nm are only recorded in the sample containing 0.5 mol% and more visible under λ_ex = 910 nm from the tunable Ti-sapphire laser. At this moment it is hazardous to assign them to Yb³⁺ ion and might be associated with an impurity only present in this specific sample. The photoluminescence intensity increases rightly with the change of excitation wavelengths from 890 nm to 925 nm in agreement with the profile of the absorption lines in Fig. 17.4.

Moreover, a continuous shift of the emission line corresponding to 5 → 4 transition occurs by changing excitation wavelength from 925 nm to 890 nm. This is another probe of the presence of multisites inside the molybdate lattice.

The emission spectra at low temperature of 10 mol% Yb³⁺-doped La₂Mo₂O₉ by pumping with Ti-sapphire laser are presented in Fig. 17.6. In case of molybdates with cubic structure are observed much broader emission lines with clearly noticeable positions at 978.8 nm (5 → 1), 1002 nm (5 → 2), 1023.5 nm (5 → 3) and 1051.6 nm (5 → 4).

The shape and the number of emission lines stay the same with changing the excitation wavelengths. The dominant features of the emission spectra recorded under laser diode excitation λ_ex = 878 and 900 nm are much intense emission lines attributed to 5 → 2 and 5 → 3 transitions than the 0-phonon line. We think that re-absorption phenomenon of the 0-phonon line 5↔1 occurs largely in such case. This results indicate evident differences for cubic molybdates from the monoclinic structure. Although the 0-phonon line is not split, the calculated value of full width at half maximum (FWHM) of this line is large, around 80.3 cm⁻¹, which indicates a distribution of un-equivalent sites. Moreover, the 0-phonon line has asymmetrical shape which is the most visible under λ_ex = 900 nm and 922 nm by pumping with a tuneable Ti-sapphire laser, leading to the hypothesis of two or three main environments of Yb³⁺ ions in this cubic lattice. Additionally, another re-absorption effect is observed on the emission spectra of Fig. 17.6 at 972 nm. It is probably the result of the high concentration of Yb³⁺ ions in Yb₂MoO₆ as seen in Fig. 17.2 for the morphology study.

We wanted to know the concentration dependence of Yb³⁺ ions on photoluminescence properties of molybdates and then we have recorded the low temperature emission spectra under the same laser diode excitation at λ_ex = 405 nm into the charge transfer band of the molybdate group (Figs. 17.7 and 17.8).

Fig. 17.7

Emission spectra of La₂Mo₂O₉:Yb³⁺ 0.5, 1 and 2 mol% solid solutions crystallizing in the monoclinic system, measured at 77 K under laser diode excitation λ_ex = 405 nm into the charge transfer band of the molybdate group

Fig. 17.8

Emission spectra of La₂Mo₂O₉:Yb³⁺ 3–10 mol% materials crystallizing in the cubic system, measured at 77 K under laser diode excitation λ_ex = 405 nm into the charge transfer band of the molybdate group. The values of wavelengths are mentioned for 8 mol% of Yb³⁺

The 0.5 mol% of Yb³⁺-doped La₂Mo₂O₉ molybdate with monoclinic structure shows two abnormal additional lines located at 989.6 nm and 992.1 nm which are still noticeable, under the laser diode excitation at λ_ex = 405 nm. As only this 0.5 mol % is concerned we have observed these two lines belong to a stoichiometric Yb₂MoO₆ phase characterized by the smallest white points, less than 1 μm, in the SEM photo of Fig. 17.5. The splitting of the 0-phonon line into three components (972.3 nm, 975 nm and 977.8 nm) are visible only for the lowest concentrations 0.5 and 1 mol% of Yb³⁺-doped La₂Mo₂O₉ molybdates. However, the profile of this 0-phonon line is questionable in the following way. Instead to assign the sharp line at 972.3 nm to one component belonging to one special site symmetry or polyhedral coordination, we also can imagine there is a hole between 972.3 and 975 nm due to the reabsorption from the Yb³⁺ rich phase as can be seen in Fig. 17.2.

It should be a way to measure the absorption line of this Yb³⁺ segregation in the sample. In such hypothesis, it seems we can keep the resolution of three components of the 0-phonon line in Fig. 17.7 connected with three polyhedra in the monoclinic structure as already interpreted previously from the absorption spectra. When concentration of Yb³⁺ ions increases, the structured shape of the 0-phonon line disappears in the new cubic structure. Moreover, the large full width at half maximum (FWHM) of this line equals to 77.6 cm⁻¹ suggests at least two or three slightly un-equivalent crystallographic sites occupied by the Yb³⁺ in substitution of La³⁺ ions of the cubic phase.

Based on the results, both monoclinic and cubic structures are pointed out in Yb³⁺-doped La₂Mo₂O₉ molybdates. The substitution of Yb³⁺ ions on several types of La³⁺ sites causes the broadening of all emission lines suggesting a disordering of the Yb³⁺ ions in the host structure. The spectral resolution of the monoclinic phase is much higher than the cubic one. More especially, three main configurations of sites are detected in the monoclinic phase and most probably in the cubic phase which might be connected with three coordination numbers.

17.3.1.5 Some Comparison Between Cubic Structure of Yb³⁺-Doped La₂Mo₂O₉ and Yb³⁺-Doped Mixed La₂MoWO₉

In order to compare spectroscopic properties of two quite similar cubic systems: Yb³⁺-doped La₂Mo₂O₉ and Yb³⁺-doped La₂MoWO₉ were set together low and room temperature emission spectra recorded under Ti-sapphire laser (Fig. 17.9). For both samples, the most intense emission broad line on emission spectra corresponds to the similar 0-phonon line (²F_5/2 (5) → ²F_7/2 (1) transition). The calculated value of full width at half maximum (FWHM) of the 0-phonon line is around 73.7 cm⁻¹ for 3 mol% Yb³⁺-doped La₂Mo₂O₉ and 72.2 cm⁻¹ for 3 mol% Yb³⁺-doped La₂MoWO₉, which suggests multisite disordered distribution of Yb³⁺ ions in both of structures. The room temperature and 77 K emission spectra shows weakly separated broad emission bands. Successfully, along with the temperature drop to 77 K, the three emission bands located at 1000 nm (5 → 2), 1020 nm (5 → 3) and 1060 nm (5 → 4) for Yb³⁺-doped La₂MoWO₉ and 1000.8 nm (5 → 2), 1024.4 nm (5 → 3) and 1047.8 nm (5 → 4) for Yb³⁺-doped La₂Mo₂O₉ were observed.

Fig. 17.9

Emission spectra of 3 mol% Yb³⁺-doped La₂Mo₂O₉ and 3 mol% Yb³⁺-doped La₂MoWO₉ solid solutions under selected excitation lines of the Ti-sapphire laser

17.3.1.6 Yb³⁺ Energy Level Diagram in La₂Mo₂O₉

In order to evaluate the crystal field splitting of ²F_5/2 and ²F_7/2 multiplets of Yb³⁺ ions in Yb³⁺-doped La₂Mo₂O₉ is recorded low temperature absorption spectra (4.2 K) and emission spectra (77 K). The low temperature measurements are necessary to determine exact location of absorption and emission lines in contrast to the room temperature measurements which are not structured enough.

The energy levels scheme of Yb³⁺ ion in La₂Mo₂O₉ was drawn in Fig. 17.10. The value of total splitting levels equal 10,795 cm⁻¹ for 8 mol% Yb³⁺-doped La₂Mo₂O₉:Yb³⁺ is slightly bigger then 10,730 cm⁻¹ in case of 10 mol% Yb³⁺-doped La₂MoWO₉. The partial substitution of Mo⁶⁺ ions by W⁶⁺ ones (in ratio 1:1) promote a bit smaller splitting of multiplets of Yb³⁺ ions.

Fig. 17.10

Low temperature Stark splitting levels of Yb³⁺ ions calculated from the experimental data

As can be seen from the Figs. 17.11 and 17.12, the position of the 0-phonon line on absorption and emission spectra are not the same. One most probable interpretation of this evident difference is the re-absorption effect associated with the resonance of absorption and emission lines. Two reasons can be involved: first, the presence of the second Yb₂MoO₆ phase with high concentration of Yb³⁺ ions giving rise to a hole in the 0-phonon line. Secondly, inside the main structure, where multisites occur, a deformation of the profile of the 0-phonon line emission spectrum showing an overlapping between the absorption line of one of the three centers with the total emission spectrum.

Fig. 17.11

Superposition of the absorption spectra at 4.2 K, emission spectra at 77 K of 8 mol% Yb³⁺-doped La₂Mo₂O₉ cubic molybdate under laser diode excitation λ_ex = 405 nm (CT band)

Fig. 17.12

Superposition of the absorption spectra at 4.2 K, emission spectra at 77 K of 10 mol% Yb³⁺-doped mixed La₂MoWO₉ cubic molybdato-tungstates under Ti-sapphire laser excitation

Additionally, to compare the Stark splitting levels of Yb³⁺ ions and also re-absorption phenomena in La₂Mo₂O₉ and La₂MoWO₉ [47, 48] was reminded the low temperature superposition of absorption and emission spectra for 10 mol% Yb³⁺-doped La₂MoWO₉ (Fig. 17.12).

17.3.1.7 Decay Analysis

Figure 17.13 presents collected decay profiles of Yb³⁺-doped La₂Mo₂O₉ series of molybdates with large range of activator concentration (0.5–10 mol% Yb³⁺) recorded at room temperature. The decay times were recorded under pulsed OPO laser laser pulsed excitation under λ_ex = 975 nm by monitoring ²F_5/2 → ²F_7/2 luminescence at λ_em = 1030 nm for the Yb³⁺-doped La₂Mo₂O₉. These decays are considered as exponential profiles with an excellent approximation. Only the points for the first μs deviate from the exponential as a result of an energy transfer between multisite Yb³⁺ ions. The values of the experimental lifetimes strongly depend on amount of Yb³⁺ ions as shown in Fig. 17.13. Among the whole series a slight increase of decay times is observed with approaching to 320 μs for 3 mol% Yb³⁺. Then, the clearly visible reduction of value lifetimes is noticed for 4 and 5 mol%, which may be caused by the appearance of a new cubic phase. Above 5 mol% of Yb³⁺ ions, the decay lifetimes increase again from 319 μs up to 346 μs related to 6 and 9 mol% and increase again up to the slightest reduction to 327 μs for 10 mol%. There is evident correlation between the structural change from the monoclinic form (Yb³⁺ ≤ 2 mol%) to the cubic form (Yb³⁺≥3 mol%) and the decay lifetimes. An increase of Yb³⁺ ions contributes to the resonant diffusion process between the Yb³⁺ ions in all compounds and manifests as self-trapping phenomenon so that Fig. 17.13 shows the same behavior of the self-trapping effect for the two phases till 10 mol% of Yb³⁺ ions. The change of monoclinic and cubic structures indicates irregular changes of lifetimes has been compared with Yb³⁺-doped La₂Mo₂WO₉ solid solutions which was described by the two usual phenomena of self-trapping effect and self-quenching effect [47]. Respectively, for the same activator concentrations are noticeable much longer decay lifetimes for Yb³⁺-doped La₂MoWO₉ than for Yb³⁺-doped La₂Mo₂O₉ solid solutions. It can suggest less number of resonant transitions of the ²F_5/2 ↔ ²F_7/2 0-phonon line and so shorter distances of light between the grains of Yb³⁺-doped La₂Mo₂O₉: solid solutions.

Fig. 17.13

Comparative concentration dependence of experimental decay times in Yb³⁺-doped La₂Mo₂O₉ and Yb³⁺-doped La₂MoWO₉ with different concentration of Yb³⁺ ions

17.3.1.8 Comparative Analysis of 3 mol% Yb³⁺-Doped La₂Mo₂O₉ and 3 mol% Yb³⁺-Doped La₂MoWO₉ Ceramics

First translucent ceramics of 3 mol% Yb³⁺-doped La₂Mo₂O₉ and 3 mol% Yb³⁺-doped La₂MoWO₉ ceramics have been successfully fabricated. The comparison turns out in favor of Yb³⁺-doped mixed La₂MoWO₉. The structural and spectroscopic differences are also significant for these materials. Figure 17.14 presents SEM images with the EDS elemental analysis performed for 3 mol% Yb³⁺-doped La₂Mo₂O₉ and 3 mol% Yb³⁺-doped La₂MoWO₉ micro-ceramics sintered at 1200 °C/6 h in vacuum. For both cases the images indicate inhomogeneous morphology consisted of quite large grains with circular shape and much smaller, white points mostly accumulated at the grain boundaries (Fig. 17.21). According to the EDS analysis, the white points contain higher amount of Yb³⁺ ions than at the grains surface. The identification of this additional phase (white points) is very difficult due to the lack in crystallographic data base of standard corresponding to this phase. The performed analysis of morphology for both ceramics shows the troubles with obtaining transparent ceramics contained only one pure phase of Yb³⁺-doped La₂Mo₂O₉ or Yb³⁺-doped La₂MoWO₉. From Fig. 17.14 we see that La₂MoWO₉ ceramics is less yellow and characterized by better translucency.

Fig. 17.14

SEM images with the EDS elemental analysis performed for 3 mol% Yb3³⁺-doped La₂Mo₂O₉ and 3 mol% Yb³⁺-doped Yb³⁺-doped La₂MoWO₉ micro-ceramics sintered at 1200 °C/6 h in vacuum

Figure 17.15 presents comparison of room temperature and 4.2 K absorption spectra of Yb³⁺-doped La₂Mo₂O₉ and Yb³⁺-doped La₂MoWO₉ ceramics prepared in the same annealing conditions at 1200 °C/6 h in vacuum. The room temperature absorption spectra reveals clearly visible differences of shape and number of components in the 850–1100 nm spectral range. Going to the low temperature absorption spectra, the differences are more pronounced. Only in case of Yb³⁺-doped La₂MoWO₉, the intense 0-phonon line (1 → 5 transition) has two components, one at 968.9 nm at 4.2 K caused by the presence of the stoichiometric Yb₂MoO₆ in white points of Fig. 17.14 (11.73 at% and 9.67 at.% respectively) and another one at 976.2 nm at 4.2 K in the regular phase of Yb³⁺-doped La₂MoWO₉. Therefore, on absorption spectra of Yb³⁺-doped La₂Mo₂O₉ is visible only one component of 0-phonon line at 976.2 nm at 4.2 K. It is worth note that the 0-phonon line is relatively wide and a full width at half maximum (FWHM) equals to 72 cm⁻¹ for molybdate and 44 cm⁻¹ for molybdato-tungstate system. It may be suggest different distribution of the crystallographic Yb³⁺ sites in both structures. Additionally, even at low temperature absorption bands of Yb³⁺-doped La₂MoWO₉ are more structured by existence of additional components primarily in the 900–970 nm spectral range.

Fig. 17.15

Room and low temperature (4.2 K) absorption spectra of 3 mol% Yb³⁺-doped La₂Mo₂O₉ and 3 mol% Yb³⁺-doped La₂MoWO₉ sintered micro-ceramics

The luminescence properties of 3 mol% Yb³⁺-doped La₂Mo₂O₉ and 3 mol% Yb³⁺-doped La₂MoWO₉ ceramics were investigated by using selective lines of Ti-sapphire laser in Fig. 17.16. The spectra were measured both at room and 77 K. As we can see, the common graph of emission spectra for Yb³⁺-doped La₂Mo₂O₉ revealed mostly the same shape emission lines recorded at low temperature by changing the excitation wavelength and quite similar emission spectra at room temperature with the most intense and broad 0-phonon line at 977 nm as well as weakly resolved (5 → 2, 5 → 3, 5 → 4) emission lines in the 980–1080 nm spectral region. In contrast to that, the low temperature (77 K) emission spectra of 3 mol% Yb³⁺-doped La₂MoWO₉ demonstrated the splitting of 0-phonon line into two components at 975 nm and 978 nm that affirmed the multisite character of Yb³⁺ ions most probably related to the two main YbO₇ and YbO₈ polyhedra. Also changes in the intensity ratio of the emission lines strongly dependent on the excitation wavelengths. Additionally, the emission spectra of 3 mol% Yb³⁺-doped La₂MoWO₉ includes one hole at 969.2 nm as signature of the reabsorption line corresponding to the additional 0-phonon line in the absorption spectra at 4.2 K presented in Fig. 17.15. This absorption line is associated with the unknown tetragonal Yb³⁺-rich phase, which is observed as white points on the grain boundaries in Fig. 17.14 [46, 47]. In the case of the excitation line at 937 nm, we observed more than three bands in the range from 990 to 1100 nm, which should correspond to transitions from the lowest Stark level of the ²F_7/2 excited state (5) to Stark levels 2, 3, or 4. The multiplicity of emission lines point out a multisite character of the samples.

Fig. 17.16

Site selective emission spectra of 3 mol% Yb³⁺-doped La₂Mo₂O₉ and 3 mol% Yb³⁺-doped La₂MoWO₉ sintered micro-ceramics, measured under different excitation lines of the Ti-sapphire laser

17.3.1.9 Conclusion for Yb³⁺-Doped La₂Mo₂O₉/Mixed La₂MoWO₉ Cubic Materials

The structural studies for the first group: La₂Mo₂O₉ activated by Yb³⁺ dopants from 0.5 mol% to 25 mol% reveal the low solubility limit of 3 mol% Yb³⁺ ions in the La₂Mo₂O₉ host lattice. A small quantity of active ions (≤2 mol% of Yb³⁺) leads to obtain the pure monoclinic structure, while when the amount of Yb³⁺ ions is above 3 mol%, the mixture of two phases, the cubic structure of Yb³⁺-doped La₂Mo₂O₉ and the additional phase of monoclinic stoichiometric Yb₂MoO₆, was detected. Depending on the concentration of Yb³⁺ ions, the obtained molybdates have different morphology. If the amount of dopant is less than 3 mol% good quality homogeneous particles, mostly in spherical shape are observed. In case of higher concentration of Yb³⁺ ions the irregular clusters of grains without visible grain boundaries covered by a second phase in the form of cube-shaped crystals are present. This second phase is corresponding to the monoclinic stoichiometric Yb₂MoO_{6 .}phase. The low temperature emission spectra of Yb³⁺ probe ions-doped La₂Mo₂O₉ show three components of 0-phonon line for monoclinic molybdates and broad but not resolved 0-phonon line for cubic molybdates. Both results indicate the multisite character of Yb³⁺ ions in each of main un-equivalent sites of the La₂Mo₂O₉ structure which are consistent with research performed for Yb³⁺-doped mixed La₂MoWO₉. Consequently, it can be assumed that the environment of all Yb³⁺ ions can be distinguished as, first of all, two types of polyhedra YbO₇ and YbO₈ like in Yb³⁺-doped La₂MoWO₉ and secondly by a third type of YbO₉ polyhedron_. Our results are confirmed by the decay time measurements. Summing up the results, it is evident strongly influence of the concentration of Yb³⁺ ions and type of crystallographic system on the luminescence properties.

In view of continuous research on application of the cubic molybdates or tungstates as cubic laser ceramics, the new Yb³⁺-doped La₂Mo₂O₉ are less promising candidates in contrast to the Yb³⁺-doped mixed La₂MoWO₉, which stabilize the pure cubic system in a whole concentration range (0.1–20 mol%). Also all the samples are very homogeneous.

We have shown that Yb³⁺ rare earth is a probe ion. It gives a new deeper contribution of the complex structure of both La₂MoO₉ and La₂MoWO₉ molybdate families. Our approach is similar with the analysis we have previously done with Nd³⁺ ion, which can be also used as a structural probe one. More generally we should bring all contributions to the knowledge of these crystallographic structures from the spectroscopic properties of rare earth optical ions. In this way, the spectroscopy of Eu³⁺ rare earth probe ion should bring complementary results in the next step.

First translucent ceramics of 3 mol% Yb³⁺-doped La₂Mo₂O₉ and 3 mol% Yb³⁺-doped La₂MoWO₉ ceramics have been successfully fabricated. The comparison turns out in favor of Yb³⁺-doped mixed La₂MoWO₉. The structural and spectroscopic differences are also significant for these materials. SEM images with the EDS elemental analysis show for both cases inhomogeneous morphology consisted of quite large grains with circular shape and much smaller, white points mostly accumulated at the grain boundaries which contain higher amount of Yb³⁺ ions than at the grains surface. So, in both cases we deal with phase segregation in first translucent ceramics.

17.3.2 Yb³⁺-Doped Y₆MoO₁₂

17.3.2.1 Structure Characterization

Basing on the article of Fournier [64] from 1970 devoted to deep research on Ln₂O₃-MoO₃ systems and Ln₆MoO₁₂ phases, we know that Y₆MoO₁₂ (3Y₂O₃:MoO₃) can exist in two forms: until 1480 °C – low temperature orthorombic phase and above 1500 °C – high temperature face cantered, cubic (Z = 4) structure, which is very stable. The compositions with chemical formula Ln₆MoO₁₂ previously investigated by Aikten [65] and Bartram [66] were attributed to the cubic and orthorhombic structures. Fournier et al. have repeated the investigations by preparing the samples at 1350 °C, excepting the Y₆MoO₁₂ and Er₆MoO₁₂ obtained at 1500 °C. Thanks to this research which we know that stoichiometric molybdates with La, Pr, Nd, Sm, Eu, Gd, Tb, Dy and Ho crystallize in the cubic phase already at 1350 °C [64]. The three last lanthanides i.e. Tm, Yb and Lu do not give the cubic Ln₆MoO₁₂ phase event at 1525 °C. Maybe this transformation could be possible at much higher temperature. It is necessary to add that all information on the structure of Ln₆MoO₁₂ described by Fournier based on the refinements of XRD powder patterns and in the literature there is no information of crystal structure from the single crystals of Y₆MoO₁₂. This is why it is a great need to obtain samples in the form of single crystals.

Nevertheless, the crystal structure of Y₆WO₁₂ crystalized in the rhombohedral system (S.G. R3 and Z = 3 for the R-centered setting) from the Rietveid refinement reported by Diot [67] can be helpful to discussion of Y₆MoO₁₂ structure. Obtained at 1300 °C/24 h ternary oxides of Y₆WO₁₂ crystallize with a three-dimensional rhombohedral structure closely related to that of the binary oxides Ln₇O₁₂ and deriving from the ideal fluorite structure. In this case the yttrium ion is sevenfold coordinated with Y-O bond length ranging from 2.19 to 2.70 Å. The coordination polyhedron may be described as a mono-capped trigonal prism. The tungsten atom is located at the center of a WO₆ octahedron with unique W-O distances of 1.98 and 1.92 Å. In the crystal structure of Y₆WO₁₂, W atoms occupy 3a (0, 0, 0) octahedral sites with six O atoms around them, forming slightly deformed WO₆ octahedra, while Y atoms occupy 18f (x, y, z) sites and they are coordinated to three O1 atoms and four O2 atoms, having C₁ point symmetry. The WO₆ and YO₇ polyhedra are connected each other by sharing corners and edges [67]. It is important to notice that at the temperature 1200 °C/12 h the Y₆WO₁₂ represents the hexagonal system. According to the literature, [66, 68, 69] Y and Mo ions be distributed randomly in the cationic sublattice, and O ions and O vacancies should be distributed randomly in the anionic sublattice. An ordered distribution of Mo and O vacancies will result in a related hexagonal phase. For the isostructural Y₆WO₁₂, the disordered cubic fluorite phase is considered to be metastable. As regard to Y₆MoO₁₂, a coexistence of cubic and hexagonal phases was obtained from solid-state reaction, while the only cubic phase was observed from the chemical solution method [54]. This indicates that the stability of the cubic phase is equivalent or surpassing that of the hexagonal phase.

The Yb³⁺-doped Y₆MoO₁₂ micro-powders were characterized by XRD method to verify the phase purity. Figure 17.17 plots experimental results as a function of the Yb³⁺ concentration compared with the simulated XRD pattern of cubic Y₆MoO₁₂ (ICSD#30-1456) from the database of inorganic crystal structures [31]. It is obvious that all the diffraction peaks of these samples are in good agreement with the pure Y₆MoO₁₂ and no other phases as impurities can be detected. This result indicates that the Yb³⁺ ions were completely incorporated into the Y₆MoO₁₂ host lattice without making significant changes to the crystal structure. As the ionic radius of Yb³⁺ (r = 0.925 Å, CN = 7) is smaller that of Y³⁺ (r = 0.96 Å, CN = 7), we suppose that Yb³⁺ ions occupy Y³⁺ sites. The un-doped and Yb³⁺-doped Y₆MoO₁₂ crystallize as disordered cubic fluorite phase with a space group of Fm-3 m and the lattice constants are calculated to be a = 5.29 Å. From literature we know also that the fluorite structure is capable to construct superstructures with its flexible nature [29, 31].

Fig. 17.17

X-ray powder diffraction patterns of Y₆MoO₁₂ and Yb³⁺-doped Y₆MoO₁₂ solid solutions with different contents of the Yb³⁺ optically active ions

The lattice parameter calculated basing on the indexing powder diffraction patterns were reported recently [49]. The substitution of Y³⁺ ions by smaller Yb³⁺ leads to systematic decrease in the lattice parameters, thus we observed linear dependence of lattice constant vs. Yb³⁺ content for cubic Yb³⁺-doped Y₆MoO₁₂. Furthermore, using the pycnometric method, an experimental density of each obtained solution was determined. The unit cell parameter and the volume calculated for each analyzed solution satisfy the Vegard law [49].

17.3.2.2 Morphology and Particle Size by SEM

According to very high sintering temperatures (1550 °C) applied in the solid-state reaction to obtain pure cubic phase the grains formed micro-crystallites with a grain size in the range from 0.5 to even 8 μm. A large impact on the size has also the fact that the reactions do not occur quickly and violently as in the case of the combustion method.

As an example in Fig. 17.18 we present the SEM (Scanning Electron Microscope) micrographs Y₆MoO₁₂ compound and Yb³⁺-doped Y₆MoO₁₂ solid solutions with a different content of the active ion. The overview pictures show a good quality materials. Aggregates composed of grains and a type of boundary between the microcrystals of powders with different concentrations of Yb³⁺ ions can be seen. The overview pictures show separated particles and loose clusters. However, they create also the bigger forms by aggregation of the smaller grains of irregular forms. For Yb³⁺-doped Y₆MoO₁₂ solid solutions no second phase formation was observed, the samples are very homogeneous.

Fig. 17.18

SEM micrographs of Y₆MoO₁₂ and Yb³⁺-doped Y₆MoO₁₂ solid solutions

17.3.2.3 Absorption Spectra

The relative energy of the Yb³⁺ Stark levels was investigated by optical absorption (4.2 K) and photoluminescence (77 K) measurements in near infrared region (NIR). Fig. 17.19 presents the absorption spectra for 20 mol% Yb³⁺ in Y₆MoO₁₂ recorded at room and liquid helium temperatures. At RT the spectra consist of broad weakly resolved bands located between 850 nm and 1100 nm corresponding to the transition from the ²F_7/2 ground state of Yb³⁺ to the three Stark components of the ²F_5/2 excited state. The absorption line relates to the so-called zero phonon line, from the lowest Stark component to the ²F_7/2 (1) ground state to the lowest Stark level of the ²F_5/2 (5) is located at around 976 nm and it is the strongest absorption line. The others lines corresponding to the ²F_7/2 (1) → ²F_5/2 (6) and to ²F_7/2 (1) → ²F_5/2 (7) transitions are at room temperature not very well formed. Going to the low temperature we are able to distinguish four components located at 875, 905, 920 and 949 nm, respectively. We supposed that the bands of highest intensities at 920 and 949 nm originated from (1) → ²F_5/2 (7) and ²F_7/2 (1) → ²F_5/2 (6) transitions respectively and those at 875 and 905 nm might possess the vibronic character, the same as the line of small intensity at 963 nm.

Fig. 17.19

Absorption spectra of Yb³⁺-doped Y₆MoO₁₂ solid solution recorded at room and liquid helium temperatures

The measurements performed at 4.2 K should lead to narrowing and better resolution of the main electronic Stark components, however it is not case for Y₆MoO₁₂ host lattice. As the ionic radius of Yb³⁺ (r = 0.925 Å, CN = 7) is slightly smaller that of Y³⁺ (r = 0.96 Å, CN = 7), we supposed that the Yb³⁺ ions introduced to the network occupy the Y³⁺ site. There is not a very large difference in the ionic radii, so that the disorder in the structure will not be significant. It will be at least smaller than in case when we substitute the La³⁺ ion (r = 1.1 Å, CN = 7) by the Yb³⁺ one [47].

According to the crystal-field theory of Kramer’s ions, the maximum of allowed components splitting for J = 5/2 state is three; therefore, for one symmetry site the absorption spectrum at liquid helium temperature should be resolved into three bands. The 0-phonon line is a sharp one and contains only one component at 4.2 K. The total splitting of the excited ²F_5/2 state 624 cm⁻¹ is similar like that observed for some other matrices like sesquioxides (about 793 cm⁻¹) [70], K₅Bi(MoO₄)₄ molybdates [71] orthohosphates (750–800 cm⁻¹) [72] but much larger than we reported recently for La₂MoWO₉ 474 cm⁻¹ [47] and 454 cm⁻¹ for CdMoO₄ [26]. The interest of high splitting is clear for solid state lasers with a near 4-levels scheme.

17.3.2.4 Concentration Dependence and Excitation Wavelength Dependences of the Photoluminescence Spectra

The emission spectra of Y₆MoO₁₂ activated with different concentrations of Yb³⁺ ion registered at room temperature under λ_ex = 915 nm of a Ti- Sapphire laser are plotted in Fig. 17.20a. The studied materials exhibit the usual luminescence lines in the near-infrared region from the ²F_5/2 → ²F_7/2 transitions of Yb³⁺ ions. At RT the lines are broad, however it is possible to distinguish the four components corresponding to the transitions from the lowest Stark level of the ²F_5/2 excited state (5) to each of the four Stark levels of the ²F_7/2 state (1, 2, 3, 4), respectively.

Fig. 17.20

Room temperature (RT) emission spectra of Yb³⁺-doped Y₆MoO₁₂ solid solutions with different concentrations of Yb³⁺ activator

The (5 → 1) component is located at around 975–977 nm. In case of Y₆MoO₁₂ solid solutions we can distinguish the lines at 974.7 nm (5 → 1), 1010 nm (5 → 2), 1030 nm (5 → 3) and 1075 nm (5 → 4). Starting from 5 mol% up to 20 mol% one can distinguish a second component of the 0-phonon (5 → 1) line located at 977 nm indicating a second type of Yb³⁺ site in this matrix. The maximum concentration of Yb³⁺ ions in this host lattice where the luminescence intensity is the highest was set at 3 mol%. For this sample, and also for 1 mol%, we can observe also very low components at 965 nm and 987 nm, which may correspond to vibronic transitions. Going to the 4.2 K the shape of the emission spectra stay the same although we should observe the narrowing of the lines. Very slight narrowing of the lines was also observed in the absorption spectra presented in the previous paragraph indicating some disorder degree of the active ions in the host. A clear change observed in the emission spectra of low Yb³⁺ concentration (1%) with decreasing temperature is the shifting of the 0-phonon line from 974.7 nm at RT to 976.6 nm at 4.2 K (see Fig. 17.20b). The multisite distribution of Yb³⁺ ions in Y₆MoO₁₂ solid solution as a probe of some disorder can be seen by the temperature dependence of both, the unusual slight shift of the 0-phonon line in luminescence spectra, and almost the same half-widths at half maximum (FWHM) of this 0-phonon line (54 cm⁻¹ at 77 K and 59 cm⁻¹ at RT). Low temperature emission spectra recorded under site selective excitation of Ti-sapphire laser are presented in Fig. 17.21 for 1% Yb³⁺. The spectra are normalized to show the differences in the shape of lines under different excitation wavelength. Nevertheless, we have performed also the comparison of the luminescence intensity of all samples by recording the spectra under the same conditions. The highest intensity has been observed for 5 → 3 transition at 1030 nm (the laser line when materials are used as laser source) under 974 nm pumping. It is between 2.8 times and 7 times more higher than for any other excitation lines of the tunable Ti-sapphire. The position of zero-phonon line from the absorption spectra stay in good agreement with that from the emission. Under the excitation of 915.9 nm four lines are clearly resolved. The main lines corresponding to the four Stark emission components are located at 976 nm (5 → 1), 1010 nm (5 → 2), 1030 nm (5 → 3) and 1075 nm (5 → 4), respectively. Under excitation at 940.6 nm the additional weak components at 1020 nm and 1025 nm is the result of electron– phonon coupling with M–O modes. When the excitation wavelength changes we can especially observe the shifting of the 5 → 3 transition to higher energy wavelengths i.e. at 1025 nm for excitation 908.7 nm and 975.8 nm or to infrared region (1037 nm) for excitation at 976.8 nm confirming the distribution of Yb³⁺ ions in, at least, three non- equivalent sites in the lattice. In addition, these bands start to be broad and the shifting from 1025 nm to 1037 nm is a result of changing the excitation line of only of 1 nm. We suppose in the case of Y₆MoO₁₂ we observe the main distribution of Yb³⁺ ions under excitation at 915.9, 940.6 and 974 nm respectively, giving the highest intensity of the usual laser line at 1030 nm. We obtain similar well resolved spectra and two slightly distorted symmetry sites of Yb³⁺ ions for other excitations yielding the shifted lines observed at 1025 and 1037 nm.

Fig. 17.21

Low temperature emission spectra of 1 mol% Yb³⁺-doped Y₆MoO₁₂ solid solution under site selective excitations of Ti-sapphire laser

17.3.2.5 Yb³⁺ Energy Level Diagram in Y₆MoO₁₂

Figure 17.22 presents the low temperature absorption and emission spectra of 10 mol% Yb³⁺-doped Y₆MoO₁₂ which allowed to propose the drawn energy level scheme of Yb³⁺ ion in this matrix. An energy level diagram can be derived from low temperature measurements. The splitting both of ground state as well as the excited state is quite large if we compare with other cubic matrix recently reported by us i.e. Yb³⁺-doped La₂MoWO₉.

Fig. 17.22

Absorption and emission spectra at low temperatures and Stark splitting levels of Yb³⁺ ion

17.3.2.6 Decay Analysis

The decay curves of x% Yb³⁺-doped Y₆MoO₁₂ solid solutions (x-0.1-50) were obtained under laser pulsed excitation at 940 nm (some of them at 980 nm) in the ²F_7/2 (1) → ²F_5/2 (6) absorption line and by monitoring 1030 nm ²F_5/2 (5) → ²F_7/2(2) emission. Table 17.2 summaries the decay times for Yb³⁺-doped Y₆MoO₁₂ with different concentration of Yb³⁺ ions recorded at room and low temperatures. The decays curves (not shown here) are exponential at low temperature for most of samples while at room temperature only for the lowest concentration (0.1 mol%) the decay curve keep the exponential shape with a fitted lifetime of about 285 μs at room temperature and 340 μs 77 K corresponding to the radiative lifetime. For the concentration of 3 and 5% energy transfer occurs between centers. Then, for the concentration above 10% self-quenching phenomena is predominant. When we compare Yb³⁺-doped Y₆MoO₁₂ solid solutions with another cubic composition evaluated recently by us as Yb³⁺-doped La₂MoWO₉ and also with tetragonal Yb³⁺-doped CdMoO₄ solid solution, we clearly see the difference [47, 73] concerning the usual self-trapping process which is not observed in this matrix. Only the self-quenching process appears here. For investigated previously micro-crystallites, also obtained by the high temperature solid state method, with similar grain sizes the transfer of energy between the Yb³⁺ ions i.e. self-trapping process was easily observed due to short distances between Yb³⁺ ions where resonant diffusion occurs (3.66 Å in CdMoO₄ and 3.15 Å in La₂MoWO₉)_. It means that in Yb³⁺-doped Y₆MoO₁₂ the unknown distance between Yb³⁺ ions should be much higher than these values to reduce this process.

Table 17.2 Luminescence decays times of cubic Yb³⁺-doped Y₆MoO₁₂ solid solutions recorded at room temperature and 77 K, λ_ex = 940 nm (some of them at 980 nm) and λ_em = 1030 nm

17.3.2.7 Conclusion on Yb³⁺-Doped Y₆MoO₁₂ Cubic Material

Other investigated group is Yb³⁺-doped Y₆MoO₁₂ solid solutions crystallizing as disordered cubic fluorite phase with a space group of Fm-3 m and the calculated lattice constants 5.29 Å. This material is thermally stable up to 1500 °C. All micro-powders obtained by high-temperature solid state reaction possess the yellow colour. The SEM images showed relatively big grains with the average size of few micrometers (0.5–8 μm) due to the high temperature (1550 °C) of synthesis. All the samples are very homogenous without other phase segregation and no significant change of the grain size with changing the activator concentration is observed. Relatively broad absorption lines in NIR region suggest disordering of the active ions in the structure. This phenomena is smaller than for cubic Yb³⁺-doped La₂MoWO₉ molybdato-tungstates, what is a consequence of the smallest difference of ionic radii between Yb³⁺ (0.925 Å, CN = 7) and Y³⁺ (0.96 Å) ions in Yb³⁺-doped Y₆MoO₁₂ than between Yb³⁺ (0.925 Å) and La³⁺ (1.1 Å) ions in Yb³⁺-doped La₂MoWO₉. Well formed in low temperature absorption spectra electronic transitions point out only one symmetry site in Yb³⁺-doped Y₆MoO₁₂, but we observe also many vibronic components, what make very difficult the interpretation of the spectra. The NIR emission spectra recorded at RT are characterized by broad lines, and it is possible to distinguish four components corresponding to the transitions from the lowest Stark level of the 2F_5/2 excited state (5) to each of the four Stark levels of the ²F_7/2 state (1, 2, 3, 4), respectively. Also in this case the band are assisted by the vibronic transitions. Very slight narrowing of the emission lines like in the absorption spectra is indicating a disorder of the active ions in the host. The multisite character in Yb³⁺-doped Y6MoO₁₂ has been confirmed by the shifting of the 0-phonon line from 974.7 nm at RT to 976.6 nm at 4.2 K under the same excitation at 915 nm. Another evidence for the multisite character can be found in the site selective excitation spectra, where change of excitation lines results in three different spectra. Thus, we observe three types of Yb³⁺ site symmetry, one main distribution of Yb³⁺ ions and two slightly distorted sites. Basing on the low temperature absorption and emission energy level diagram has been proposed. When we compare with other cubic matrix recently reported by us, i.e. Yb³⁺-doped La₂MoWO₉, one can notice the advantage of the largest splitting both of the ²F_7/2 ground state as well as the ²F_5/2 excited one with Y₆MoO₁₂ molybdate what is an important property in laser materials. The decays curves are exponential at low temperature for most of samples while at room temperature only for the lowest concentration (0.1 mol%) the decay curve keep the exponential shape with a fitted lifetime of about 285 μs and 340 μs 77 K corresponding to the radiative lifetime. When the concentration of Yb³⁺ ions increases the usual energy transfer occurs between centers so that the self-quenching phenomena is predominant. We didn’t observe any self-trapping effect in this matrix meaning higher distances between Yb³⁺- Yb³⁺) ions than in cubic Yb³⁺ − doped La₂MoWO₉ and tetragonal Yb³⁺-doped CdMoO₄ previously analyzed by us.

Concluding, the first studies have shown many advantages of this material and in the step of our program we will prepare the nano-powdered samples to succeed the fabrication in the form of transparent optical ceramics.

17.4 General Conclusion

A purpose of the paper is to summarize the first results obtained for Yb³⁺-doped molybdates and molybdato-tungstates synthesized by the high-temperature solid-state reaction and crystallizing in the cubic system: La₂Mo₂O₉ /La₂MoWO₉ /Y₆MoO₁₂ for future new transparent optical ceramics. We investigate the correlation between structural and spectroscopic properties of mentioned above compositions. Yb³⁺ rare earth dopant has been used as a structural probe in addition of the intrinsic interest to be a laser ion.

This probe ion gives a new deeper contribution of the complex structure of both La₂MoO_9, La₂MoWO₉ and Y₆MoO₁₂ molybdate families. Our approach is similar with the analysis we have previously done with Nd³⁺ laser ion, which can be also used as a structural probe one. More generally we should bring all contributions to the knowledge of these crystallographic structures from the spectroscopic properties of rare earth optical ions. In this way, the spectroscopy of Eu³⁺ rare earth probe ion should bring complementary results in the next step.

The first studies of Yb³⁺-doped Y₆MoO₁₂ have shown many advantages of this material and in the step of our program we will prepare the nano-powdered samples to succeed the fabrication in the form of transparent optical ceramics.

The research is under progress to get high quality optical transparent ceramics from the technique of sintering nano-crystalline powders by using both the fast SPS (Spark Plasma Sintering) and known HIP (Hot Isostatic Pressing) techniques.