Luminescence spectroscopy of Ce³⁺-doped ABaPO₄ (A=Li, Na, K) phosphors

Abstract

Ce³⁺ doped ABaPO₄ (A=Li, Na, K) phosphors were prepared by conventional high temperature solid-state reaction. The phosphors were investigated by XRD, photoluminescence excitation and emission spectra, and luminescence decay curves. The five 5d levels corresponding to the 4f¹→4f⁰5d¹ transition of Ce³⁺ ions were identified. The spectroscopic parameters, e.g., the 5d barycenter, the crystal-field splitting, and the Stokes shift, were discussed. LiBaPO₄:Ce³⁺ phosphor could be efficiently excited by the near-UV lights (330–420 nm) and showed a broad emission band in the range of 430–620 nm with the maximum wavelength at 468 nm. In contrast, Ce³⁺-doped NaBaPO₄ and KBaPO₄ showed only excitation bands in a limited UV region (230–370 nm) and have blue emission at 385 nm and 416 nm, respectively. The temperature quenching of luminescence and the chromaticity coordinates were reported. The luminescence properties were discussed by analyzing the crystal structure and the local surroundings of Ce³⁺ ions on the Ba²⁺ sites.

1 Introduction

Rare-earth (RE) activated inorganic materials have been established as useful luminescent materials in fabricating the optoelectronic devices. Nowadays, more and more interest has been focused on the synthesis and luminescence properties of the new RE-doped phosphors due to the theory researches and the potential applications [1–5].

The 5d-level spectroscopy of Ce³⁺ ion is simple, which has only one electron in the 4f shell and exhibits strong absorption and efficient fluorescence from the allowed inter-configurational 4f¹–4f⁰5d¹ transition. The spectroscopic parameters of Ce³⁺, e.g., the positions of 5d level, can be used to predict the 5d energies of other lanthanides, since the influence of the crystal field and covalence of the host lattices on the 4f5d levels are approximately equal for all the RE ions [6, 7]. Efficient energy transfer from the Ce³⁺ ions (as an energy donor) to acceptors (RE or transition metal ions), for example, Ce³⁺ to Tb³⁺ ions [8], Ce³⁺ to Nd³⁺ [9], Ce³⁺ to Mn²⁺ [10], Ce³⁺ to Eu³⁺ ions [11], and Ce³⁺ to Eu²⁺ ions [12], can be realized in many inorganic materials excited by the VUV–UV. Consequentially, Ce³⁺-doped materials are attractive for numerous applications, such as white light emitting diodes (W-LEDs) [13, 14], and fast scintillators [15], etc. The luminescence spectra of the doped Ce³⁺ ion have been investigated in many hosts [16, 17].

The phosphates with ABPO₄ formula (A and B are mono- and divalent cations, respectively) are in a family of mono-phosphates with the different structure types strictly depending on the relative size of the A and B ions [18]. These compounds have been considered to be efficient luminescent hosts due to the excellent thermal and hydrolytic stability [19–21]. The variety in these structures makes it possible to tailor the physical and chemical properties of ABPO₄ hosts. It has been confirmed that LiBaPO₄ is suitable for second harmonic generating (SHG) responses and shows larger SHG effects relative to quartz [22]. Upadeo and Moharil [23] investigated radiation-induced valence changes (Eu³⁺→Eu²⁺ conversion) in Eu-doped NaBaPO₄. Waite et al. [24] firstly report the Eu²⁺ luminescence doped in LiBaPO₄.

Recently, Eu²⁺-doped ABPO₄ phosphates have received much attention for the potential applications as new white light emission diodes (W-LEDs) phosphors [19]. Wu et al. have reported that LiBaPO₄:Eu²⁺ is a good candidate phosphor applied in W-LEDs and exhibits higher thermal stability than that of commercially available Y₃Al₅O₁₂:Ce³⁺ [25]. ABaPO₄:Eu²⁺ (A=Na, K) phosphors were reported to emit a strong blue light under UV excitation [26]. NaBaPO₄:Eu³⁺ [27] has been reported to be potential red-emitting phosphor. KBaPO₄:Eu²⁺ show an emission band with a maximum at 420 nm [28]. Lin et al. [29] reported the full-color band phosphors, Eu²⁺, Tb³⁺, and Sm³⁺ doped KBaPO₄, which could generate blue, green, and orange-red light, respectively. The thermal stabilities of KBaPO₄:Ln (Ln=Eu²⁺, Tb³⁺, Sm³⁺) are higher than the commercially available YAG:Ce³⁺. KBaPO₄:Eu²⁺ coated by silica on the phosphor surface is very potent as a blue-emitting phosphor for PDPs (plasma display panels), CCFLs (cold cathode fluorescence lamp), and phosphor-converted (pc) W-LEDs based on near ultraviolet LED [30]. However, there are few reports on the luminescence properties of Ce³⁺ ions doped into ABaPO₄:Eu²⁺ (A=Li, Na, K).

In this study, Ce³⁺-doped ABaPO₄ (A=Li, Na, K) phosphor were prepared by conventional high temperature solid-state reaction. The structures were confirmed by XRD measurements. The photoluminescence excitation and emission spectra, and luminescence decay curves, the temperature quenching and chromaticity coordinates of luminescence were investigated. The luminescence properties of the Ce³⁺ ion were investigated by spectroscopic parameters such as emission bands, 5d components, Stokes shift, barycenter of Ce³⁺ ions doped in ABaPO₄ (A=Li, Na, K) host. This research can be referential for the luminescence investigation of other RE ions doped into these phosphates.

2 Experimental

The preparation of Ce³⁺ ions doped ABaPO₄ (A=Li, Na, K) were carried out by conventional solid state synthesis. The raw materials were high-purity (4N) A ₂CO₃ (A=Li, Na, K), BaCO₃, NH₄H₂PO₄ and CeO₂. The starting materials with stoichiometric amounts were ground together in an agate mortar. The mixture was firstly heated up to 350 ^∘C and kept at this temperature for 5 h. After a second homogenization in the mortar, the samples were heated up to 750 ^∘C and kept at this temperature for 10 h. After that, the samples were mixed and finally heated at 1000 ^∘C for 10 h in crucibles along with the reducing agent (active carbon).

The X-ray diffraction (XRD) patterns were collected on a Rigaku D/Max-2000 diffractometer operating at 40 kV, 30 mA with Bragg–Brentano geometry by using Cu Kα radiation (λ=1.5418 Å) and analyzed using Jade-5.0 software. The optical excitation and emission spectra were recorded by a Perkin–Elmer LS-50B luminescence spectrometer and a Hitachi F-4500 fluorescence spectrophotometer. The luminescence decay was measured using the fourth harmonic of a Nd:YAG laser (266 nm). To study thermal quenching of the phosphors between 25 to 150 ^∘C the same spectrofluorimeter was equipped with a homemade heating cell under the excitation of a 266 nm Nd:YAG laser.

3 Results and discussion

Figure 1 show the XRD patterns of LiBaPO₄:Ce³⁺, NaBaPO₄:Ce³⁺ and KBaPO₄:Ce³⁺. The peak positions and the relative intensities of the samples are the same as the PDF2 standard cards in the International Centre for Diffraction Data (ICDD) database: No. 14-0270 for LiBaPO₄, No. 33-1210 for NaBaPO₄ and No. 33-0996 for KBaPO₄. No impurity lines were observed. The sharp peaks in each XRD pattern indicate the good crystallizability of the samples.

Fig. 1

XRD patterns of LiBaPO₄:Ce³⁺, NaBaPO₄:Ce³⁺ and KBaPO₄:Ce³⁺ compared with the corresponding standard JCPDs cards. The insets show the schematic views of the structures

Inset in Fig. 1 are the corresponding schematic views of the structures. This structure of LiBaPO₄ (Fig. 1a) involves the existence of a 1:1 ordering between the cross-linked LiO₄ and PO₄ tetrahedra on the basis of an isotopic with the β-SiO₂ trydimite [31]. The hexagonal phase structure shows mixed rings with alternately “up” and “down” tetrahedral of LiO₄ and PO₄.

NaBaPO₄ has a structure (Fig. 1b) isotypic with that of glaserite with space group$P\overline{3}m1$ [32, 33]. The network is built up with PO₄ tetrahedra, NaO₆ octahedra, BaO₁₂, and YO₁₀ polyhedra (Y=Na or Ba). It contains chains of polyhedra parallel to c (Fig. 1b). These alternately involved NaO₆ and BaO₁₂ oxygen atoms. The remaining cations (Y=Na or Ba) lie at sites with 10 neighboring atoms in another B chain [32, 33].

KaBaPO₄ have the β-K₂SO₄ type structure (Fig. 1c), which is made up of two types of string along the a-axis [18]. One string consists of alternating [PO₄] tetrahedra and alkaline atoms. Another is made up of only alkaline earth atoms parallel to the a-axis. K⁺ lies in a site with 10-fold coordination, much like the Y site in NaBaPO₄, and Ba²⁺ in a 9-fold coordinated site [34]. The difference between β-K₂SO₄-type KBaPO₄ and NaBaPO₄ structure is that the Ba²⁺ ions have the 9-fold coordinated sites in the former and have an equal number of 10- and 12-fold coordinated sites in the latter.

Figure 2 shows the normalized excitation and emission spectra of ABaPO₄:Ce³⁺ (A=Li, Na, K) phosphors. The three distinct excitation spectra were taken by monitoring emission at 468 nm for LiBaPO₄:Ce³⁺, 385 nm for NaBaPO₄:Ce³⁺, and 416 nm for KBaPO₄:Ce³⁺. Each spectrum contains five excitation bands which are characteristic absorption transitions of direct 4f¹(²F_5/2)→4f⁰5d¹ of Ce³⁺ ions. It is well known that the 4f configuration of Ce³⁺ ion has only one electron, and a UV photon can excite the 4f electron into the 5d orbital, leaving the 4f shell empty. So the excitation spectrum of Ce³⁺ shows the direct splitting information of 5d orbital in the crystal field [35].

Fig. 2

The excitation (a) and emission spectra (b) of LiBaPO₄:Ce³⁺, NaBaPO₄:Ce³⁺ and KBaPO₄:Ce³⁺. The dot lines in (b) are the two decomposed Gaussian components of the spectra. The texts in (b) are the two Gaussian components of wavelength position (xc, nm), FWHM (w, nm) and integrated areas (A)

The each broad excitation band in Fig. 2 actually is composed of several sub-bands, which can be assigned to five transitions of 4f¹5d¹(²F_5/2)→4f⁰5d¹ of Ce³⁺. It indicates that the mean energy of the 5d configuration shift downward relative to the free ion value and the 5d levels split into the five levels 5d(1), 5d(2), 5d(3), 5d(4), and 5d(5), which could be roughly estimated from the excitation spectra as shown in Table 1. The 5d barycenter values in LiBaPO₄:Ce³⁺, NaBaPO₄:Ce³⁺, and KBaPO₄:Ce³⁺ are 34,157, 37,130, and 34,690 cm⁻¹, respectively.

Table 1 The spectroscopic data of Ce³⁺ doped ABaPO₄ (A=Li, Na, K)

The excitation bands show a red shift in the order of NaBaPO₄:Ce³⁺ (maximum wavelength position 324 nm) < KBaPO₄:Ce³⁺ (340 nm) < LiBaPO₄:Ce³⁺ (390 nm). The results show that LiBaPO₄:Ce³⁺ could be efficiently excited by the incident lights of 330–430 nm, which well matched with the emissions wavelength of near-UV LEDs (360–400 nm). This is necessary for the potential application in the W-LEDs fabricated with near-UV chips. However, the application as a potential LED phosphor is limited for NaBaPO₄:Ce³⁺ and KBaPO₄:Ce³⁺.

Generally, the emission spectra of Ce³⁺ ions have a doublet character due to the spin–orbit splitting of ground state (²F_5/2 and ²F_7/2). The emission spectra were recorded under the excitation of 380 nm (LiBaPO₄:Ce³⁺), 324 nm (NaBaPO₄:Ce³⁺), and 363 nm (KBaPO₄:Ce³⁺), which corresponds to excitation into the lowest energy d-level of Ce³⁺. The two 5d(1)→²F _J (J=5/2, 7/2) sub-bands can be resolved by using the deconvolution method based on their curve shapes shown in Fig. 2b. The Gaussian peak-fittings suggest that the L–S splitting value of Ce³⁺ 4f level is 1,900 cm⁻¹ for LiBaPO₄:Ce³⁺, 2,125 cm⁻¹ for NaBaPO₄:Ce³⁺, and 1,900 cm⁻¹ for KBaPO₄:Ce³⁺. These values are in agreement with the normal ground-state splitting of Ce³⁺ ions (2,000 cm⁻¹) [6, 7].

The CIE (Commission Internationale de l’Eclairage) coordinates were calculated from emission spectra shown in Fig. 3. Three phosphors show different luminescence colors, which are consistent with the emission spectra. LiBaPO₄:Ce³⁺ shows bright blue-green color with CIE coordinates (0.142, 0.298). The CIE chromaticity coordinates of NaBaPO₄:Ce³⁺ (0.134, 0.08) and KBaPO₄:Ce³⁺ (0.135, 0.141) are in the blue region.

Fig. 3

The CIE chromaticity coordinates of LiBaPO₄:Ce³⁺, NaBaPO₄:Ce³⁺ and KBaPO₄:Ce³⁺ calculated from the emission in Fig. 2(b)

The fluorescence decay curves for the Ce³⁺ emission under excitation of 266 nm pulsed YAG laser are shown in Fig. 4. The curves can be fitted into a single exponential function: I=I ₀exp[−t/τ], where I ₀ is the initial emission intensity for t=0, τ is lifetime. The lifetime of LiBaPO₄:Ce³⁺ (468 nm), NaBaPO₄:Ce³⁺ (385 nm) and KBaPO₄:Ce³⁺ (416 nm) are calculated to be 41.4 ns, 49.8 ns, and 51.8 ns, respectively, characteristic of the allowed Ce³⁺5d¹→4f¹ transition.

Fig. 4

The decay curves of LiBaPO₄:Ce³⁺ (468 nm) (a), NaBaPO₄:Ce³⁺ (385 nm) (b) and KBaPO₄:Ce³⁺ (416 nm) (c). The excitation source was the fourth harmonic of a Nd:YAG laser (266 nm)

The approximate positions of Ce³⁺5d sublevels, emission bands and Stokes shift values are reported in Table 1. The Stokes shift is increased in the order of KBaPO₄: Ce³⁺ (2,920 cm⁻¹) < NaBaPO₄:Ce³⁺ (4,330 cm⁻¹) < LiBaPO₄:Ce³⁺ (4,810 cm⁻¹).

The lowest 5d energy level of Ce³⁺ in a specific compound, the 5d energy levels of other rare earth ions in the same compound can be predicted. Dorenbos [6, 7] proposed that the crystal-field depression D(Ln,A) of the energy of lowest. The 4fⁿ⁻¹5d levels of a lanthanide ion in compound “A” relative to the level energies in the free ion is almost independent of the type of lanthanide ion:

$$ D(\mathrm{Ln},A) = E(\mathrm{Ln},\mathrm{free})-E(\mathrm{Ln},A)$$

(1)

where E(Ln,free) is the energy of the first f–d transition of Ln³⁺(Ce³⁺) in a free ion and E(Ln,A) the energy of 4fⁿ⁻¹5d-levels of a lanthanide ion in compound A. The d level energy of free Ce³⁺ is 49,340 cm⁻¹ [36]. It could be calculated that the f–d transition energy of Ce³⁺ in LiBaPO₄ D(Ce³⁺, LiBaPO₄) is depressed by 7,840 cm⁻¹, in NaBaPO₄ D(Ce³⁺, NaBaPO₄) is depressed by 6,790 cm⁻¹, in KBaPO₄ D(Ce³⁺, KBaPO₄) is depressed by 6,970 cm⁻¹ (Table 1).

The 5d manifold Ce³⁺ ions doped in a crystal field undergoes an energetic depression on the basis of the nephelauxetic effect and a splitting due to the crystal field. According to the nephelauxetic effect, the stronger the covalency is, the smaller the energy difference between the ground state 4f¹ and activated band 5d of the Ce³⁺ ions is. The nephelauxetic effect shifts the center of gravity of 5d bands to lower energy. From the calculations of the crystal-field depression effects, it can be suggested that the covalency effects are stronger in LiBaPO₄:Ce³⁺ than those in NaBaPO₄:Ce³⁺ and KBaPO₄:Ce³⁺.

In Fig. 2b it is obvious that the emission wavelength increases from 385 nm in NaBaPO₄:Ce³⁺, to 416 nm in KBaPO₄:Ce³⁺, to 468 nm in LiBaPO₄:Ce³⁺. The observed variation of emission center should also result from the position of the 5d band, which can be shifted by the crystal field. ABaPO₄ (A=Li, Na, K) exhibit similar stoichiometries, but the crystal structures are different. However, the Ce³⁺ 5d¹ crystal-field splitting should be dependent upon both the Ce³⁺–O²⁻ bond length and the distortion. Figure 5 illustrates the comparison of the coordination geometries around the Ba in LiBaPO₄:Ce³⁺, NaBaPO₄:Ce³⁺, and KBaPO₄:Ce³⁺. In LiBaPO₄, Ba is surrounded by the cross-linked LiO₄ and PO₄ tetrahedra with an average Ba–O (BaO₉) distance of 2.74 Å [31]. The average Ba–O distances in NaBaPO₄ (BaO₁₂) and KBaPO₄ (BaO₉) are 3.018 [32, 33] and 2.82 Å [18], respectively. For LiBaPO₄:Ce³⁺, the smaller Ba(Ce)–O bond length and the larger distortion should lead to a larger crystal-field splitting. This indicates that the crystal-field strength of LiBaPO₄ is stronger than that of KBaPO₄:Ce³⁺ and NaBaPO₄:Ce³⁺. Consequently, the emission peak of LiBaPO₄:Ce³⁺ should shift to a long wavelength band.

Fig. 5

The comparison of the coordination geometries around the Ba in LiBaPO₄:Ce³⁺ (a), NaBaPO₄:Ce³⁺ (b) and KBaPO₄:Ce³⁺ (c)

The concentration dependence of the emission intensity is also investigated. The strongest emission intensity has been obtained for three samples with about 3 % Ce³⁺ doping (figure omitted). The position of the emission bands does not change with increasing Ce³⁺ concentration in NaBaPO₄:Ce³⁺ and KBaPO₄:Ce³⁺, however, a red shift in the emission spectrum and its maximum is observed in LiBaPO₄:Ce³⁺ as shown in Fig. 6. The red shift is about 15 nm. The red shift may be mainly related to the crystal-field effects upon the 5d levels of the Ce³⁺ ions due to the modification of the crystal field as well as the structural relaxation. This trend is in good accordance with the degree of crystalline volume constriction mentioned above. No significant emission band shifts in NaBaPO₄:Ce³⁺ and KBaPO₄:Ce³⁺ might be caused by a weak shrinkage of the unit cell volume due to the bigger Ba²⁺ sites accommodating Ce³⁺ ions.

Fig. 6

The normalized emission spectra of LiBaPO₄:xCe³⁺ (x=0.01, 0.03, 0.05)

The temperature-dependent luminescence of Ce³⁺-doped ABaPO₄ (A=Li, Na, K) phosphors were investigated. With the increasing of temperature the thermal quenching can be observed in three phosphors. Figure 7 represents the temperature dependence of the integrated emission intensities normalized at the at 25 ^∘C values. With the increasing of temperature up to 150 ^∘C, the normalized emission intensity is decreased to 49 % for LiBaPO₄:Ce³⁺, 56 % for NaBaPO₄:Ce³⁺ and 62 % for KBaPO₄:Ce³⁺. The thermal quenching of Ce³⁺-doped phosphors is usually affected by many factors, such as the rigidity of the crystal structure, crystallinity, defect levels, Stokes shift, etc. [37]. This stronger thermal quenching in LiBaPO₄:Ce³⁺ might be due to the lower 5d energy position and the relatively large Stokes shift.

Fig. 7

The integral emission intensity as a function of temperature of LiBaPO₄:Ce³⁺, NaBaPO₄:Ce³⁺ and KBaPO₄:Ce³⁺ normalized with respect to the values at 25 ^∘C

4 Conclusions

The luminescence properties of Ce³⁺ are reported in three monophosphates ABaPO₄ (A=Li, Na, K) with similar stoichiometries, but different structures. The spectroscopic parameters and luminescence properties are different.

(1) The crystal-field splitting of Ce³⁺ ions increases going from about 11,690 cm⁻¹ for NaBaPO₄:Ce³⁺ to about 14,820 cm⁻¹ for KBaPO₄:Ce³⁺ and about 15,185 cm⁻¹ for LiBaPO₄:Ce³⁺; The Ce³⁺ 5d center of gravity decreases to lower energy with increase in covalency (nephelauxetic effect) in the series: NaBaPO₄:Ce³⁺ (37,130 cm⁻¹) > KBaPO₄:Ce³⁺ (34,690 cm⁻¹) > LiBaPO₄:Ce³⁺ (34,157 cm⁻¹); The Stokes shift decreases in the order: LiBaPO₄:Ce³⁺ (4,810 cm⁻¹) > NaBaPO₄:Ce³⁺ (4,330 cm⁻¹) > KBaPO₄:Ce³⁺ (2,920 cm⁻¹).

(2) LiBaPO₄:Ce³⁺ shows a bright blue–green color (468 nm) with CIE coordinates (0.142, 0.298). The CIE chromaticity coordinates of NaBaPO₄:Ce³⁺ (385 nm) (0.134, 0.08) and KBaPO₄:Ce³⁺ (416 nm) (0.135, 0.141) are in the blue region. The position of the emission bands does not change with increasing Ce³⁺ concentration in NaBaPO₄:Ce³⁺ and KBaPO₄:Ce³⁺, however, a significant red shift in the emission spectrum and its maximum is observed in LiBaPO₄:Ce³⁺.

(3) A large Stokes shift causes quenching at relatively low temperatures. With the increasing of temperature up to 150 ^∘C, the normalized emission intensity is decreased to 49 % for LiBaPO₄:Ce³⁺, 56 % for NaBaPO₄:Ce³⁺, and 62 % for KBaPO₄:Ce³⁺.

(4) The efforts for the further exploration of the applications in Ce³⁺ doped ABaPO₄ (A=Li, Na, K) can be suggested. An intense absorption and excitation band in the near-UV range (300–420 nm) in LiBaPO₄:Ce³⁺ is a hint that this material is promising as regards conversion phosphors for white-light LEDs. However, the application as a potential LED phosphor is limited for NaBaPO₄:Ce³⁺ and KBaPO₄:Ce³⁺ because the phosphors only show blue emission and the absorption of Ce³⁺ is limited in UV region. Taking into account of the emission properties (in the purplish blue region) and the possible energy transfer from Ce³⁺ ions in NaBaPO₄ and KBaPO₄ hosts, high efficient phosphors could be obtained by co-doping Ce³⁺ ions with other ions, for example, co-doping of Tb³⁺, Eu^3+,2+, Mn²⁺ ions, to get green, red and deep red phosphors, respectively.