Electron paramagnetic resonance and photoluminescence properties of α-Al₂O₃:Cr³⁺ phosphors

Abstract

The red emitting Cr³⁺ activated α-Al₂O₃ powder phosphor has been prepared by easy combustion reactions from mixed metal nitrate reactants and urea with ignition temperatures of 500 °C. The as-synthesized powder was characterized by X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared techniques. The X-ray diffraction pattern reveals that the phosphor crystallized in the hexagonal α-Al₂O₃ phase directly from the combustion reaction. The EPR spectrum exhibits an intense resonance signal with effective g value at g=3.33 along with a few weak resonance signals with effective g values at g=13.7, 2.34, 1.95, 1.49, and 1.26. The spin concentration (N) and its paramagnetic susceptibility (χ) have been evaluated. The excitation spectrum consists of two broad intense bands at 415 nm and 555 nm and are assigned to ⁴A_2g (F)→⁴T_1g (F) and ⁴A_2g (F)→⁴T_2g (F) transitions, respectively. The intense fluorescence peak around 691 nm is attributed to ²E _g →⁴A_2g transition of Cr³⁺ ion. By correlating EPR and optical data, the crystal field splitting parameter (Dq), Racah inter-electronic repulsion parameter (B) have been evaluated and discussed. The EPR and optical studies reveal that Cr³⁺ ions are occupying in Al³⁺ sites in octahedral coordination.

1 Introduction

In recent years, significant efforts have been made by several research groups on the synthesis and characterization of various Cr³⁺ doped host materials [1–5]. The α-Al₂O₃ has been considered as a significant host material for rare-earth or transition metal ions because of its significant technological importance such as the large optical transparency from an ultraviolet to near infrared wavelength, excellent mechanical properties, and good chemical stability [6]. Alumina shows a very high melting point (2050 °C), hence there are several advantages of Al₂O₃ as a luminescent phosphor host material. After performing various studies on rare-earth ion doped aluminium oxide (Al₂O₃), it has been concluded that α-Al₂O₃ is the only the stable phase. One of the main applications of α-Al₂O₃ in optics is determined by its excellent emitting properties when doped with Cr³⁺ ions. The ruby (Al₂O₃:Cr³⁺) is industrially, technically important material and is being used as a laser rod. Therefore, studies on ruby powder phosphor would be favorable for novel laser applications using powdered media [7–9].

Mehta et al. [10] published the first study on Al₂O₃:Si,Ti as a sensitive thermoluminescent phosphor. Recently, Bhatt et al. [11] investigated the optically stimulated luminescence characteristics of Al₂O₃:Si,Ti along with its thermoluminescence and photoluminescence studies. More recently, Eckert et al. [12] reported the sol-gel deposition of multiply doped thermographic phosphor coatings Al₂O₃:Cr,Dy, Tm for the wide range surface temperature measurement application. Singh et al. [13] synthesized Al₂O₃:Er³⁺ phosphor co-doped with Yb³⁺ and Zn²⁺ and reported on the frequency upconversion in the visible region and the emission in the IR region upon excitation with a CW laser lasing at ∼980 nm. In recent years, several reports appeared on rare-earth doped aluminium oxide [14–17]. However, transition metal ions incorporated aluminium oxide is limited. Among the few transition metal ions, Cr³⁺ is a very useful probe to have better information about local surrounding of the ion in the hosts [18, 19].

In the present study, α-Al₂O₃:Cr³⁺ powder phosphors were prepared by the relatively low temperature combustion method, in which we employed urea as the fuel. The combustion synthesis technique has been proved to be a fast, extremely facile, reduced time consumption and low process temperature route for the preparation of ultrafine powders. During the exothermic decomposition of organic fuel and metal nitrates at low temperature, sufficient heat is generated locally to form direct crystalline materials. Elimination of secondary processing is a great advantage of the combustion process. The combustion prepared sample was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), the excitation, and emission spectroscopic techniques. In continuation of our interest on EPR spectra of paramagnetic impurities in different hosts, in the present work, we also performed EPR studies on Cr³⁺ ions in Al₂O₃ host.

2 Experiment

Powder phosphors of Al_1.98O₃:Cr_0.02 {prepared using [Aluminum nitrate (10 g, Al(NO₃)₃⋅9H₂O), Sigma-Aldrich], [Chromium nitrate (0.1077 g, Cr(NO₃)₃⋅9H₂O), Sigma-Aldrich], [Urea (4.0393 g, NH₂CONH₂), Sigma-Aldrich]} were prepared by using the solution combustion method. The oxidizer: fuel ratio was calculated based on oxidizing (O) and fuel (F) valencies of the reactants, keeping $\mathrm{O/F} = 1$, as reported earlier [20]. The starting materials were dissolved with a small amount of de-ionized water in a China dish of approximately 300 ml capacity. Then the solution was introduced into a muffle furnace preheated to 500 °C. Combustion took place with the introduction of the solution along with the evolution of gases. The entire combustion process was completed in about 5 to 8 min. The resultant fluffy masses were crushed into a fine powder. This prepared phosphor powder was utilized without any further treatments.

X-ray diffraction data were recorded at room temperature on an X’Pert Pro Diffractometer (Panalytical) using CuKα ₁ radiation (λ=1.5418 Å) in the 2θ range of 05° to 80°. The morphology of the powders was determined using a JEOL-JSM-700F scanning electron microscope (FE-SEM). FT-IR spectra were taken using a Perkin-Elmer R×1 instrument in the range from 4000 to 400 cm⁻¹. The room temperature photoluminescence (PL) of the prepared phosphors was studied using a Hitachi F-4500 FL Spectrophotometer. A powdered sample of 100 mg was taken in a quartz tube for the EPR measurements. The EPR spectra of all the samples were recorded on a JEOL FE1X ESR Spectrometer, operating in the X-band frequencies, with a field modulation of 100 kHz.

3 Results and discussion

3.1 X-ray diffraction

The phase purity and crystal structure of the as-combustion synthesized Al₂O₃:Cr³⁺ phosphors were analyzed using the XRD pattern. The X-ray diffraction pattern shown in Fig. 1 is consistent with the hexagonal α-Al₂O₃ phase referenced in the JCPDS file No. 46-1212. The presence of sharp and intense diffraction peaks indicates the crystalline nature of the phosphor material. Fujita et al. [21] prepared α-Al₂O₃ by the metal-salt-derived sol-gel method. Their XRD analyses show that dried gels as well as gels heat-treated at 700 °C are amorphous. As the heat-treatment temperature is increased over 700 °C, crystalline γ-Al₂O₃ is precipitated at 800 °C, a mixture of θ- and α-Al₂O₃ phases appears at 1100 °C, and a single phase of α-Al₂O₃ is obtained at 1200 °C. However, our XRD analyses show that the crystalline hexagonal phase of α-Al₂O₃ could be obtained directly by the combustion method even at furnace temperatures as low as 500 °C within a few minutes. This is the main advantage of the present method.

Fig. 1

Powder XRD pattern of α-Al₂O₃:Cr³⁺ phosphor

3.2 FT-IR

Figure 2 shows the FT-IR spectrum of as-combustion synthesized Al₂O₃:Cr³⁺ phosphors. Common bands can be seen in the spectrum such as the broad O–H band around 3446 cm⁻¹ and the 1643 cm⁻¹ H₂O vibration band. However, there is no signature of $\mathrm{NO}_{3}^{-}$ in the as-combustion synthesized sample. This indicates that the nitrates used in the starting materials were completely eliminated during the combustion process. Strong and sharp peaks observed in the 400–900 cm⁻¹ region are due to several metal–oxygen (M–O) stretching and bending vibrations. Although no post heat treatment was performed, M–O bands could be obtained through the combustion process. It is significant that the formation of M–O bands could be formed even at furnace temperatures as low as 500 °C, which is several hundred degrees lower than the conventional solid-state reaction. The XRD results are well matched with this FT-IR investigation.

Fig. 2

FT-IR spectrum of α-Al₂O₃:Cr³⁺ phosphor at room temperature

3.3 Scanning electron microscopy

Both low and high magnification scanning electron microscope (SEM) images of as-combustion synthesized Al₂O₃:Cr³⁺ powders are shown in Fig. 3. Similar to previous reports [22, 23], the powder particles are faceted plates and of varying sizes and shapes (Figs. 3A, 3B, and 3C). There are some plates containing pores (Fig. 3A) while some do not (Fig. 3B). The pores are probably due to the rapid release of gaseous by-products. It is presumed that irregularity in porosity, sizes and shapes is due to the inhomogeneous heat and the mass transport in the reaction chamber during the combustion process. Figure 3D, which is a magnified view of Fig. 3C (zone c), shows the presence of micro and nanoparticles within the plates. The micro and nanoparticles close to the pores occurs during the combustion process when gases escape under high pressure.

Fig. 3

FE-SEM images of α-Al₂O₃:Cr³⁺ phosphor at low and high magnification. (A) Shows that the powder particles are faceted plates (B) Faceted plates without pores (C) Some of plates containing pores and (D) is a magnified view of C (zone c)

3.4 Electron paramagnetic resonance

No resonance signals were observed in the undoped sample indicating that the starting materials used in the present work are free from paramagnetic impurities. When Cr³⁺ ions are introduced in the host matrix, the sample exhibits resonance signals. The EPR spectrum of α-Al₂O₃:Cr³⁺ powders at room temperature and at 110 K are shown in Figs. 4(a) and (b), respectively. It is observed that the EPR spectrum exhibits an intense resonance signals with effective g value at g=3.33 along with a few weak resonance signals with effective g value at g=2.34, 1.95, 1.49, and 1.26. The appearance of weak resonance signal at g=13.7 may be due to unwanted iron impurity present in chromium nitrate sample used in the present work. The other resonance signals are due to chromium ions.

Fig. 4

EPR spectrum of α-Al₂O₃:Cr³⁺ at (a) room temperature and (b) 110 K

Cr³⁺ ions belong to d³ configuration and it is known that the axial distortion of octahedral symmetry gives rise to two Kramers doublets |±3/2〉 and |±1/2〉. Application of Zeeman field lifts the spin degeneracy of the Kramers doublets. The resonances observed are due to transitions within the Zeeman field split Kramers doublets. The resonance signals on the low field region have been attributed to resonance within the lower Kramers doublet of isolated Cr³⁺ ions in strongly distorted sites while the resonance signals on the high field region have been attributed to a combination of isolated Cr³⁺ ions in weakly distorted sites and exchange coupled Cr³⁺ pairs [24].

In each of the above cases, the chromium ions are assumed to be octahedrally coordinated. The intense line appear in the range of low magnetic fields with a resonance field around 200 mT (g=3.33) and a weak resonance signal around 300 mT (g=2.34). This indicates that the zero-field splitting 2D of the ⁴A_2g orbital state is relatively large in comparison with the energy of the microwave radiation used in the X-band ESR spectrometer [25].

If the Cr³⁺ ions occur in pairs, the resonance signal at g≈2.0 will be intense. Carman and Kroenke [26] studied ESR of α-chromia-Alumina solid solutions and Patra et al. [7] studied ESR of Cr³⁺ in Al₂O₃ nanocrystals. Both the authors observed an intense resonance signal at g≈2.00 and they explained this resonance signal due to pair-formation mechanism. In the present work, the resonance signal at g=1.95 is not intense compared to the resonance signal at g=3.33, and hence the authors conclude that Cr³⁺ ions have not paired at this concentration (Al_1.98O₃:Cr_0.02), and hence the resonance signal at g=1.95 is attributed entirely due to isolated Cr³⁺ ions [24].

Carman and Kroenke [26], Di Biasi and Rodrigues [27], and Patra et al. [7] studied ESR spectra of Cr³⁺ ions in Al₂O₃ in solid solutions, powders, and nanocrystals, respectively. All the three authors observed two resonance signals around 450 mT and 540 mT. In the present work also, the authors observed resonance signals around that region and the g values observed at g=1.49 and g=1.26 are attributed to Cr³⁺ ions weakly distorted sites.

The population difference (N) between Zeeman levels can be calculated by comparing the area under the absorption curve with that of a standard (CuSO₄⋅5H₂O in this study) of known concentration. Weil et al. [28] gave the following expression which includes the experimental parameters of both sample and standard:

(1)

where A is the area under the absorption curve, which can be obtained by double integrating the first derivative EPR absorption curve, Scan is the magnetic field corresponding to a unit length of the chart, G is the gain, B _m is the modulation field width, g is the g factor, S is the spin of the system in its ground state and P is the power of the microwave source. The subscripts “x” and “std” represent the corresponding quantities for the α-Al₂O₃:Cr³⁺ phosphor and the reference (CuSO₄⋅5H₂O), respectively. The value of N has been calculated for the studied phosphor at room temperature as well as at 110 K and is found to be 0.63×10¹⁸ and 2.54×10¹⁸, respectively. It is observed that as the temperature is lowered the spin population increases obeying the Boltzmann law. The EPR data can be used to calculate the paramagnetic susceptibility of the sample using the formula [29].

$$\chi =\frac{N g^{2} \beta^{2} J (J+1)}{3k_{B} T}$$

(2)

where N is the number of spins per m³ and the other symbols have their usual meaning. N can be calculated from Eq. (1) and g is taken from EPR data. Since the resonance signal at g=3.33 is intense, this g value is used in calculating the paramagnetic susceptibility. The paramagnetic susceptibility (χ) evaluated from EPR data at room temperature as well as at 110 K and are found to be 0.42×10⁻⁶ emu mol⁻¹ and 1.72×10⁻⁶ emu mol⁻¹, respectively.

3.5 Photoluminescence studies

Cr³⁺ with 3d³ configuration typically occupies octahedral symmetry sites in crystalline hosts [30]. The unfilled 3d³ electronic shell of the Cr³⁺ ion has a number of low-lying energy levels, among which optical transitions can occur generating luminescent emission. As the 3d electrons are outside the ion core, the optical spectroscopic properties of the Cr³⁺ ions are directly affected by the static and dynamic properties of their environments and the optical associated spectra are characterized by both sharp and broad emission bands. Figure 5(a) shows the excitation spectrum of Cr³⁺ doped α-Al₂O₃ phosphors at room temperature. The main features of the excitation spectrum consist of two broad intense bands with barycenter around 415 nm (24100 cm⁻¹) and 555 nm (18000 cm⁻¹), and these two bands are assigned to spin allowed electronic transitions from ⁴A_2g (F)→⁴T_1g (F) and ⁴A_2g (F)→⁴T_2g (F), respectively. Besides these spin allowed bands, a sharp band is observed at 465 nm (21500 cm⁻¹). The sharp band in the excitation spectrum is characteristic for many Cr³⁺ bearing crystals. The spin allowed transitions are broad because they are associated with a change in the electronic configuration as seen below

Fig. 5

Photoluminescence spectra of the α-Al₂O₃:Cr³⁺ phosphor (a) excitation spectrum (λ _em=691 nm) and (b) emission spectrum (λ _ex=555 nm)

For d³ configuration, the two lowest excited levels in the energy level diagram of Cr³⁺ ion are ²E and ⁴T₂. In a weak crystal field, the ⁴T₂ state is at a lower energy than the ²E state. In this case, the Cr³⁺ exhibits a broad luminescence band due to the lattice transition from the ⁴T₂ to the ground ⁴A₂ level. This transition is strongly coupled to the lattice. In a strong crystal field, the first excited state of Cr³⁺ the ²E level weakly coupled to the lattice and the optical transitions are characterized by sharp zero-phonon lines (R lines) with a phonon side band. Figure 5(b) shows the emission spectrum of Cr³⁺ doped α-Al₂O₃ phosphor upon 555 nm excitation. The intense fluorescence peak around 691 nm (14470 cm⁻¹) is attributed to the zero-phonon lines in the ²E _g →⁴A_2g transition of Cr³⁺ion in α-Al₂O₃ phosphor, i.e., the R line which has been assigned to the ²E _g (G)→⁴A_2g (F) transition of Cr³⁺ ions. The weaker lines on either side of the intense lines at 658 nm (15200 cm⁻¹), 668 nm (14970 cm⁻¹), 702 nm (14245 cm⁻¹) and 712 nm (14045 cm⁻¹) are associated to phonon assisted transitions [31]. The R line position of a Cr³⁺ in bulk Ruby crystal is around 693 nm at room temperature [32]. The R line observed in the present work is very close to that of single crystal. Moreover, in Ruby crystal, Cr³⁺ ions are in a strong ligand field (Dq/B=2.8). In the present work, the authors obtained Dq/B=3.07 which will be discussed below. This indicates that the crystal field around the Cr³⁺ ions inside present phosphor is indistinguishable from that of single crystal materials.

The Cr³⁺ ion in α-Al₂O₃ phosphor has predominantly octahedral crystal field and the strength of the crystal field interaction is usually expressed in terms of the parameter Dq expressed in energy units.

$$\mathit{Dq} = \frac{E(^{4}\mathrm{A}_{2g} \to{}^{4}\mathrm{T}_{2g} )}{10}$$

(3)

The crystal field splitting parameter (Dq) for Cr³⁺ ion in α-Al₂O₃ phosphor in the present work is 1798 cm⁻¹. The value of the Racah parameter B can be evaluated from the expressions [33, 34].

(4)

where

$$\lefteqn{x = \frac{E(^{4}\mathrm{A}_{2g} \to{}^{4}\mathrm{T}_{1g} ) - E(^{4}\mathrm{A}_{2g}\to{}^{4}\mathrm{T}_{2g} )}{\mathit{Dq}}}$$

(5)

The calculated Racah B parameter in the present work is 585 cm⁻¹ which is considerably lower than the free ion value of Cr³⁺ ion (918 cm⁻¹) [35]. A comparison of two values indicates that B value is decreased by 64 % from its free ion value. This decrease is due to the effect of covalency of bond. In weak crystal field sites, the values of Dq/B≪2.3 and for strong fields the values of Dq/B≫2.3 [36]. For intermediate crystal fields, the value of Dq/B≈2.3. The Dq/B value obtained in the present work is 3.07, which indicates that the Cr³⁺ ions are in the strong ligand field sites. The Dq and B values are of the same order reported for Cr³⁺ ions in strong field sites [37]. The full-width at half maximum (FWHM) of the ⁴A_2g (F)→⁴T_1g (F) and ⁴A_2g (F)→⁴T_2g (F) band are 3340 cm⁻¹ and 2715 cm⁻¹, respectively. These values are in good agreement with the value observed by Maiman et al. [32]. The EPR, optical absorption, and photoluminescence studies reveal that Cr³⁺ ions occupy Al³⁺ sites in octahedral coordination and Cr³⁺ ions are in strong crystalfield environment.

4 Conclusions

Powder phosphor of Cr doped α-Al₂O₃ was prepared by combustion reactions from metal nitrate reactants and urea (NH₂CONH₂) as organic fuel. The XRD results confirm that crystalline hexagonal phase of α-Al₂O₃ could be obtained directly by the combustion method. The samples showed strong and sharp peaks in the FT-IR spectrum are due to several M–O stretching and bending vibrations. SEM images of as-synthesized powders showed that particles are faceted plates and of varying sizes and shapes. The resonance signals at g=3.33, g=2.34 and g=1.95 are attributed to isolated Cr³⁺ ions in strongly distorted sites, while the resonance signals on the high field region at g=1.49 and g=1.26 are attributed to isolated Cr³⁺ ions in weakly distorted sites. The resonance signal at g=13.7 is due to unwanted iron impurity. The paramagnetic susceptibility (χ) evaluated from EPR data at room temperature as well as at 110 K and it is found to be 0.42×10⁻⁶ emu mol⁻¹ and 1.72×10⁻⁶ emu mol⁻¹, respectively. The excitation spectrum exhibits two broad intense bands characteristic of Cr³⁺ ions in octahedral symmetry. Upon 555 nm excitation, an intense red emission peak is observed and is attributed to ²E _g →⁴A_2g transition of Cr³⁺ion. The Dq/B value obtained in the present suggests that Cr³⁺ ions are in strong crystal field environment. The EPR and optical studies reveal that Cr³⁺ ions occupy Al³⁺ sites in octahedral coordination.