Nature of WO₄ tetrahedra in blue light emitting CaWO₄ probed through the EXAFS technique

Abstract

Blue emission due to self trapped exciton recombination in CaWO₄ is believed to be very sensitive to the nature of the WO₄ structural units. The present manuscript deals with the probing of structural differences existing in WO₄ tetrahedra of CaWO₄ particles having varying average crystallite sizes and different blue light emission characteristics. Based on XRD, W L₁ edge XANES and W L₃ edge EXAFS studies, it is inferred that factors like average W–O and Ca–O bond lengths, average number of oxygen atoms around W⁶⁺ ions and disorder in WO₄ tetrahedra do not have any effect on the blue luminescence intensity from the samples. The lifetime value of excitons is lower for the nanocrystals/nanoparticles of CaWO₄ compared to the bulk sample. The lower lifetime of self trapped excitons and the associated decrease in blue luminescence intensity for nanoparticles/nanocrystals (compared to bulk) has been explained based on competing non-radiative processes involving the interaction of holes with surface hydroxyl groups and an associated decrease in the extent of radiative exciton recombination.

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1. Introduction

CaWO₄ belongs to the family of alkaline earth tungstates having a scheelite structure. Compounds of this family exhibit interesting phenomena like excitonic luminescence, thermoluminescence, electro-optical properties etc.^1–5 CaWO₄ is also a very promising material for applications based on the scintillation phenomenon due to its very high light output.^6–10 However, this light output is a strong function of the quality or nature of the samples as well as the synthesis conditions. In most of the luminescence studies involving CaWO₄, a blue emission centered in the range of 420–450 nm and a weak green emission that strongly overlaps with the blue emission are observed. Based on time resolved spectroscopy, Leonelli and Brebner^11,12 have confirmed that visible emission from such materials is arising due to recombination of self trapped excitons. It is now generally accepted that the blue emission is arising from recombination of self trapped excitons associated with WO₄²⁻ species in the lattice and weak green emission is due to defect levels present in the optical gap of CaWO₄.^13–16 Campos et al.¹⁷ have carried out detailed studies on the luminescent properties of both crystalline and amorphous CaWO₄ and CaMoO₄ powders. Authors have suggested that in the case of amorphous CaWO₄ or CaMoO₄, emission is due to the presence of localized bands in the optical gap. Such bands are arising due to oxygen vacancy complexes of type [MO₃·], [MO₃·] and [MO₃·] (where M is Mo or W and , , represent oxygen vacancy with neutral, singly positive and doubly positive charges respectively). Unlike this, in the case of crystalline material emission is arising due to slight distortion of WO₄ or MoO₄ tetrahedra. These inferences are further supported by detailed theoretical calculations.¹⁸ In an another study, Hu et al.¹⁹ prepared Ca_(1−x)Zn_xWO₄ nano-crystals at room temperature based on solution chemistry and characterized their structural and optical properties. It is observed that iso-valent substitution of Ca²⁺ by Zn²⁺ in CaWO₄ lattice leads to quenching of intrinsic blue emission. This has been explained based on the shorter Zn–O distance compared to Ca–O distance leading to lattice contraction and associated constraints in the generation of self trapped excitonic states (STEs). Pôrto et al.²⁰ based on structural and luminescence studies on Ca_xSr_1−xWO₄ samples, confirmed that certain extent of disorder as well as the coexistence of WO₄ and WO₃ clusters in CaWO₄ lattice improve their luminescence intensity. Emission due to defects observed along with the blue emission from CaWO₄ crystals has been found to be very sensitive to annealing temperatures and the atmospheres under which the annealing is carried out.²¹

Thongtem et al.²² prepared nano-crystalline CaWO₄ and SrWO₄ by microwave method using CTAB as stabilising ligand. It was observed that luminescence intensity increased with increase in ligand concentration in the synthesis medium. However, reason for improved luminescence with increase in ligand concentration is not clearly understood. Li et al.²³ observed an improved symmetry in terms of c/a ratio of tetragonal unit cell of CaWO₄ by reducing size of CaWO₄ nanoparticles to less than 5 nm. These authors have also observed that luminescence intensity decreases with decrease in particle size. Quantum confinement effects in the excitation spectrum corresponding to ¹A₁ → ¹T₁ transition of W⁶⁺ ions has been reported for CaWO₄ nanoparticles by Su et al.²⁴

Earlier findings mentioned above suggests that blue emission from CaWO₄ samples depends on the nature of WO₄ structural units as well as the local environment around W in it. Above inference has been arrived based on techniques like XRD, IR and Raman spectroscopy. A more quantitative approach will be probing the structure of the system with X-ray absorption techniques. Techniques like X-ray Absorption Near Edge Structure (XANES) combined with Extended X-ray Absorption Fine Structure (EXAFS) can give direct information regarding the local environment around W⁶⁺ in CaWO₄ samples. XANES, which extends up to 50 eV above the absorption edge (of W), gives qualitative information regarding the chemical state of the system. The region extending from 50–1000 eV above the edge, corresponds to EXAFS region. Accurate theoretical modeling and fitting of observed oscillations in this region gives quantitative information like bond distance and coordination number of the nearby shells, disorder factor in the system etc. For example, Buysser et al.,²⁵ based on EXAFS studies on polyoxotungstate, confirmed that more intense blue and less intense green emissions from polyoxotungstates arise due to the presence of WO₄ and WO₆ species respectively.

In the present manuscript, nature of WO₄ structural units present in CaWO₄ particles having range of crystallite size (obtained by varying annealing temperatures) is investigated in detail using XANES, EXFAS and FTIR techniques. Steady state luminescence and lifetime measurements were also carried out. These studies were carried out to understand the structural changes taking place with WO₄ structural unit of CaWO₄, subjected to annealing at different temperatures and its dependence on the technologically important blue emission from CaWO₄ particles.

2. Experimental

2.1. Synthesis of CaWO₄ particles

For synthesis of CaWO₄ particles Ca(NO₃)₂·4H₂O and Na₂WO₄·2H₂O were used as starting materials. Around 0.5 g Ca(NO₃)₂·4H₂O was dissolved in 20 ml of ethylene glycol while stirring. To this solution, 0.8 g of Na₂WO₄·2H₂O was added and stirring was continued for two hours. A white precipitate was formed which was separated by centrifugation and then washed with methanol and acetone to remove unreacted species. The samples thus obtained were dried under ambient conditions for overnight. These samples were divided into five parts and heated at different temperatures in air for 5 hours each.

2.2. Characterization

X-ray diffraction (XRD) studies were carried out using a Philips powder X-ray diffractometer (model PW 1071) with Ni filtered Cu-Kα radiation. The lattice parameters were obtained from Rietveld refinement of the XRD patterns using GSAS software.^26,27 Cosine Fourier series function has been chosen for background shape and type-2 function of GSAS software has been chosen for peak profile during refinement. Average crystallite size (D) was calculated by using Scherrer formula D = 0.9λ/(β cos θ), where β is the full width at half maximum in radians, λ is the wavelength of X-rays and θ is the Bragg angle. All luminescence measurements were carried out at room temperature with a resolution of 5 nm, using Edinburgh Instruments' machine (FLSP920) having a 450 W Xe lamp, a microsecond flash lamp and a nanosecond flash lamp as the excitation sources for steady state and time resolved luminescence measurements. Quantum yield measurements have been carried out using an integrating sphere. Transmission Electron Microscopic (TEM) images of the samples were recorded by using 200 keV electrons in a JEOL 210 UHR TEM microscope. Fourier Transform Infrared (FTIR) spectra were recorded for thin pellets of the samples made with KBr using a Bomem MB102 FT-IR machine. Thermo-gravimetric Analysis (TGA) of the as prepared sample was carried out in platinum crucibles using a Setaram, 92-16.18 make TG-DTA instrument. In this analysis, the sample was heated under Ar environment up to 1000 °C at a heating rate of 10°C min⁻¹.

XANES and EXAFS measurements were carried out using the Energy scanning EXAFS beam line (BL-9) at the INDUS-2 Synchrotron Source (2.5 GeV, 120 mA) at the Raja Ramanna Centre for Advanced Technology (RRCAT), Indore, India.²⁸ XANES measurements were carried out at W L₁ edge (12 100 eV) and EXAFS measurements were performed at W L₃ edge (10 207 eV). In EXAFS measurements, initially absorption coefficient (μ(E)) is measured as a function of energy (E). In order to take care of the oscillations in the absorption spectra, the energy dependent absorption coefficient μ(E) has been converted to absorption function χ(E) defined as follows,²⁹

where E₀ is the absorption edge energy, μ₀(E₀) is the bare atom background and Δμ₀(E₀) is step in the μ(E) value at the absorption edge. After converting the energy scale to the photoelectron wave number scale (k) as defined by eqn (2),(where “m” is the electron mass) the energy dependent absorption function χ(E) has been converted to wave number dependent absorption function χ(k). Finally, χ(k) is weighted by k to amplify oscillations at high k values and the resulting χ(k)k functions are Fourier transformed in R space to generate χ(R) versus R (or FT-EXAFS) spectra, which represent the variation of absorption function in terms of real distances from center of the absorbing atom.

It should be mentioned here that a set of EXAFS data analysis program available within the IFEFFIT software package³⁰ have been used for reduction and fitting of the experimental EXAFS data. This includes data reduction and Fourier transform to derive the χ(R) versus R spectra from the absorption spectra (using ATHENA subroutine), generation of the theoretical EXAFS spectra starting from an assumed crystallographic structure (using ATOMS subroutine), and finally fitting of the experimental data with the theoretical spectra using the FEFF 6.0 code (using ARTEMIS subroutine). In the present study theoretical modeling for the samples has been carried out with lattice parameters a = b = 5.243 Å and c = 11.376 Å with atomic positions of different atoms as mentioned in ICSD database.³¹ The bond distances, co-ordination numbers (including scattering amplitudes) and disorder (Debye-Waller) factors (σ²), which give the mean-square fluctuations in the distances, for W–O nearest neighbor shell of WO₄ tetrahedra have been used as fitting parameters. The fitting has been performed in the k range of 3–8 Å⁻¹ and R range of 0.9–2 Å.

3. Results and discussion

Fig. 1 shows Rietveld refined XRD patterns of CaWO₄ samples, as prepared and heated at different temperatures. The average crystallite size calculated from the line width of the diffraction patterns is shown in Table 1. All the diffraction patterns are characteristic of tetragonal structure of CaWO₄. For the as prepared sample, lattice parameters are found to be a = b = 5.230(1) Å and c = 11.392(7) Å with c/a ratio of 2.178 (Table 1). With increase in heat treatment temperature to 300 °C, c/a ratio slightly decreases (c/a = 2.168) indicating an increase in the order taking place with the lattice (c/a = 2.0 for ideal tetragonal lattice). On further increase in heat treatment temperature, c/a ratio remains same (Table 1). The as-prepared CaWO₄ sample is found to have crystallite size in the range of 5–10 nm and the average crystallite size is found to be same for both as prepared and 300 °C heat treated samples, whereas for samples heated above 300 °C, crystallite size increases as can be seen from Table 1. Representative TEM images of the as prepared sample (both high and low resolution) along with selected area electron diffraction (SAED) pattern are shown in Fig. 1 of ESI.† Particle sizes are found to be in the range of 5–10 nm. Nano-crystalline nature of the sample is confirmed by high resolution transmission electron microscopic (HRTEM) image and selected area electron diffraction patterns. The distance between lattice fringes, 3.1 Å, matches well with inter-planar distance between (112) planes of CaWO₄ lattice. Upon heat treatment there is significant aggregation taking place with the sample as can be seen from the representative TEM image for the 500 °C heated sample shown in Fig. 2 of the ESI.† The particle sizes are found to be in the range of 25–70 nm.

Fig. 1 Rietveld Refined XRD patterns of (a) as prepared CaWO₄ sample along with that heated at (b) 300 °C, (c) 500 °C, (d) 700 °C and (e) 900 °C.

Table 1 Different bond length values of Ca–O and W–O bonds present in different CaWO₄ samples. For the purpose comparison c/a ratio is also given

Sample	Average crystallite size (nm)	Ca–O (Å)	W–O (Å)	c/a ratio
CaWO4 as prepared in ethylene glycol	7	2.441(4)	1.845(3)	2.178
CaWO4 annealed at 300 °C	8	2.433(2)	1.814(1)	2.168
CaWO4 annealed at 500 °C	26	2.447(2)	1.815(2)	2.170
CaWO4 annealed at 700 °C	90	2.423(5)	1.857(5)	2.169
CaWO4 annealed at 900 °C	99	2.424(5)	1.839(5)	2.171

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Fig. 2 Emission spectra (a) from CaWO₄ nanoparticles as prepared and annealed at different temperatures. Corresponding decay curves are shown in (b). Samples were excited at 250 nm and emission was monitored at 420 nm.

Fig. 2(a) shows emission spectrum obtained after 250 nm excitation from as prepared CaWO₄ nanoparticles along with those heat treated at different temperatures, namely 300, 500, 700 and 900 °C. For the as prepared sample, emission spectrum is found to have very poor signal to noise ratio with a peak maximum around 420 nm. The emission from CaWO₄ around 420 nm has been thoroughly investigated by different groups.^{11–14,17,18} Based on those extensive investigations, it is inferred that the peak at 420 nm is arising due to the electronic transition from lowest triplet states to ¹A₁ level of W⁶⁺ ions in WO₄ tetrahedron having D_2d symmetry. To our surprise, it is observed that the line width and peak maxima of emission spectra of samples heat treated at different temperatures are identical to that of the as prepared sample, except that signal to noise ratio increases with increase in heat treatment temperatures. A similar trend is also observed in the excitation spectrum (λ_max = 254 nm and line width 28 nm) as can be seen from the excitation spectrum corresponding to blue emission from representative samples shown in Fig. 3 of the ESI.† To check colour purity of emission, CIE (Commission Internationale de l'éclairage) coordinates have been calculated from the corresponding emission spectra shown in Fig. 2(a). The value, (0.18, 0.14), is found to be same for all the samples (as prepared and heated at different temperatures) and is indicated in standard CIE diagram shown in Fig. 4 of the ESI.† From the figure it is clear that the emission from the as prepared and heat treated samples corresponds to bright blue region in CIE plot. To get further insight regarding nature of luminescent species existing in the samples, decay curves corresponding to blue emission from the samples were recorded and are shown in Fig. 2(b). For the as prepared sample, the decay is found to be multi-exponential with an average lifetime value of 2.08 μs. This is because WO₄²⁻ species feels a variety of environments in the nanoparticles due to the large surface to volume ratio. The situation is same in the case of 300 °C heat treated sample. However, for samples heat treated at 500 °C and above, decay curves are nearly single exponential with higher average lifetime values. The lifetime values are found to be ∼6.5 μs for 500 °C heated sample and ∼8 μs for both 700 and 900 °C heat treated samples (the variation in the lifetime values from 8.2 μs to 7.8 μs, as can be seen from Fig. 2(b), is within experimental error limits). Single exponential nature of decay curves for heated samples can be attributed to increase in the particle size and lack of distribution in the environment around WO₄²⁻ species compared to that in the nanoparticles. In other words, the values remained same with heat treatment temperatures at 700 °C and above. The quantum yield of luminescence has been found to be 17% for as prepared sample. The corresponding values for 300, 500, 700 and 900 °C annealed samples are, 20%, 53%, 66% and 64% respectively. Based on these quantum yield values, radiative lifetime has been calculated (extrapolating the quantum yield values to 100% in quantum yield versus lifetime plots) and found to be around 12.3 μs, with a rate constant of 8.13 × 10⁴ s⁻¹. To get insight into the origin of increased lifetime (quantum yield) values for samples heat treated at higher temperatures, further structural investigations were carried out on the samples based on the Rietveld refinement of XRD patterns, W-L₁ edge XANES and W-L₃ edge EXAFS measurements. The details are described below.

Fig. 3 CaO₈ polyhedron (a) and WO₄ polyhedron (b) present in as prepared CaWO₄ nanoparticles. Colour codes: Green spheres represent Ca, blue spheres represent W and red spheres represent oxygen atoms.

Fig. 4 XANES spectra (a) for CaWO₄ samples at W L₁-edge (μ(E) vs. E). The corresponding EXAFS spectra for the samples (μ(E) vs. E) recorded at W L₃-edge are shown in (b).

Based on bond length and bond angle values obtained from Rietveld refinement of XRD patterns, WO₄ and CaO₈ polyhedra are constructed for all the samples and representative polyhedra for the as prepared sample is shown in Fig. 3a and b. The value of Ca–O bond length in CaO₈ polyhedron is found to be identical for CaWO₄ samples heated at different temperatures. Unlike this average W–O bond length value initially decreases for 300 °C annealed sample and then increases (and remains same) to the value corresponding to that of as prepared nanoparticles (Table 1). The results suggest that the bond length variation do not have any effect on the variation in the luminescent lifetimes as a function of annealing temperatures. As Rietveld refinement involves number of fitting parameters, it might be quite possible that observed trend in the values generated can be somewhat subjective. Hence to further substantiate the trends observed in W–O bond length values, XANES and EXAFS experiments were performed on the samples and the results are described below.

To identify the chemical state of W atom in CaWO₄ samples annealed at different temperatures, XANES measurements were performed at W L₁ edge (12 100 eV) and are shown in Fig. 4(a). The presence of the pre-edge peak of considerable intensity in all the samples confirms that W in CaWO₄ annealed at different temperatures are existing mainly as WO₄ tetrahedra (in WO₄ clusters) in accordance with other reports.³² Small amounts of WO₃ cluster may be present but its relative concentration is unaffected with annealing temperatures as revealed by the identical peak position and line shape observed for samples subjected to different annealing temperatures. These results suggest that the environment around WO₄ structural units is same and independent of annealing temperatures. The EXAFS spectra, absorption coefficient (μ(E)) measured as a function of energy (E), for the as-prepared and the heat treated samples are shown in Fig. 4(b). Clear oscillations after the edge can be seen for all the samples suggesting the existence of long range order in the samples. To get further information from these curves, the energy dependent absorption coefficient μ(E) has been converted to the wave number dependent absorption function χ(k), weighted by k, and Fourier transformed in R space to generate the χ(R) versus R (or FT-EXAFS) plots. These plots are discussed in the following section.

Fig. 5 shows the experimental χ(R) versus R spectra of CaWO₄ samples derived from the EXAFS spectra measured at room temperature at W L₃-edge along with the best fit obtained from theoretical modeling as discussed above. The best fit parameters obtained are presented in Table 1 of the ESI.† Variation of these parameters as a function of annealing temperature is shown in Fig. 6. For the as prepared sample the average W–O bond length is found to be 1.74 Å and it decreases to 1.69 Å up on heating the sample to 300 °C. A similar trend is also observed in the W–O bond length values obtained from the Rietveld refinement of the XRD patterns (Table 1). Up on increasing the annealing temperature to 500 °C, the average W–O bond length increases slightly and remained same with further increase in the annealing temperatures (i.e. for 700 and 900 °C annealed samples). Number of oxygen atoms around W (co-ordination number) is found to be exactly 4 for the as prepared sample. However, this parameter decreases systematically for samples annealed up to 500 °C and then remained same with further increase in annealing temperatures. A similar trend is also observed for disorder parameter of WO₄ tetrahedron (Fig. 6 of the main manuscript and Table 1 of ESI†). Decrease in oxygen coordination number suggests an increase in oxygen vacancies in the system with heat treatment. As expected with increase in annealing temperatures, the static disorder (disorder of WO₄ tetrahedra which is extended throughout the CaWO₄ lattice) decreases. From these results, it is established that the average lifetime values and luminescence intensity are not related to W–O bond length, oxygen coordination number around W and the extent of disorder in WO₄ polyhedra. The other possible reason for the variation in the lifetime values (and associated variation in the luminescence intensities) with annealing temperatures can be the stabilizing ligands which can interact with the self trapped excitons. To confirm this FTIR studies were carried out on the samples and the results are described below.

Fig. 5 The experimental χ(R) versus R spectra and corresponding theoretical fits for CaWO₄ samples at W L₃-edge.

Fig. 6 Variation of W–O bond length (a), nearest neighbor oxygen coordination (b) and W–O bond disorder factor (c) for CaWO₄ samples annealed at different temperatures.

Fig. 7 shows the FTIR patterns of CaWO₄ samples annealed at different temperatures. Free WO₄²⁻ ion has T_d symmetry and its IR active modes are represented as A₁, E and F₂.²¹ However, in CaWO₄, symmetry changes to S₄ with IR active vibrational modes ν₂(E), ν₃(F₂) and ν₄(F₂).²¹ For as prepared CaWO₄ nanoparticles, strong band is observed around 800 cm⁻¹ which is characteristic of W–O asymmetric stretching vibrations (represented as ν₃(F₂)). The corresponding bending mode appears around 439 cm⁻¹ (ν₂(E)) (also indicated as A_u mode of WO₄²⁻). The peak at 315 cm⁻¹ is also a bending mode and is represented as ν₄(F₂). The above mentioned modes of CaWO₄ sample remained unaffected with increase in annealing temperatures. In addition to this, there is a broad peak around 3423 cm⁻¹ for as-prepared sample (Fig. 7(a)) which is characteristic of OH stretching vibrations of the ethylene glycol moiety attached on the surface of nanoparticles. The corresponding bending vibrations appear around 1636 cm⁻¹. As temperature increases to 300 °C, intensity (also the peak maximum) of the peaks corresponding to bending and stretching vibrations of OH groups remains same (Fig. 7(b)). However, for samples heated at 500 °C and above, intensity of peaks corresponding to OH groups drastically decreases. In other words, for the sample heated at 500 °C, OH groups are not present with sample and this can lead to aggregation of particles. This is further supported by the higher average crystallite size observed for high temperature heated samples as can be seen from Table 1. The above inferences are further supported by thermo gravimetric (TG) analysis results on as prepared CaWO₄ samples which are described below.

Fig. 7 FTIR patterns of (a) as prepared CaWO₄ nanoparticles along with that annealed at (b) 300 °C (c) 500 °C and (d) 700 °C.

Fig. 8 shows both weight loss and differential weight loss as a function of temperature. The peak around 100 °C is attributed to removal physisorbed water molecules present with nanoparticles. Up on subsequent heating the decomposition of C–H and O–H linkages in ethylene glycol moiety attached with the nanoparticles takes place. This decomposition is complete around 568 °C and thereafter there is no change in the weight with increasing temperature. These results confirm the fact that there is no OH groups attached with CaWO₄ samples heated at 700 and 900 °C.

Fig. 8 TG-DTG pattern corresponding to as prepared CaWO₄ nanoparticles.

From XRD, lifetime measurements, IR and thermo-gravimetric studies, it is clear that OH groups present with nanoparticles are responsible for faster decay of luminescence in as prepared and 300 °C heated samples compared to that heated at 500 °C and above. However, vibration energy of the OH groups (3423 cm⁻¹) is significantly smaller than energy corresponding to blue emission (420 nm equivalent to ∼23 810 cm⁻¹) and hence quenching based on dipole–dipole interaction cannot be responsible for decrease in lifetime values. The observed decrease in lifetime values can be explained based on the trapping of photo-generated holes in the valence band of CaWO₄ by hydroxyl groups attached/present on surface of nanoparticles. This is schematically shown in Fig. 9. Trapping of holes by OH groups of hydroxides and hydrous oxides to form hydroxyl radicals is a well known process.^33,34 This is a competing process for the radiative recombination of electrons and holes leading to reduction in luminescence. Once OH groups are removed, extent of trapping will be less (negligible) and this leads to the improved emission intensity (Fig. 9). Since quantum yield of emission can be considered as ratio of measured lifetime to radiative lifetime, increase in quantum yield of emission is associated with increase in measured lifetime values, provided the radiative lifetime is same. It is indeed observed that samples heated at higher temperatures show higher lifetime (quantum yield) values compared to as prepared and 300 °C heated samples (Fig. 2). Hence from XRD, FTIR and lifetime measurements, it is confirmed that surface hydroxyl groups present on CaWO₄ particles are responsible for decrease in lifetime values for as prepared and 300 °C annealed samples compared to high temperature (500, 700 and 900 °C) annealed samples.

Fig. 9 Schematic representation for the decrease in blue luminescence intensity from CaWO₄ samples brought about by the interaction of holes with hydroxyl groups.

4. Conclusions

CaWO₄ nanoparticles having size in the range of 5–10 nm have been synthesised at room temperature and annealed at different temperatures to vary the average crystallite size. Based on XRD, XANES and EXAFS studies it is concluded that average W–O and Ca–O bond lengths, average number of oxygen atoms around W⁶⁺ ions and disorder in WO₄ tetrahedron depend on the crystallite size of CaWO₄ particles and those parameters do not have any effect on the blue luminescence intensity from CaWO₄ samples. The increase in measured lifetime values and associated luminescence intensities with increase in annealing temperature is due to removal of surface hydroxyl groups form CaWO₄ particles. The OH groups traps holes and inhibit the formation of self trapped excitons and their recombination, leading to decrease in blue emission intensities.

Acknowledgements

Authors thank Dr V. K. Jain, Head, Chemistry Division, BARC, Dr N. K. Sahoo, Head, Atomic and Molecular Physics Division, BARC and Dr B. N. Jagatap, Director, Chemistry Group, BARC for their constant encouragement. Mr. S. Kolay, Chemistry Division, BARC, is thanked for carrying out TG-DTA experiments.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra47619g

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