Doping-Induced Room Temperature Stabilization of Metastable Î²

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Doping Induced Room Temperature Stabilization of Metastable #-Ag2WO4 and Origin of Visible Emission in # and #Ag2WO4: Low Temperature Photoluminescence Studies Santosh Kumar Gupta, Kathi Sudarshan, Partha Sarathi Ghosh, Saurabh Mukherjee, and Ramakant Mahadeo Kadam J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00078 • Publication Date (Web): 22 Mar 2016 Downloaded from http://pubs.acs.org on March 23, 2016

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The Journal of Physical Chemistry

Doping Induced Room Temperature Stabilization of Metastable β-Ag2WO4 and Origin of Visible Emission in α and β-Ag2WO4: Low Temperature Photoluminescence Studies Santosh Kumar Guptaa*, Kathi Sudarshana, Partha Sarathi Ghoshb, Saurabh Mukherjeea, Ramakant Mahadeo Kadama a. Radiochemistry Division, Bhabha Atomic Research Centre b. Materials Science Division, Bhabha Atomic Research Centre

Abstract: A new strategy of synthesizing hexagonal metastable β-Ag2WO4 at room temperature based on aliovalent Eu3+ doping induced orthorhombic-to-hexagonal phase transition using coprecipitation method is offered. Both α-Ag2WO4 and β-Ag2WO4 phase were characterized systematically using X-ray diffraction (XRD), Raman Spectroscopy, Fourier transformed infrared spectroscopy (FTIR) and Time resolved photoluminescence (TRPL). Emission spectra at low temperature (77K) show two clear bands in the case of α-Ag2WO4 and βAg2WO4 which are designated as PL1 (low wavelength region) and PL2 (high wavelength region). PL1 (~430-440nm) is attributed to charge transfer transition within tungsten octahedra in α-Ag2WO4 where as it is attributed to similar transition within the tungsten tetrahedral in β-Ag2WO4. Temperature dependent studies showed that origin of PL2 emission in α-Ag2WO4 and β-Ag2WO4 might be different. PL2 in the case of α-Ag2WO4 is because of presence of F+ centre (singly ionized oxygen vacancy) within the band gap which is supported by density function theory measurement (DFT). Doping Eu3+ in α-Ag2WO4 diminishes PL2 emission. Lifetime measurement supports that in the case of β-Ag2WO4, both PL1 and PL2 might have common origin i.e. charge transfer transition. Keywords: α and β-Ag2WO4, aliovalent doping, charge transfer, photoluminescence

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1. Introduction: Transition metal tungstates form an important class of functional materials because they possess a combination of covalent, ionic and metallic bonding. Because of their unique symmetry dependent and spontaneous polarization properties which render them with important properties like ferroelectricity, conductivity and photoluminescence, they have attracted significant attention among scientific community1. To mention a few; CuWO4 is used for photo-electrochemical splitting of water2, Y2WO6 as a self activating luminescence material3, ZrW2O8 as negative thermal expansion material4, ZnWO4 for super capacitor and photocatalysis5, CdWO4 as a luminescence host6 etc. Among them silver tungstate has emerged as a new class of inorganic tungstate material which has potential applications in various areas. These include sanitary ceramics as antibacterial agent7, high temperature tribological applications as solid state lubricants8, plasmonics9, catalyst for water splitting and dye degradation10,11, electrochemical sensor12, optical material13, antibacterial property14,15, luminescence host16 etc. It exists in three different polymorphs: α- (orthorhombic, thermodynamically stable), β- (hexagonal, metastable) and γ- (cubic) depending on the pH condition17. Most of the studies reported in literature are related to α-Ag2WO4 due to its ease of synthesis while not many studies are reported on β-Ag2WO4. The β-phase tends to transform easily to thermodynamically stable α-phase. Metastable phases are very interesting cases due to their unique physico-chemical characteristics which can be entirely different from those of the thermodynamically stable phase.18, 19 Main challenge thus is to synthesize the metastable beta phase, as conventional route would only result in the α-Ag2WO4. There is only one report on the room temperature synthesis of metastable β-Ag2WO4 using poly(methacrylic acid) assisted precipitation reaction between AgNO3 and Na2WO4.11 Doping is a fundamental process in materials science technology to produce functional materials. In nanodomain; it is highly significant in stabilizing unusual phases as well as in modifying fundamental properties such as magnetism, electronic and luminescence20. Stabilization of different phases by aliovalent substitution is a well known phenomenon21-24. In the present case 2.0 mol % doping of Eu3+ stabilized beta phase at room temperature. This is indeed the only work where metastable silver tungstate is stabilized using aliovalent substitution by Eu3+. Due to the direct wide band gap of 3.55 eV, high exciton binding energy, interesting electronic properties, and high physical and chemical stability, silver tungstate is considered 2


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as a promising candidate for the next-generation ultraviolet light emitting devices. As far as photoluminescence (PL) is concerned; room temperature photoluminescence is observed in both α-Ag2WO49,25 and β-Ag2WO426 which is attributed to intrinsic charge transfer within tungstate group. Nevertheless, the investigations on the evolution of photo luminescence with temperature in α-Ag2WO4 and β-Ag2WO4 are still lacking. As far as our work is concerned we have synthesized both thermodynamically stable α-Ag2WO4 phase as well as metastable β-Ag2WO4 phase by aliovalent doping of 2.0 mol % Eu3+. Also we have investigated the emission characteristics and the lifetime variation of α-Ag2WO4 and β-Ag2WO4 using PL measurements from 77 to 300 K. The aim of this study is to understand the mechanisms involved in the temperature induced evolution of PL. Temperature dependent luminescence (77 K to room temperature) is not reported at all for these phases. Such investigations are very important firstly; it would be possible to propose some mechanisms of luminescence if emissions were performed under the simplified conditions of small thermal energy. Secondly, the role of thermal vibrations in luminescent process would become more evident at low temperature and establishing some interaction mechanisms between the luminescent process and thermal vibrations might be possible.

2. Experimental: 2.1. Synthesis of α and β- Ag2WO4 microparticles α-Ag2WO4 microparticles were synthesized using highly efficient co-precipitation method similar to the one used for silver molybdate27. In this method 100 mL solutions each of 20.0 mM of Na2WO4.2H2O (99.995 %, Sigma Aldrich, St. Louis, USA) and 40 mM of AgNO3 (Analytical reagent, 99.9 %, Chemco Fine chemicals, Mumbai) in deionized water were prepared separately. The two solutions were then mixed slowly at room temperature which resulted in the formation of yellow precipitate instantaneously. Within few minutes; pale yellow color of the precipitate turned white. The precipitate was allowed to settle down. The two components of the mixture (precipitate and supernate) are separated by decanting. The white precipitate so obtained was washed thoroughly using acetone (2-3 times) and dried at room temperature. For β-Ag2WO4 microparticles, 88 mg of Eu2O3 was dissolved in dil HNO3 and solution is evaporated to dryness. The solution was made upto 50 mL. From this solution, 2 mL was added to AgNO3 solution before mixing it with Na2WO4 solution. 3



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2.2. Characterization of silver tungstate The product obtained was identified by X-ray diffraction analysis using a Miniflex 600 X-ray diffractometer (Model: Rigaku D/Max 220VPC, Japan) with CuKα radiation at a scan rate of 4°/min. Time resolved PL measurements were carried out on an Edinburgh CD-920 unit equipped with M300 grating monochromators (placed on either side of sample). The data acquisition and analysis were done by F900 software. A 150 W Xenon flash lamp having variable frequency range of 10 to100 Hz was used as the excitation source. Multiple emission and excitation scans (at least five) were taken to minimize the fluctuations in peak intensity and maximize signal-noise ratio. Approximately 25 mg of powder sample mixed with few drops of 4 % collodion solution in the form of slurry was pasted over a glass plate. This was dried under ambient temperature and used for further studies. Emission spectra and lifetime at low temperature (77-300K) were measured with a nitrogen bath cryostat (Oxford Instruments, Optistat DN) and a temperature controller (Oxford Instruments, intelligent temperature controller, ITC 503). Raman spectrum was recorded on Seki's STR300 Raman spectrometer using an excitation wavelength of 532 nm from a fibre coupled diode-pumped solid-state laser (DPSS) source. 2.3. Computational Methodology: All calculations in this study are based on density functional theory (DFT) in conjunction with projector augmented wave (PAW) potentials, which is implemented in the plane wave based Vienna Ab-initio Simulation Package (VASP).28,

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The generalized

gradient approximation (GGA) parameterized by Perdew-Burke-Ernzerhof (PBE)30 was used as the exchange-correlation functional. The projector augmented wave (PAW) potentials31 were used for the ion-electron interactions including the valence states of Ag (4d, 5s – 11 valence electrons), W (5d, 6s - 6 valence electrons) and O (2s, 2p – 6 valence electrons). In our calculations, the Kohn-Sham single particle wave functions were expanded in a plane wave basis with kinetic energy cutoff 500 eV and the results were well converged at this cut off. For cubic α-Ag2WO4 structure, optimization was carried out with respect to plane wave cut-off energy and k-point meshes to ensure convergence of total energy to within a precision 0.1 meV/atom. The total energy of α-Ag2WO4 were optimized with respect to volume (or lattice parameter and b/a, c/a ratio) and atomic positions. The structural relaxations (b/a, c/a 4


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ratio and atomic positions) were performed for each structure using the conjugate gradient algorithm until the residual forces and stress in the equilibrium geometry were of the order of 0.005 eV/Å and 0.01GPa, respectively. The Brillouin-zone (BZ) integrations were performed on an optimized Monkhorst-Pack32 k-point grid of 10x10x14 for α-Ag2WO4. The final calculation of total electronic energy and density of states (DOS) were performed using the tetrahedron method with Blöchl corrections33.

3. Results and discussion: 3.1. Phase purity: Powder X-ray diffraction (PXRD) The XRD pattern of the as-synthesized Ag2WO4 sample is shown in Figure 1a. All the diffraction peaks are indexed to the orthorhombic structure of α-Ag2WO4 with space group Pn2n which matches well with the reported value of JCPDS No 34-0061. The small peak at 23.72o is assigned to presence of small amount of silver oxide. The average crystallite size from peak broadening has been determined to be about 80 nm. Figure 1b displays the XRD patterns of 2.0 mol % Eu3+ doped α-Ag2WO4 sample obtained which shows reflections peaks that can be indexed to the hexagonal structured β-Ag2WO4 with space group P63/m which matches well with the reported value of JCPDS No 33-1195. It is reported34 that till 1.0 mol % of europium doping there is no change in structure of Ag2WO4 and it is the α-Ag2WO4 phase which is stabilized. We have used doping level to 2.0 mol %. Based on ionic radii analogy, europium should occupy silver site ( = 1.26 Å and = 1.08 Å) although charge compensating defects will arise which may affect the optical properties of silver tungstate. As result of this aliovalent substitution silver vacancy is generated which may distort the basic structure of α-Ag2WO4. It is reported that α-Ag2WO4 consists of various silver polyhedra such as AgO2, AgO4, AgO6 and AgO7 and relative positions of Ag, W and O are usually variable14. Although it is difficult to predict which polyhedra is predominantly occupied by Eu3+; but as a dopant it will certainly cause severe distortion in the basic network by creating negatively charged silver vacancy. Lighter lanthanide such as europium ion has strong affinity towards electron cloud distortion because of increased polarizability and thus favouring hexagonal structure20. This may be the plausible cause of transformation from orthogonal α-Ag2WO4 to hexagonal β-Ag2WO4. The addition of the precursor solution of Eu dopant may also have changed the pH of the reaction solution and increased the kinetics of the reaction resulting in formation of metastable βAg2WO4. 5



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To confirm the europium doping, Energy Dispersive Spectroscopy (EDS) measurement son α-Ag2WO4 and β-Ag2WO4 (ESI# Figure S1a & S1b) have been carried out. The distinct Xray absorption edge binding energy for Ag, W, and O in the EDS pattern of α-Ag2WO4 and that of Ag, W, Eu and O in β-Ag2WO4 confirms the high purity of both samples. The EDS analysis clearly indicated the presence of europium ion in the β-Ag2WO4 (ESI# Figure S1b).

Figure 1: XRD pattern of (a) α- and (b) β-Ag2WO4. The vertical lines indicate the position and relative intensity of the data from JCPDS No 34-0061 and 33-1195 for α and β-Ag2WO4 phase respectively. 3.2. Raman spectroscopy: Figure 2a and 2b shows the Raman spectrum of α-Ag2WO4 and β-Ag2WO4 respectively. In Raman spectrum of α-Ag2WO4; we have observed ten active Raman modes: 153 cm-1 (A2g), 272 cm-1 (B1g), 284 cm-1 (A2g), 300 cm-1 (A2g), 320 cm-1 (B2g), 578 cm-1 (A1g), 758 cm-1 (B2g), 837 cm-1 (A2g), 880 cm-1 (A1g), 920 cm-1 (A1g) 34. This is an indication of long range ordering in silver tungstate sample as also seen in XRD. Vibrational modes observed in Ag2WO4 can be categorised into two parts (a) internal modes and (b) external modes. Internal mode is due to vibration of intrinsic tungstate group whereas external mode is because of interaction between silver and tungstate ion. Raman modes before 100-500 cm-1 is attributed to vibration of silver ion (external modes) and can be due to various polyhedra such as AgO2, AgO4, AgO6 and AgO7 whereas the one from 500-1000 cm-1 is attributed to various vibration modes of tungstate (internal)10.

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Figure 2: Raman spectra of (a) α- and (b) β-Ag2WO4.

The weak bands at 320, 837 and 920 cm-1 is attributed to stretching and bending mode of WO42- moiety, whereas the intense Raman band at 880 cm-1 is due to the bending modes of the Ag–O–W cluster 8. This particular Raman mode (at 880 cm-1) is weaker in intensity as well as of asymmetric/broader characteristics in case of α-Ag2WO4 compared to β-Ag2WO4. This is in correspondence with XRD peaks of α-Ag2WO4 phase which is also broader than βAg2WO4. Such variation can be caused by variation in crystallite size and different level of structural distortion in these samples. The doping of trivalent Eu3+ ion at monovalent Ag+ site creates a lot of negatively charged silver vacancy which should have distorted the basic network. The fact highly symmetric and sharp Raman band at 880 cm-1 (due to bending modes of the Ag–O–W cluster) could be seen in β-Ag2WO4 indicated the fact that europium 7



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doping causes short range structural ordering possibly by destroying the oxygen vacancies that are generated in the α-Ag2WO4 (as indicated by DFT calculation in the later section 3.4) and eventually reduces the tensions in the material, which causes a better relaxed state of crystal lattice resulting in a better definition of the Raman modes. Similar behavior is also observed by Pinatti et al 34. The fact that this mode is intense in beta phase compared to alpha one is related to trivalent Eu3+ ion substituting monovalent Ag+ site, which in turn can change this bending mode. The FT-IR spectrum of as synthesized α- and β-Ag2WO4 sample is shown in Figure 3. In both the samples FTIR spectra show strong absorption around 3400 and 1650 cm-1 is indicating the vibration of -OH group on the surface of water molecule. The intense peak at 867 cm−1 for both the samples is ascribed to asymmetric stretching vibrations of the O-W-O bonds within distorted [WO6] clusters. Interestingly this particular absorption in β-Ag2WO4 is highly asymmetric with a shoulder at 861 cm-1 which is another proof of different degree of distortion in tungstate cluster in β-Ag2WO4 relative to α-Ag2WO4.

Figure 3: FTIR spectra of α- and β-Ag2WO4. 3.3. Photoluminescence spectroscopy α- Ag2WO4 Figure 4a depicts temperature dependent (77-300 K) emission spectra of α-Ag2WO4 under the excitation wavelength of 230 nm. It can be seen from the Figure that low temperature emission spectra (till 120 K) clearly consists of two distinct bands; one at around 430 nm (PL1, blue region) and another broad band at 550 nm (PL2, green region). Such multicolor emission is indicative of participation of various energy states within the band gap of material. This characteristic is related to the structural disorder of α-Ag2WO4 and indicative of additional electronic levels in its band gap which is further supported by density function 8


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theory (DFT) calculation (section 3.4). It is reported that emission spectra of silver tungstates are usually decomposed into blue- and green-light components.12, 35 The PL bands at ~430 and ~550 nm will be referred to in the further discussion as PL1 and PL2, respectively. The spectral features of PL excitation (PLE) spectra monitored at 430 nm (PLE1) and 550 nm (PLE2) at 77 K (ESI #Figure S2) are different from each other, implying that the different emission centres are responsible for PL1 and PL2. The PL1 in violet-blue region is attributed to intrinsic oxygen to tungsten charge transfer transition with in WO6 cluster where as the PL2 in green-yellow region is attributed to presence of structural defects. There is a general consensus in all the literature pertaining to tungstates or molybdates about attributing the lowest energy emission bands (PL2) to the presence of additional defects in the band gap of material. These defects can either be some structural distortions in metal-oxygen frame work, or oxygen vacancies37. The PL1 is attributed to radiative transition from the O2p states to the W5d states while the PL2 is due to electronic transition from Ag+-VO defect level to valence band. At lower temperature tungstate centres capture electrons from the conduction band and then relax to Ag+-VO defect level and finally to the VB. Therefore, we get both blue as well as green bands compared to one only band at higher temperature. From the temperature dependent emission profiles in Figure 4a it is seen that emission intensity is reduced with increase in temperature. Such reduction due to thermal quenching wherein emissive centres are thermally activated through the crossing point between the ground and the excited levels are well known. The emission spectra are fitted as sum of two peaks and the individual intensities are shown in Figure 4b. It may be pointed out that the two peak fitting was best upto 160 K and fitting became inferior at higher temperature due to fine structure in emission bands (ESI # Figure S2) coupled with lower intensities. The intensity of PL2 is about 6.6 times of PL1 at 77 K. Emission intensities of both PL1 and PL2 decreases with raise in temperature (Figure 4b) although the rate of decrease is higher in PL2 than PL1. The temperature dependent photoluminescence intensity is usually described by a modified Arrhenius equation38:

=

∆

γ

1

Where ∆Ea is activation energy for thermal quenching, I0 represents the initial emission intensity, and γ is constant related to the ratio of radiative to non-radiative lifetime of the carriers in the host and k is the Boltzmann constant. 9



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Equation (1) was used to fit the temperature dependent emission intensities shown in Figure 4b. For the fitting process I(T)/I(77) was fitted with ∆Ea and γ as free parameters. The activation energy (∆Ea) was obtained from fitting as 20.1±2.5 meV and 33.4 ±1.9 meV for PL1 and PL2 respectively while γ was 7.27 ±.63 and 32.1 ± 3.4 for the two peaks. The emission intensity of PL1 is thermally quenched at lower rate compared to PL2: the quenching temperature, at which the initial emission reduces to half of the initial value (I77), is 120 K for PL2 and 160 K for PL1. The steeper reduction in the intensity with the temperature can be caused by lower value of ∆Ea or higher value of γ. The more sensitive nature of PL2 intensity to temperature here is caused by higher value of γ though ∆Ea is marginally higher than PL1. Higher value of γ is due to shorter nonradiative lifetime of the carriers associated with PL2 emission.

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Figure 4: (a) The temperature dependence of the emission spectra of the α-Ag2WO4. The excitation wavelength was 230 nm. (b) Temperature dependence of the emission intensity corresponding to PL1 and PL2 bands. The line shows the fit to Equation 1. The mean energy and full width at half-maximum (FWHM) in energy obtained from twopeak-fitting of the spectra are shown in Figures 5a and 5b. It is seen that PL2 is broader than PL1 also suggesting that the origin of PL2 is due to the recombination of holes/electrons trapped in defects. Both PL1 and PL2 peaks broaden at higher temperature. PL1 shows regular trend whereas nonmonotonous behavior is observed for PL2 peak for all samples with a maximum at 150 K. There are several contributions which can leads to luminescence peak broadening (a) inhomogeneous and (b) homogenous broadening. Inhomogeneous broadening arises due to variation in size and shape crystallites whereas homogenous broadening is due to electron-phonon interaction. The inhomogeneous contribution is temperature independent and each emission peak is broad even at a low temperature due to multi-peak overlapping from different sized α-Ag2WO4 particles. Normally acoustic phonon contribution is dominant at low temperature whereas optical phonons contribute only at higher temperatures39. In fact, at higher temperatures, phonon absorption starts to provide an additional non-radiative pathway which causes decrease of PL intensity. Consequently, the usual multiple phonon coupling and thermal broadening dominate, leading to the net broadening of the peak. However, the broadening of PL2 with increase in temperature in the temperature range below 150 K is too large to be attributed to the increased phonon density and electron–phonon interaction at elevated temperatures. The mean energy of PL2 also shows similar increase 11



below 150 K. Closer study of the PL2 peak shows that it might contain more than one overlapping emission band and the changes in the relative contributions of these bands with the temperature to the designated PL2 band are the reasons behind abnormal changes in the peak energy and width obtained in fitting the emission spectra. It may be noted that quality of deconvolution of the PL spectra was distinctly different above 160 K (ESI # Figure S3). 0.9

(b)

(a)

3.0

0.8 2.8

0.7

PL1 PL2

2.6

PL1 PL2

2.4

0.6 0.5

FWHM (eV)

Emission peak energy (eV)

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0.4 2.2 100

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300

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Temperature (K)

Figure 5: Peak energy and full width at half maxima of the emission peaks obtained by twoGaussian fitting of the emission spectra and designated as PL1 and PL2 in α-Ag2WO4. To avoid this complication, the emission intensity changes in wavelength widows of 20 nm are monitored as a function of temperature. The intensity changes in different wavelength windows beyond 550 nm follow same trend while they differ from those of lower wavelength windows (ESI# Figure S4). The temperature dependent photoluminescence intensity in various wavelength windows are fitted to Eq.1 to evaluate ∆Ea and γ and are shown in Figure 6. It is seen from the Figure 6 that the emission spectrum can be divided into three regions (marked in the figure) which differ in their activation energies and γ-parameter. It is appears that that the emission in region II has contributions from the levels responsible for the emission in region I and region III. At temperatures >160 K, the emission in the region III is negligible and emission spectra in region I and II follow same trend in the integrated intensities. In fitting the emission spectra to two peaks, region II and region III are taken together as PL2 (ESI # Figure S3). Faster reduction in the emission intensity in region III compared to region I or II along with the fitting procedure used is responsible for shift of peak of PL2 towards higher energy. Overall, the emission spectra consist of two overlapping bands with higher energy band intensity being less temperature sensitive than lower energy band.

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400

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PL Intensity (a.u.)

20000

(A)

77 K 300K

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∆ E (meV) a

50 0

γ -parameter (a.u.)

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40

0

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(C)

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1 400

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W avelength (nm)

Figure 6: (A) PL Emission spectra of α-Ag2WO4 at 77K and 300 K, (B) activation energy and (C) γ-parameter obtained by fitting the emission intensities as function of temperature to Equ.1. Emission intensities in 20nm wavelength widow are fitted to Equ.1. 3.4. DFT calculations The α-Ag2WO4 phase is having orthorhombic structure characterized by the space group (Pn2n) with eight molecular formula per unit cell (Z=8). The PAW-PBE calculated equilibrium lattice parameters (a = 10.2427, b = 12.7268, c = 6.1841 Å) and volume (806.14 Å3) are matching well with previous Rietveld refinement15. In this structure, W atoms are coordinated to six O atoms in the distance ranging from 1.80-2.14 Å forming distorted octahedral [AgO6] clusters with Oh symmetry group. In this structure, Ag atoms are coordinated to 2, 4 and 6 O atoms at 1.79 Å whereas molybdenum forms a tetrahedral [MoO4] clusters with Td symmetry group (shown in ESI# Table S1). Comparison of PAWPBE calculated equilibrium atomic positions as well as previous experimentally obtained atomic positions by Rietveld refinement15 are mentioned in Table S2 (ESI#). PAW-PBE calculated atomic positions are in well agreement with experimentally obtained values. 13



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Previously, Pereira et al.40 calculated equilibrium bulk structure of α-Ag2WO4 using the same theoretical methodology as we adopted and their reported lattice parameters a = 11.234, b = 12.574, c = 5.812 Å and V0 = 820.98 Å3 is different from our PAW-PBE calculated values of a = 10.2427, b = 12.7268, c = 6.1841 and V0 = 806.14 Å3. Our PAWPBE calculated V0 is 4.6% higher than experimentally reported value. But PAW-PBE calculated values reported by Pereira et al.40 is 6.5% compared to experimentally reported value15. In Table S2 (ESI#) we have compared equilibrium atomic positions determined by our study at V0 = 806.14 Å3 and atomic positions obtained at V0 = 820.98 Å3. In Table S1 (ESI#) we have also compared cohesive energy calculated at V0 = 806.14 Å3 and V0 = 820.98 Å3. We found that the cohesive energy at equilibrium volume calculated by our PAW-PBE study is 0.58 eV lower compared to that of V0 obtained by Pereira et al.40. So, the structure obtained by this study is more energetically favorable and for the further study we have considered only this equilibrium structure. In order to simulate order-disorder present in the system as well as complex structural vacancies associated with them three structural models were built: (i) by displacement of Ag atom (α-Ag2WO4_Ag); (ii) by displacement of W atom (α-Ag2WO4_W) and (iii) by displacement of both Ag and W atoms (α-Ag2WO4_Ag/W). α-Ag2WO4_Ag model was built by displacing Ag atom in one of the [AgO2] cluster towards an O atom by keeping O-Ag-O angle almost constant. After displacement, Ag-O bond lengths were 2.09 and 2.22 Å compared to 2.14 Å at equilibrium position. In undistorted α-Ag2WO4 two kind of [WO6] clusters were found to have W-O bond-length distribution of 1.81, 1.82, 1.82, 2.13, 2.14, 2.14 Å and 1.80, 1.80, 1.93, 1.93, 2.21, 2.21 Å. Secondly α-Ag2WO4_W model was built by displacing W atom in one of the [WO6] clusters such that W-O bond length distribution becomes 1.77, 1.78, 2.00, 2.01, 2.06, 2.2 Å from equilibrium W-O bond length distribution of 1.80, 1.80, 1.93, 1.93, 2.21, 2.21 Å. To generate α-Ag2WO4_Ag/W distortion model both Ag and W atoms were displaced simultaneously, as described above. The DFT-GGA calculated total and angular momentum projected electronic density of states (DOS) of optimized α-Ag2WO4 is presented in Figure 7 (a). From Figure 7 (a) it can be noted that the lower part of the valence band (VB) from -8 to -3 eV is formed mainly by strong hybridization of the W 5d and O 2p states. Upper part of the valence band from -3 eV to 0.0 eV is composed by mixing of Ag 4d states and O 2p states. The d-states of W predominantly and s-states of Ag (with small contribution) are present in the conduction band (CB) region. The DFT-GGA calculated electronic band-gap of optimized α-Ag2WO4 is 2.30 eV. 14


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Experimentally determined band-gap of α-Ag2WO4 is 3.2 eV25 from UV–visible absorbance spectra (measured at room temperature). The DFT-GGA calculated band-gap is underestimated with respect to the experimentally determined values. Such an underestimation of the band gap and overestimation of equilibrium volume is well-known for the GGA in the DFT calculations41,42. But simple GGA reproduces structural properties and insulating nature of α-Ag2WO4 very well. In this study, we aim to provide a qualitative explanation of photoluminescence properties of α-Ag2WO4 by using a distortion model with DFT calculations. So change in electronic density of state (DOS) features compared to ideal α-Ag2WO4 is important to understand the origin of impurity states in the vicinity of Fermi level and lower part of conduction band. In other words, effects of oxygen vacancy clusters in the photoluminescence properties of α-Ag2WO4 can be explained qualitatively from DFT calculations even though numerical values of electronic band-gaps are underestimated. Fig 7(b), 7(c) and 7(d) shows DFT-GGA calculated total and angular momentum projected electronic DOS of α-Ag2WO4 with Ag atom displaced (α-Ag2WO4_Ag), W atom displaced (α-Ag2WO4_W) and both Ag/W atom displaced (α-Ag2WO4_Ag/W), respectively. Displacements of Ag atom, W atom individually and both Ag/W atoms simultaneously in the unitcell of α-Ag2WO4 generate intermediary energy levels in the upper edge of VB and lower edge of CB resulting in reduction of electronic band-gap from 2.30 eV to 1.40 eV, 1.20 eV and 0.9 eV, respectively. Displacement of Ag atom and W atom in the unitcell of α-Ag2WO4 expands VB width but overall bonding features remain unaltered. Displacement of W atom causes more effect on the electronic structure by decreasing the band gap more in comparison to displacements performed on Ag atom. The perturbation in octahedral [WO6] clusters (Oh symmetry) creates new intermediate levels just below the CB minimum in the forbidden region, promoting different electronic transitions and decreasing the electronic band-gap. Defect states are mainly contributed by the Ag-d states. The perturbation in [AgO2] cluster (C2v site symmetry) creates new states just above the VB maximum. Defect states are mainly contributed by the Ag-d and O-p states. Perturbation in both [AgO6] and [AgO2] clusters creates defect states both in below the CB minimum and VB maximum. When a defect in the structure (αAg2WO4_Ag/W model) was caused by the displacement of Ag and W atoms, two kinds ([WO6]o′ and [WO6]•d as well as [AgO2]o′ and [AgO2]•d) of clusters were observed, in which the superscript ' is the cluster with one electron, and • is the cluster with one hole. As a result, 15



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the DOS revealed that the VB has a predominance of O 2p orbitals. Therefore, our DFT based calculations qualitatively explains PL2 host emission of α-Ag2WO4 in green region (see Fig. 4(a)) due to presence of oxygen vacancy clusters. Thus PL2 is attributed to recombination of trapped electron/holes in oxygen vacancy.

Figure 7: DFT-GGA calculated total and angular momentum decomposed density of states (DOS) of pure α-Ag2WO4 (a), α-Ag2WO4_Ag distortion model (b), α-Ag2WO4_W distortion model (c), and α-Ag2WO4_Ag/Mo distortion model. The vertical lines at 0 eV represent Fermi energy. 3.5. Photoluminescence spectroscopy β- Ag2WO4 Emission spectra of α and β- Ag2WO4 at 77 K under the excitation wavelength of 230 nm are depicted in Figure 8. It can be very well seen from the Figure that emission spectrum of both the sample consists of two peaks; one in lower wavelength region (PL1) and another in higher wavelength region (PL2). The position of PL1 is common in both the sample i.e. approximately around 440 nm although FWHM is much smaller in case of β- Ag2WO4 compared to that of α- Ag2WO4. This reflects the common origin of PL1 in both the sample 16


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i.e. Charge transfer transition. On the other hand the PL2 is blue shifted by approximately 45 nm to 505 nm in β- Ag2WO4 as compared to 550 nm in case of α- Ag2WO4. The shift could be due to differences in the structural distortions in crystal lattice of orthogonal α-Ag2WO4 compared to hexagonal β- Ag2WO4. However, narrower and closer emission bands in βAg2WO4 could also be due to involvement of shallow traps. To account for increased FWHM of emission band in α-Ag2WO4 compared to β-Ag2WO4 it must be realized that the width of the PL peak at low temperature has a major contribution from inhomogeneous broadening. There can be various shallow traps for electrons in a relatively small sized (nano) α- Ag2WO4 (as indicated by broad XRD and Raman peak) due to small perturbations caused by surface states and defects in the nanocrystal. Furthermore, the large surface area in a nanoparticle gives rise to an increased inhomogeneous broadening for any emission band compared to that of a bulk β- Ag2WO4 due to the (relatively) large number of defect states present at the surface of a nanocrystal. After the excitation of the αAg2WO4 nanocrystals an electron can be trapped in a shallow trap state from which it cannot escape due to the insufficient thermal energy at cryogenic temperatures. The luminescence must therefore occur through recombination of an electron in such a trap level with a hole trapped in a nearby oxygen vacancy. In a nanocrystalline α-Ag2WO4 there will be many different trap levels, each with a slightly different trapping energy. As a result, the emission band in such a nanocrystalline α- Ag2WO4 will have a large inhomogeneous width compared to β-Ag2WO4.

Figure 8: Low temperature (77 K) emission spectra of α and β- Ag2WO4 under excitation wavelength of 230 nm. 17



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In case of alkaline earth tungstates (AWO4) coordination number of A (= Ca, Sr and Ba) and tungsten atom is fixed i.e. 8 and 4 respectively where tungstate exists in the form of WO4 tetrahedra. The situation is entirely different in α- Ag2WO4 whose structure is studied in detail by Cavalcante et al.14. His studies suggested that in α- Ag2WO4 there are three types of tungsten atoms and all of them are oriented in the form of distorted WO6 octahedra whereas silver can have various coordination numbers and exist in various polyhedra such as AgO2, AgO4, AgO6 and AgO7. Due to differences in bond angle; both WO6 octahedra as well as AgOn (n= 2, 4, 6 and 7) polyhedra are distorted. As far as structure of β- Ag2WO4 is concerned; it is a hexagonal allotrope of silver tungstate. This particular phase has never been characterized extensively and its structure remains unknown for the last 33 years, although De Santana et al.36 speculated that its structure is closer to δ-Na2WO4. If it is so it will be constructed, similarly to the spinel-type γ-Ag2WO4 allotrope, from isolated WO4 tetrahedra. So the basic difference between alpha and beta phase is the orientation of tungstate network; which is tetrahedra WO4 in beta phase and octahedra WO6 in alpha phase. One of the reasonable assumptions is to suppose that the structural differences in tetrahedral WO4 and octahedral WO6 group’s results in the different relaxed configurations of the excitons generated in α- Ag2WO4 and β- Ag2WO4 leading to the difference in position of peak maxima of PL2 and as well as the difference in FWHM. The luminescence property of the silver tungstate microcrystal is greatly influenced by temperature. To explore the luminescence property of the hexagonal β- Ag2WO4 crystal, we have carried out photoluminescence measurements under Xe lamp at different temperatures, as shown in Figure 9. Emission spectrum consisted to two distinct peaks till 90 K. With increase in temperature, PL1 is red shifted while unusual blue shift of PL2 peak is seen. Beyond 90 K these two peaks are merged and a strong blue-green emission peak around 480 nm is seen. Beyond 160 K, the emission profile of β- Ag2WO4 is found to be similar to that of α- Ag2WO4 but for the additional peak due europium in β-Ag2WO4 at 615 nm (5D0-7F2) (ESI# Figure S5). The relative emission intensities of the both the phases are shown in Figure 10. As seen from the figure, below 160 K the emission intensity steeply rises in the case of β-Ag2WO4. Peak energies of the two peaks in the emission spectra of β-Ag2WO4 in low temperature region where two distinct peaks are clearly seen are shown as inset of Figure 10. It is clear from the figure that PL2 shows blue shift with increase in temperature while the 18


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PL1 shows red shift. The blue shift in PL2 along with increase in the intensity of the PL2 at low temperature, suggest the population of excited levels corresponding to PL2 emission band by the cross over from PL1 or the redistribution of the population between the shallow traps. The intensity ratio of PL2 to PL1 increased from 2.8 at 77 K to 4.9 at 89 K. The emission intensities as a function of temperature couldn’t be fitted to Equ.1 as the intensity of PL2 increased with temperature below 90 K and transfer of exciton population from PL1 band to PL2 band was evident. Fitting of the intensities in various windows was also not feasible due to shift and broadening the peak positions. However, it has been found the changes in the relative intensities in the 400-450 nm window and 500-550 nm window are similar (ESI# Figure S6) showing that the two peaks might have origin from levels of reasonable similarity in thermal properties. The temperature dependent emission intensities in these wavelength windows could be reasonably fitted with activation energy of 0.30-0.35 eV and γ-parameter of 500-600. The results suggest that origin of PL2 emission in β-Ag2WO4 might be different that that of α-Ag2WO4, where vacancies are thought to be responsible for PL2 emission.

Figure 9: The temperature dependence of the emission spectra of the β-Ag2WO4. The temperature of the sample was changed from 77 to 300 K and the excitation wavelength was 230 nm.

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Figure 10: Relative emission intensities of α-Ag2WO4 and β-Ag2WO4 as a function of temperature. The Intensities at 300 K are normalised to unity. Inset shows the peak position of the two emission peaks in β-Ag2WO4, in the temperature regions where they are clearly resolved. 3.6. Photoluminescence lifetimes Lifetime of the photoluminescence spectra has been monitored using well known single photon counting technique. In the case of α-Ag2WO4, lifetime spectra are recorded at emission wavelength of 430 nm and 550 nm corresponding to peak maxima in the emission spectra. In the case of β-Ag2WO4, since there was red shift in PL1 and blue shift in PL2, the lifetime spectra are recorded fixing the emission wavelength at the peak maxima till 100K. Beyond this temperature, since there has been only broad peak as discussed in the previous section, the decay spectrum is recorded at emission wavelength of 480 nm. PL decay curve in all the cases was fitted to triexponential (n=3) equation $

= ∑&' !" # %

(2)

where I(t) is intensity at time t, Ti and Ai are luminescence lifetimes and their relative magnitudes. The average lifetime in each case is calculated as ()* =

∑ + ,+-

(3)

∑ + ,+

20


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Calculated average photoluminescence lifetimes are shown in Figure 11. It is seen from the figure that the photoluminescence lifetimes in general decreased with increase in temperature which is due to an increase in the nonradiative-decay rate, which also induces a reduction in the PL intensity at higher temperatures. From Figure 11a, it is also seen that the lifetimes corresponding to PL1 emission and PL2 emission are quite distinct in the case of α-Ag2WO4. Contrarily, the lifetimes corresponding to PL1 and PL2 in the case of β-Ag2WO4 (upto 100 K where the two peaks are clearly distinguishable) are similar in magnitude and temperature dependence. Once again the photoluminescence lifetimes in α-Ag2WO4 supports our earlier observations based on DFT calculations that PL1 is due to charge transfer transitions in the tungstate group while PL2 is due to the presence of oxygen vacancies which lead to levels in the band gap leading to lower energy emission. In the case of β-Ag2WO4, both PL1 and PL2 seem to have common origin i.e. charge transfer transition. The energy of PL2 emission of βAg2WO4 is very close to the region-II of α-Ag2WO4 as marked in Figure 6 where there is a significant contribution from PL1 like emission. In the case of β-Ag2WO4, two emission peaks at 77 K are quite sharp probably because of larger crystallite size and involvement of shallow traps while they are much broader where α-Ag2WO4 is in nanophase. DFT calculations in α-Ag2WO4 showed that there is no significant effect of cation vacancies on the band structure while oxygen vacancies introduce intermediate levels below the valence band. The absence of emission corresponding to excitons in oxygen vacancies in the case of β-Ag2WO4 is due to the compensation of oxygen vacancies by 2% aliovalent doping of Eu3+; the procedure adopted to stabilize Ag2WO4 in the β phase. The faster reduction in the intensity of PL2 in the case of α-Ag2WO4 shows that the excitons are weakly bound.

15

15

10

10

Lifetime (µ s)

PL1 PL2 20 Mid

PL1 PL2

20

25

(Β) β-Ag2WO4

(Α) α-Ag2WO4

25

Lifetime (µ s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 5 100

150

200

250

300

100

150

200

250

300

Temperature (K)

Figure 11: Photoluminescence lifetimes in (A) α-Ag2WO4 (B) β-Ag2WO4. The emission wavelength is fixed at 430 nm (PL1) and 550 nm (PL2) for α-Ag2WO4. For β-Ag2WO4, the 21



emission wavelength is fixed at peak maxima of individual peaks till 97 K and at 480 nm above 97 K. If indeed, the presence of oxygen vacancies is the reason for PL2 emission αAg2WO4 and

compensation of oxygen vacancies by 2% Eu3+ doping is the reason for

absence of very low energy emission in β-Ag2WO4, then it should be possible to remove the PL2 emission in α-Ag2WO4 by Eu3+ doping. To check this hypothesis, 0.5% Eu3+ doped Ag2WO4 was prepared by the same procedure. It has been found that at this doping Ag2WO4 stabilizes in α-phase (ESI# Figure S7). Emission spectra of pure α-Ag2WO4, 0.5% doped αAg2WO4 and β-Ag2WO4 stabilized by doping 2% Eu3+ recorded at 77 K and 100 K; and at excitation wavelength of 230 nm are shown in Figure 12. It is clearly seen from the figure that 550 nm emission attributed the presence of oxygen vacancies in the case of pure αAg2WO4 is absent in 0.5% doped sample most likely due to removal of oxygen vacancies due to aliovalent doping of Eu3+.

Emission Intensity (a.u.)

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α-Ag2WO4

α-Ag2WO4-0.5% Eu

(A) 77 K

400

500

600

700

β-Ag2WO4-2% Eu

(B) 100 K

400

500

600

700

Emission wavelength (nm) Figure 12: Emission spectra of α-Ag2WO4, α-Ag2WO4 doped with 0.5% Eu and β-Ag2WO4 stabilized by doping 2% Eu at (A) 77 K and (B) 100 K. The emission colour coordinates of the samples are calculated from their respective emission spectra and are shown in Figure 13. The arrow indicates the increasing direction of temperature. It is seen from the figure that emission is in the bluish-region for β-Ag2WO4 below 130 K while it is in the red-green region in α-Ag2WO4. Above 160 K, the emission

22


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spectra of both phases match. The blue shift in the emission of β-Ag2WO4 with increase in temperature from 77 K to 90 K is also evident.

Figure 13: CIE color coordinate diagram of α-Ag2WO4 and β-Ag2WO4 at different temperatures (77 - 300 K). The arrow indicates the increase in direction of temperature. In recording the emission spectrum for β-Ag2WO4 (both 0.5 and 2.0 mol %) we have used excitation wavelength of 230 nm; which is characteristics of host. In this case very weak peak corresponding to 5D0-7F2 electric dipole transition at 612 nm could only be seen because host could not efficiently sensitize europium ion. But when we directly excite europium ion using 395 nm which is characteristics of Eu3+ 7F0-5L6 transition we could clearly see (ESI# Figure S8) the europium emission corresponding 5D0-7FJ (J=1, 2, 3 and 4).

Conclusion:

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α-Ag2WO4 was synthesized using simple and rapid coprecipitation method and metastable βAg2WO4 was synthesized by aliovalent doping of 2.0 mol% Eu3+ in α-Ag2WO4. XRD and Raman measurement shows that α-Ag2WO4 is in nano domain whereas β-Ag2WO4 is closer to being a bulkier counterpart. Luminescence spectra at 77K in both these phases consisted of two emission bands. The lower wavelength band (PL1) coincides in both the samples whereas the second band (PL2) is blue shifted in beta phase by 45 nm. PL1 in both the phases is attributed to intrinsic O→W charge transfer whereas PL2 although has similar origin in beta phase but different in α-Ag2WO4. DFT calculation has confirmed the origin of PL2 in alpha phase is due to presence of oxygen vacancy within the band gap of materials. Nanodomain nature of alpha phase is also depicted in its broad emission peak compared to beta phase. Based on CIE index diagram it was observed that emission is in the bluish-region for β-Ag2WO4 below 130 K while it is in the red-green region in α-Ag2WO4. Above 160 K, the emission spectra of both phases merge. Our results also suggest that the doping-induced structural transition, demonstrated here in Ag2WO4, could be extended to other lanthanidedoped nanocrystal systems for applications ranging from bioimaging to lighting display.

Supporting Information Available: Energy dispersive spectra (EDS) of α-Ag2WO4 and aliovalent Eu3+ doped β-Ag2WO4 to check the doping of euoropium Excitation spectra of α-Ag2WO4 at emission wavelength of 430 and 550 nm; Two peak fitting of emission spectra of α-Ag2WO4 at different temperatures, Relative PL intensities as a function of temperature from αAg2WO4 in different emission wavelength widows of 20nm ; Comparison of emission spectra from α-Ag2WO4 and β-Ag2WO4 at different temperatures; Relative PL intensities as a function of temperature from β-Ag2WO4 in different emission widows; PXRD pattern of α-Ag2WO4 doped with 0.5% Eu; Lattice parameters, unit cell volume, bond-lengths and cohesive energies obtained from DFT based theoretical calculations and experiment (structural refinement by the Rietveld Method, DFT calculated atomic positions of Ag, W and O atoms in α-Ag2WO4 unitcell are compared with

experimentally reported values. This information is available free of charge via the Internet at http://pubs.acs.org

Author Information: Corresponding Author- Santosh Kumar Gupta Email: [email protected], [email protected] Telephone- +91-22-25590636, Fax- +91-22-25505151 24


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Notes The authors declare no competing financial interest

Acknowledgement: The authors would like to thank Dr. Apurva Guleria (Radiation and Photochemistry Division, BARC) for Raman measurement, Smt. Kusum Vats (Radiopharmaceuticals Section, BARC) for FTIR and Pradeep Samui (Product Development Division, BARC) for XRD measurements.

References: 1. Andres, J.; Gracia, L.; Navarrete, P. G.; Longo, V. M.; Avansi, W.; Volanti, D. P.; Ferrer, M. M.; Lemos, P. S.; Porta, F. A. L.; Hernandes, A. C. et al. Structural and Electronic Analysis of the Atomic Scale Nucleation of Ag on α-Ag2WO4 Induced by Electron Irradiation Sci. Rep. 2014, 4, 5391 (1-7). 2. Velenti, M.; Dolat, D.; Biskos, G.; Schmidt-Ott, A.; Smith, W. A. Enhancement of the Photoelectrochemical Performance of CuWO4 Thin Films for Solar Water Splitting by Plasmonic Nanoparticle Functionalization. J. Phys. Chem. C, 2015, 119, 2096-2104. 3. Ding, B.; Qian, H.; Han, C.; Zhang, J.; Lindquist, S.E.; Wei, B.; Tang. Z. Oxygen Vacancy Effect on Photoluminescence Properties of Self-Activated Yttrium Tungstate. J. Phys. Chem. C, 2014, 118, 25633-25642. 4. Sanson, A. Toward an Understanding of the Local Origin of Negative Thermal Expansion in ZrW2O8: Limits and Inconsistencies of the Tent and Rigid Unit Mode Models. Chem. Mater.

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42. Gupta, S. K.; Ghosh, P. S.; Arya A.; Natarajan V.

Origin of Blue Emission in ThO2

Nanorods: Exploring it as a Host for Photoluminescence of Eu3+, Tb3+ and Dy3+. RSC Adv. 2014, 4, 51244-51255.

For Table of content only

Metastable β-Ag2WO4 was synthesized by aliovalent doping of 2.0 mol% Eu3+ in αAg2WO4. Origin of dual band emission in α and β silver tungstate is demonstrated using time resolved photoluminescence and DFT calculations.

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Doping-Induced Room Temperature Stabilization of Metastable Î²

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