Article pubs.acs.org/JPCC
Oxygen Vacancy Effect on Photoluminescence Properties of SelfActivated Yttrium Tungstate Bangfu Ding,† Haijiao Qian,† Chao Han,† Junying Zhang,*,† Sten-Eric Lindquist,‡ Bin Wei,§ and Zilong Tang∥ †
Department of Physics, Beihang University, Beijing 100191, China Department of Physical Chemistry, Uppsala University, Uppsala S-75121, Sweden § Key Laboratory of Advanced Display and System Applications, Shanghai University, Shanghai 200072, China ∥ State Key Laboratory of New Ceramic and Fine Processing, Tsinghua University, Beijing 100084, China
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‡
S Supporting Information *
ABSTRACT: A series of single-phase yttrium tungstate powders were synthesized through solid-state reaction under air or argon atmosphere. All powders showed broad band emission in the visible light region, and the argon-calcined samples presented strong near-infrared luminescence. Moreover, the long-wave excitation bands peaking at 340, 378, 380, 490, and 523 nm depended critically on the calcination atmosphere and temperature. The emergence of these new excitation bands was ascribed to different oxygen vacancy concentrations with the analysis of the first-principle calculation, Raman and X-ray absorption fine structure spectra. The oxygen vacancies caused the reduction of the average coordination number of tungsten, and the position of the localized energy band changed with the oxygen vacancy concentration. Finally, a schematic photoluminescence excitation model was proposed via anion and cation charge transfer. The obtained results promise to be very useful in interpreting self-activated tungstate luminescence mechanism. They can also serve as guide line for tuning the luminescence performance of yttrium tungstate and related materials.
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INTRODUCTION The tungstate and molybdate (MxW(Mo)yOz) families are an important class of self-activated luminescent matters and have been turned into an extensive research subject during the past century.1 The interest for these compounds originates from their excellent optical properties, which form the fundamentals of their application as phosphors, laser materials, and scintillation detectors.2−4 Tungstate compounds also have many other advantages such as large X-ray absorption coefficient, high chemical stability, and high light yield.5,6 With the increasing interest and their application as scintillator detector and phosphors, it becomes urgent to understand their luminescence and scintillation mechanism.7−11 The physical and chemical properties of inorganic materials are strongly dependent on their particle size, morphologies, chemical compositions, and internal structures.12,13 The photoluminescence properties of these materials correlate with their matrix structures, defects, impurities, and activated ions in the host.14 Hence, there is considerable experimental and theoretical literatures reporting on possible luminescence centers in tungstates.15,16 Pure and doped MWO4 (M = Pb, Ca, Ba, Zn, and Cd) have been studied widely,17−31 and many luminescence models were proposed. Different synthesis methods and conditions lead to different intrinsic defects in tungstate materials, such as oxygen vacancy (VO), M vacancy © 2014 American Chemical Society
(VM), tungsten vacancy (VW), interstitial oxygen ion (Oi), M interstitial ion (Mi), and tungsten interstitial ion (Wi).17,18,21 Some rare earth tungstates, such as Lu2WO6 and Eu2W2O9, have been synthesized and investigated focusing on the microstructures and the luminescence properties.7,8 The first principle methods were employed to reveal the luminous origin of several tungstates.22−25 In addition, the quantum chemical methods such as discrete vibrational Xα, Hartree−Fock, and quantum chemical cluster approaches were applied to derive the electronic structure properties of tungstates.26−29 However, there were only a few detailed studies of the photoluminescence excitation of tungstates in the literature. For example, nonstoichiometry, ion doping, and use of different surfactant concentrations in the synthesis17−20 endowed tungstates with different excitation spectrum characteristics. The available literature lacks adequate characterization of the local crystal field to study the origin of the excitation and emission processes.31 Only a few authors mentioned luminous properties of Re2WO6 (Re = rare earth elements) compounds among them, e.g., Eu3+ emission in Gd(Y)2WO6.32 As far as we know, no profound theoretical calculation has been performed Received: June 4, 2014 Revised: October 9, 2014 Published: October 10, 2014 25633
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Table 1. Formation Energy of Supercell Structures with Oxygen Vacancy in Different Sites Oxygen Rich Eformation(eV)
Eformation(eV) Eformation(eV)
VO(l)
VO(2)
VO(3)
VO(4)
VO(5)
VO(6)
1.9316
1.6098
1.7617 2.3243 Oxygen Poor
2.7071
2.6213
VO(1,2)
VO(l,3)
VO(1,4)
VO(1,5)
VO(l,6)
VO(2 3)
VO(2,4)
VO(2,5)
−2.8845 VO(2,6) −0.6557
−2.0455 VO(3,4) −1.4793
−1.3255 VO(3,5) −1.7749
−0.8697 VO(3,6) −1.2168
−1.8646 VO(4,5) −0.6773
−1.6101 VO(4,6) −1.2599
−2.1700 VO(5,6) −0.0847
−3.4347
Calculation Methods. Lattice dynamics, total energy, and electronic structure properties calculations were performed with the pseudopotential plane wave method incorporated in the VASP.44,45 The generalized gradient approximation (GGA) was used to represent the exchange and correlation energy.46 The constituted atomic valence states were dealt with the projector augment wave (PAW) scheme.47,48 The Monk-horst Pack scheme was applied to calculate the integration with a special set of k-points.49 In order to get agreement between the theoretical and experimental gap values, in calculating the electronic structure properties we performed the first-principle calculation with GGA+U method on the basis of Dudrev’s methods.50−52 An effective Ueff of 9.9 and 4.5 eV was applied for W and O, respectively. The calculations were completed in the following three steps. First, a series of test for pure Y2WO6 conventional cell was carried out in different volumes in order to get the minimum energy in certain suitable structure parameters. The plane wave cutoff energy of 500 eV, and a 3 × 5 × 2 grid of Monk-horst Pack points were employed to ensure good convergence. Second, three kinds of 1 × 2 × 1 supercell, namely, Y2WO6, Y2WO5.875 with single oxygen vacancy, VO(i) (there are six sites for oxygen atom in the single Y2WO6 cell, and VO(i) indicates the case where the ith oxygen atom is removed to form an oxygen vacancy), and Y2WO5.75 having a pair of oxygen vacancies, VO(i, j) (i ≠ j) (forming two oxygen vacancies by removing a pair of oxygen atoms in the vicinity of a tungstate atom) were built, and optimized with 3 × 2 × 2 k-points. For Y2WO5.875 containing a VO(i) and simulating samples calcined in air, we calculated the defect formation energy in six oxygen positions according to reduced formula53 Ef[Vo(i)] = Edefect(i) − Eperfect + μO. Here, Edefect and Eperfect are the total energies of the Y2WO5.875 and Y2WO6 supercells, respectively. For oxygen-rich environments, μO is chemical potential of O, which is determined by the energy of oxygen molecule (μO = (μ(O2))/2). For oxygen-poor conditions, the chemical potential of oxygen atom was determined by 3μO + μW = μ(WO3). The oxygen vacancies formation energies in different sites both in oxygen-rich and oxygen-poor conditions were presented in Table 1. The formation energy value of Y2WO5.875 was lowest when the oxygen vacancy was positioned in the second site. In Y2WO5.75 with a pair of oxygen vacancies, presenting the samples calcined in Ar, the ith and jth (i, j = 1, 2..., 6, i ≠ j) oxygen atoms next to a tungstate atom were both removed. Finally, the k-points number was increased up to a 6 × 4 × 4 grid for density of state (DOS) calculation of Y2WO6, Y2WO5.875 and Y2WO5.75, and then some special high symmetric points were chosen to compute the band structure.
on the luminescence properties of Re2WO6 from the density function theory (DFT).33 Recently, yttrium tungstate (Y2WO6) has been studied as a useful negative thermal expansion and luminescent material.34−37 For the photoluminescence properties of Y2WO6 crystal, most authors focus on uranium and lanthanum (Ln=Er, Sm, Eu, and Dy) doped Y2WO6 systems.38−41 The luminescence origin of these systems was ascribed to WO66− groups and doping ions. By combining emissions from both WO66− group and Sm3+, white emission was obtained.38,39 However, self-activated luminescence of the matrix was not investigated in detail. On the basis of the absence of sufficient experimental and theoretical studies upon luminous mechanism of Y2WO6, we synthesized a series of single-phased monoclinic Y2WO6 powders by calcining mixtures of Y2O3 and WO3 powders at different temperatures under the air or argon (Ar) atmospheres. We found that the long-wave excitation bands were strongly dependent on the calcination atmosphere and temperature. Similar peaks have been found in Sm-doped Y2WO6,39 nominally pure MWO4 (M = Pb, Ca)17−19 and some other materials,42 but their origin has not been explained clearly. In this work, we found that the choice of atmosphere and calcination temperature induced the changes of oxygen vacancy concentration and tungsten coordination number. These changes affected the appearance of long-wave excitation bands. The obtained results can give guidance for how to prepare the compounds with adjustable luminescence performance.
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EXPERIMENT AND CALCULATION METHODS Experimental Details. Y2WO6 powders were prepared by mixing stoichiometric amounts of Y2O3 and WO3 powders in an agate mortar. The powders were ground for 30 min to a homogeneous mixture. Then the mixture was transferred to an alumina crucible and fired for 5 h in a muffle furnace in the atmosphere of air or Ar. After firing, the samples were cooled to room temperature in the furnace, and ground into powder again for subsequent use. The composition and phase purity of the samples were studied by X-ray diffraction (XRD) using a Panalytical X’Pert PRO diffractometer that was operated in diffraction mode with Cu Kα radiation (λ = 1.54 Å). The photoluminescence excitation and emission measurements were recorded with a Hitachi F-7000 and FLS-920 fluorescence spectrometers. W LIII-edge X-ray absorption fine structure (XAFS) spectra were collected on Beamline 1W1B at the Beijing Synchrotron Radiation Facility. The analysis and theoretical calculations of XAFS spectra were performed using IFEFFIT package.43All the measurements were performed at room temperature. 25634
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25635
0.388 11(22) 0.250 0000(0) 0.250 0000(0) 0.0776(4) 0.5296(19) 0.3926(19) 0.4372(32) 0.2673(22) 0.4000(23) 0.2241(23) 11.3783(5) 90 a
Efremov et al. (ref 54).
z y
0.2529(7) 0.7340(18) 0.6864(12) 0.1941(8) 0.161(4) 0.365(4) 0.508(4) −0.006(4) 0.019(5) 0.459(4) 5.3385(4) 104.343(1) 446.64 0.280 53(28) 0.000 0000(0) 0.500 0000(0) 0.1999(5) 0.3573(29) 0.5009(30) 0.1590(34) 0.2901(35) 0.064(4) 0.211(4) 7.5894(3) 90
x z
0.388 05(18) 0.250 0000(0) 0.500 0000(0) 0.078 79(29) 0.5586(28) 0.3907(17) 0.4311(29) 0.2689(19) 0.4035(19) 0.2288(21) 11.3613(3) 90 0.2510(6) 0.7367(15) 0.6845(11) 0.1918(7) 0.138(4) 0.367(4) 0.531(4) −0.013(4) 0.051(5) 0.456(4) 5.3334(1) 104.3750(18) 445.29
y x
0.280 40(23) 0.000 0000(0) 0.500 0000(0) 0.1994(4) 0.3713(27) 0.5114(30) 0.1510(33) 0.2945(33) 0.0639(32) 0.219(4) 7.5849(2) 90 0.3879(1) 0.25 0.25 0.0782(1) 0.549(1) 0.389(1) 0.432(1) 0.272(1) 0.396(1) 0.232(1) 11.354(2) 90
z y
0.2524(1) 0.7366(4) 0.6842(4) 0.1921(3) 0.150(2) 0.362(2) 0.523(2) −0.012(2) 0.035(2) 0.463(2) 5.334(1) 104.41(2) 445.15 4g 2e 2f 4g 4g 4g 4g 4g 4g 4g
x site symmetry atom
W Yl Y2 Y3 O1 O2 O3 O4 O5 O6 a, b, c (Å) α, β, γ (deg) V (Å3)
0.2800(1) 0 0.5 0.1994(2) 0.367(2) 0.513(2) 0.146(2) 0.290(2) 0.065(1) 0.216(1) 7.589(1) 90
1200 °C in Ar 1200 °C in air
Luminescence Dependence on the Calcination Conditions. The monoclinic crystal structure of pure Y2WO6 (space group number and symbol, 13-P12/C1−C2h4) was reported by Efremov.54 Its lattice constants and atomic coordinates are shown in Table 2. Through the hightemperature solid phase method, we synthesized Y2WO6 phosphors at different temperatures in the air or Ar environment. These powders were confirmed to be singlephase, and the crystallographic parameters agreed well with those of the monoclinic Y2WO6 unit cell employed in simulation.54 The crystal parameters of samples calcined in the air or Ar at 1200 °C refined by Rietveld refinement are shown in Table 2.55 The R (reliable factor) factor Rp and weighted R factor of profile Rwp, and goodness of fit χ2, are Rp = 3.85, Rwp = 5.34, χ2 = 3.508 and Rp = 9.92, Rwp = 8.01, χ2 = 3.115, respectively. Note that, at the same calcination temperature (1200 °C), the crystal volume of the sample calcined in Ar is slightly larger than that of the air-calcined sample.39 The room-temperature photoluminescence spectra recorded under the ultraviolet excitation at λ = 320 nm (3.88 eV) are presented in Figure 1a,b. In samples calcined in the air at 1400 °C, the emission band peak position is equal to 441 nm (2.81 eV). When the calcination temperature in the air decreases, the emission peak slightly shifts to long wavelength 446 nm. A similar phenomenon is also found in the samples calcined in Ar where the emission peak displacement becomes larger. The red shifts may be caused by an increased phonon−electron interaction in the crystal lattice which perhaps changes with temperature and calcining atmosphere.56 Thus, it is experimentally shown that the emission band peak position depends on the calcination atmosphere, peaking at 441 nm for samples calcined at 1200 °C in the air and at 446 nm for the samples calcined in Ar, respectively. These emission bands around 450 nm under ultraviolet irradiation mainly originate from charge transfer transitions in the WO66− groups.35−40 With the changing of calcination temperature and atmosphere, the shape of the emission band remains basically the same. Moreover, the powders calcined in Ar also show strong nearinfrared emission in the range 1500−1700 nm under nearultraviolet or visible light excitations, while the samples calcined in the air have very weak infrared luminescence, as shown in Figure 2a,b. Figure 1c shows the photoluminescence excitation spectra of as-prepared samples by monitoring the emission at 520 nm. In the same calcination atmosphere, the shape of excitation spectra is similar. The samples calcined in Ar have only two short wavelength excitation bands, as shown by curves 4 and 5, and the intensity of the excitation band increases with increasing calcination temperature. The samples calcined in the air show three excitation bands at 280 nm (4.43 eV), 310 nm (4.00 eV), and 340 nm (3.65 eV) corresponding to curves 1, 2, and 3. As shown in Figure 1c, the intensity of the band at 340 nm increases when the calcination temperature decreases. The appearance of the new excitation band peak at 340 nm is very interesting. It may originate from defects in the structure of Y2WO6.17,18 Previously, it was found that a new excitation peak appeared in CaWO4 and PbWO4. The appearance of this peak was related to the surfactant concentration and the (excess) quantity of WO3 used in the preparation procedure. For PbWO4, tungsten atoms could enter into normal lattice
expta
RESULTS AND DISCUSSION
Table 2. Crystal Parameters and Atomic Coordinates of Monoclinic Y2WO6 Unit Cell Employed in Simulation, and Prepared in the Air or Ar Calcined at 1200°C by Rietveld Refinement
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Figure 1. Spectral energy distribution of the emission for Y2WO6 phosphors burning in Ar (a) and air (b) conditions measured under 320 nm excitation. (c) Excitation spectra of Y2WO6 powders by monitoring emission at 520 nm.
Figure 2. Emission spectra of Y2WO6 phosphors burning in (a) air and (b) Ar conditions measured under different excitation wavelength. Related excitation spectra (c, d) of Y2WO6 powders by monitoring emission in near-infrared region.
Internal Coordination Analysis. To determine the origin of these excitation bands, we performed the local crystal field characterization. The vibration characterizations of tungstates Re2WO6 and molybdates Re2MoO6 (Re = La−Lu, including Y and Bi) have been performed in detail.57 It was found that tungsten and molybdenum have different nearest coordination numbers with oxygen atoms for different cations Rex+ (x, valence state). The Y2WO6 unit cell includes four formula units (Z = 4).54 Since it has space group P12/C1−C2h4, the number of lattice points (LP) is LP = 1. Hence the number of tungstate groups per physical Bravais space cell is ZB = (Z/(LP)) = 4. The group theory calculation58 shows 3N = 3 × 36 = 108 possible vibrations in Y2WO6, which can be written as
positions, and thus, the Pb vacancy was formed in the process of crystallization due to the stoichiometric ratio between Pb and W being smaller than 1. According to charge compensation mechanism, the Pb vacancy was compensated by the O vacancy.17 For our samples, the new excitation peak at 340 nm depends critically on the calcination atmosphere, which is undoubtly related to point defects. When the fluorescent emission is recorded in the wavelength range 1500−1700 nm, as shown in Figure 2c,d, the samples calcined in Ar show the longer wavelength excitation bands peaking at 380 nm (3.26 eV), 490 nm (2.53 eV), and 520 nm (2.38 eV). For air-calcined samples, a new excitation band at 378 nm (3.28 eV), except for 340 nm band, is detected in near-ultraviolet region. 25636
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The Journal of Physical Chemistry C Y2WO6 (R) (IR) Γ vibration = 26A(R) + 28BIR g + 28Bg + 26A u u
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pentacoordinated Sm2WO6.57 The far-most two weak peaks at 973 and 948 cm−1 are probably a combined tone such as 563 + 412 and 763 + 180 cm−1. From Table 3, the tungsten atoms in the powders calcined in Ar are abundantly tetrahedrally coordinated, which may be caused by oxygen vacancy aptly formed in oxides under oxygen-deficient atmosphere. The attenuation factors of samples calcined at different temperatures and atmospheres are presented in Figure 3c. The average coordination number (CN) of tungsten and oxygen can be calculated as CN = (((attenuation factor) × 6)/A), where A is the value of W−O in standard sample (generally 0.7−0.9) and 6 is the theoretical coordination number. The average CNs are less than six for all samples. Increasing the calcination temperature in the air or altering the calcination atmosphere from air to Ar each decreases the coordination number further, agreeing well with the results from Raman spectra. Therefore, we can assume that oxygen atoms are deficient in all these samples, and many more oxygen vacancies are created in the samples calcined at higher temperature or in the Ar atmosphere. Photoluminescence Mechanism Analysis. In accord with the analysis of the experimental results, two supercells with one and a pair of oxygen vacancy were constructed and optimized as illustrated in Figure 4. As seen from Figure 4, there is a relatively large space available in region 1. Therefore, it is easy to form interstitial oxygen atom defect (Oi) in these samples calcined in the air because of the oxygen-rich atmosphere.59,60 If there is Oi in air-annealed powders, the coordination number of tungsten atom is larger than 6, which contradicts the results of XAFS and Raman spectra. In line with previous studies17,18 and the suggested electronic compensation mechanism, oxygen vacancy defect can be proposed in powders calcined in the air, as displayed in Figure 4. VO formed in the air that induces the reduced tungsten coordination number. In Ar, Oi cannot be formed owing to oxygen-poor environment,61 and oxygen atoms can easily form molecular oxygen (O2) by escaping from the solid oxides in reducing environment. Furthermore, the formation energies of Y2WO5.875 (VO(2)) and Y2WO5.875 (VO(3)) are smaller than in the cases when oxygen vacancy is located at the other four sites (as shown in Table 1). The pair of oxygen atoms next to a tungsten atom is likely to form O2 and then get lost under Ar atmosphere condition. Thus, the coordination number of tungsten by oxygen decreased to 4.61 The formation energy of the oxygen vacancy pair at the vicinity of a tungsten shows nonmonotonic variation with the distance (Oi−Oj) increase. Therefore, we calculated the electronic structure properties of Y2WO5.75 (VO(2,5)) with the lowest Eformation. Figure 5a shows the single-electron energy band dispersion curves along with four directions connecting the special k points in the Brillouin zone of monoclinic supercell crystal. The maximum value of valence band (VB) is located at high symmetry G point, while the minimum value in conduction band (CB) is approximately placed at G point. Therefore, yttrium tungstate is a direct-gap material, and its band gap value (3.75 eV) belongs to wide band gap semiconductor.62 In experiment, the optical band gap (Egap) established by the the Wood and Tauc63 method was 3.76 eV (Figure 5b). Thus, the theoretical value agrees well with the observed value, and selected U values are reasonable. The total and partial DOS of a perfect Y2WO6 supercell crystal are presented in Figure 6a. The zero of energy has been taken at the top of the last occupied band of the Y2WO6 crystal.
(1)
The superscripts R and IR denote the Raman and infrared modes, while the subscripts g and u indicate the parity under inversion in center-symmetric Y2WO6 crystals. The A and B modes are nondegenerative. In fact, because of weak coupling between the WO66− molecular group and the Y3+ cation, the vibration modes can be divided into internal vibration modes of group WO66− and external vibration modes relating to Y3+ interacting with WO66−. For the internal molecular vibration of WO66− having Oh symmetry, factor group analysis gives the following expression 6−
WO6 Γ internal vibration = A1g + Eg + F2g + 2F1u + F2u
(2)
WO66−
Because there are four ions in every Y2WO6 Bravais space cell, the total number of internal vibrations is at least 60 in theory. Figure 3a,b presents the Raman spectra of Y2WO6 calcined at different temperatures and under different atmospheres. The
Figure 3. Raman spectra of as-prepared phosphors at different temperatures in the air (a) and Ar (b). (c) Attenuation factor connected to average coordination numbers of tungsten measured using synchrotron radiation.
Raman peak parameters are shown in Table 3. There is only one strong peak at 835 cm−1 in the three samples calcined in the air, corresponding to the WO symmetrical stretching A1g mode and having the full width at half-maximum (fwhm) 20 cm−1.57 This characteristic band indicates that tungsten atoms are octahedrally coordinated with six oxygen ions. The remaining 27 peaks at 710, 693, 670, 621, 597, 550, 522, 499, 460, 446, 428, 393, 366, 340, 310, 292, 282, 270, 253, 238, 225, 199, 180, 142, 128, 119, and 104 cm−1 can be assigned to internal vibration modes between tungsten and oxygen (eq 2). These peak positions evidenced that the tungsten coordination number is six. However, the other four spectral bands (563, 480, 217, and 154 cm−1) could eventually be induced by fivecoordination tungsten atoms.57 From Figure 3b and Table 3, one can see that the Raman spectra of the samples calcined in Ar atmosphere are more complicated, and differ significantly by peak intensity and positions from these recorded from the samples calcined in the air. For example, the middle strong peaks at 874, 801, and 763 cm−1 indicate that the tungsten atom has tetrahedral coordination in analogy to that of La2MoO6 crystal.57 The peaks at 737, 563, and 480 cm−1 are similar to those of the 25637
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Table 3. Raman Vibration Spectra Peak Values of Y2WO6 Samples Calcined in the Air and Argon Atmospheresa no.
Ar
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
973(w) 948(w) 874(m)(La2MoO6b) 834(m) 801(m)(La2MoO6b) 763(m)(La2MoO6b) 737(m)(Sm2WO6b) 695(w) 669(sh) 626(w) 597(w) 563(m)(Sm2WO6b) 528(s) 497(s) 480(m)(Sm2WO6b) 461(m) 444(sh) 430(m) 412(m)
air
835(s)
710(sh) 693(w) 670(sh) 621(w) 597(w) 563(sh)(Sm2WO6b) 550(sh) 522(w) 499(w) 480(w)(Sm2WO6b) 460(w) 446(sh) 428(w) 412(w)
no.
Ar
air
22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
393(w) 373(w)
393(w) 375(sh) 366(w) 340(w) 310(w) 292(w) 282(w) 270(m) 253(w) 238(w) 225(w) 217(w)(Sm2WO6b)
340(w) 291 (w) 270(w) 253(w) 237(sh)
208(w) 199(sh) 180(sh) 142(w) 128(w) 119(w) 104(w)
199(sh) 180(w) 154(w)(Sm2WO6b) 142(w) 128(w) 119(w) 104(w)
a Values in cm−1. Tetracoordinated W(Mo)6+, La2MoO6; pentacoordinated, Sm2WO6; hexacoordination, Y2WO6. Abbreviations: w, weak; s, strong; sh, shoulder; m, medium. bBode et al. (ref 57).
Figure 5. (a) Electronic bands structure along with high symmetric points in the first Brillouin zone of perfect supercell Y2WO6 structure within the energy range −5 to 7 eV. (b) Band gap obtained using UV− vis absorption spectra of powder calcined in the air at 1200 °C.
Figure 4. Optimized supercell crystal structure of yttrium tungstate, and atomic positions are denoted by spheres and elemental symbols. The schematic diagram of coordination between tungsten and oxygen represents the samples calcined in the air and Ar (Y2WO5.875 and Y2WO5.75).
properties of Y2WO6 phosphors are mainly ascribed to anions groups WO66−. In the CB, there are two high density state regions separated by the energy gap 0.73 eV, which originates from W 5d and Y 4d states. According to the molecular orbit theory, the five degeneracy W 5d orbits are separated into eg(dz2, dx2−y2) and t2g(dxy, dyz, dxz) orbits in regular octahedron.4 For two supercells Y2WO5.875 and Y2WO5.75, the total and partial DOS are plotted in Figure 6b,c. In comparison with the pristine Y2WO6, new local energy levels 1−5 appear in the forbidden band for the above-mentioned two supercells. These local energy levels are mainly composed of W 5d state (below CB) and O 2p states (above VB). The presence of the local energy band 1 just above VB causes the appearance of the 340 nm excitation band in Y2WO6. The local state 5 under the CB induces the disappearance of the 340 nm excitation band and
The VB is mainly composed by O 2p states with a small contribution of W 5d states. The contributions of W 5p, 6s and Y 4p, 4d, 5s to VB are rather small. The O 2p, W 5d, and Y 4d states form the bottom and upper of the CB which ranges from 3.67 to 7.2 eV. Therefore, the contributions of Y3+ electronic states are similar to that in other wolframite and scheelite type tungstates and molybdates.64−68 From the empirical electronegativity consideration, since the Y (1.22) electronegativity is lower than that of W (2.36) on the Pauling scale, the Y 4d states should lie above the W 5d states, which agrees well with the calculation results.69 It is concluded that the luminescence 25638
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excitation bands centered at lower energy than the band to band transition. From Figure 7 and Table 4, it is seen that the position of local energy level changed with the variation of oxygen vacancy concentration, which is similar to the extended Zni state.70 Though the influence of oxygen vacancy in tungstates, boron phosphates, and zinc oxides17,18,42,71 on luminescence properties has been investigated intensively, studies of oxygen vacancy effect on photoluminescence excitation are limited. In our samples calcined in the air and Ar, respectively, there are localized energy bands in the forbidden band gap. When the samples are irradiated with light with the energy of 4.43 eV (280 nm) and 4.0 eV (310 nm) (larger than Egap), electrons jump to CB, and then relax to the lowest excitation. They finally emit photons and return to ground state, as shown in Figure 8a. The samples calcined in the air can be excited by the Figure 6. Total DOS and partial DOS of constituted atoms for (a) supercell Y2WO6, (b) Y2WO5.875, and (c)Y2WO5.75 from −5.5 to 8 eV.
appearance of new excitation band at 380 nm. In powders calcined in the air and Ar, the band gaps are direct and in-direct, respectively (Figure 7 and Table 4). It is interesting to point
Figure 8. Scheme diagram of energy transition of self-activated luminescence Y2WO6 calcined in the air and Ar from (a) energy band and (b) molecular orbit theory.
light energy 3.64 eV, smaller than Egap, due to the electron jumps from the local O 2p state to the CB. In powders calcined in the Ar, electrons also jump from the local O 2p state to local W 5d state under lower exciting energy (
