Correlation between Luminescence Quantum Efficiency and

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Correlation between Luminescence Quantum Efficiency and Structural Properties of Vanadate Phosphors with Chained, Dimerized, and Isolated VO4 Tetrahedra Tomohiko Nakajima,*,† Masahiko Isobe,‡ Tetsuo Tsuchiya,† Yutaka Ueda,‡ and Takaaki Manabe† National Institute of AdVanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Materials Design and Characterization Laboratory, Institute for Solid State Physics, UniVersity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan ReceiVed: NoVember 16, 2009; ReVised Manuscript ReceiVed: February 16, 2010

The internal luminescence quantum efficiency and color properties of AVO3 (A: Li, Na, K, Rb, and Cs), M2V2O7 (M: Mg, Ca, Sr, Ba, and Zn), and M3V2O8 (M: Mg, Ca, Sr, Ba, and Zn) have been investigated. These vanadate phosphors exhibited broadband emission from 400 nm to over 800 nm due to the one-electron charge transfer transition in the VO4 tetrahedra, and the color of the luminescent materials ranged from green to yellow-orange via white, corresponding to 0.277 < x < 0.494 and 0.389 < y < 0.488 on the CIE chromaticity diagram. We found that the luminescence quantum efficiency of the vanadate phosphors with VO4 tetrahedra was strongly enhanced by the strong interaction between V ions and the weak interaction between V and A(M) ions in the crystal structures. We hypothesize that the long exciton diffusion lifetime induced by these structural features enhanced luminescence, leading to high quantum efficiency. 1. Introduction In recent years, white light-emitting devices (LEDs) have attracted considerable attention as a potential light source to replace fluorescent and incandescent lamps. At present, white LEDs typically consist of blue LEDs and yellow-red phosphors1,2 or ultraviolet (UV) LEDs and blue-green-red phosphors.3,4 Many inorganic or organic phosphors have been developed for use in white LEDs.5–7 In contrast with most phosphors, metavanadates AVO3 (A: Rb and Cs) exhibit intense broadband emission from 400 nm to more than 700 nm under UV excitation.8–10 This broadband luminescence in the visible light range is essential for realizing lighting devices with good color rendering properties. Moreover, we have recently investigated a new roomtemperature fabrication process for RbVO3 films on organic substrates;10,11 this process has the potential to greatly reduce the cost of fabricating flexible lighting devices. It should be noted that not only this peculiar case of room-temperature synthesis, but also the general properties of vanadate oxides, which can be crystallized at relatively low temperature in air. Such facile conditions of the fabrication process are expected to be highly beneficial for actual applications for lighting devices. In addition to AVO3, several vanadates have been investigated as broadband-emitting phosphors.12–14 The broadband emission spectra of vanadate phosphors are due to the charge transfer (CT) of an electron from the oxygen 2p orbital to the vacant 3d orbital of V5+ in tetrahedral VO4 with Td symmetry.12 The luminescence is attributed to the 3T2 f 1A1 (Em1) and 3T1 f 1 A1 (Em2) transitions, as shown in Figure 1. However, the factors that affect the luminescence efficiency have yet to be elucidated, while vanadate phosphors have a wide range of internal quantum efficiency (η) values, for example, 4, 79, and 87% for AVO3 (A: K, Rb, and Cs), respectively.10 Therefore, we aimed to * Corresponding author. E-mail: [email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ University of Tokyo.

Figure 1. Schematic model of absorption and emission processes of VO4 tetrahedron with Td symmetry in vanadate phosphors. Ex1 and Ex2 represent excitation processes 1A1 f 1T1 and 1A1 f 1T2, respectively. Em1 and Em2 represent emission processes 3T2 f 1A1 and 3T1 f 1A1, respectively.

determine the essential factors that give rise to vanadate phosphors with high η values in order to advance the development of broadband-emitting vanadate phosphors for use in white LEDs. In this study, we focused on three material systems, namely, AVO3 (A113, A: Li, Na, K, Rb, and Cs), M2V2O7 (M227, M: Mg, Ca, Sr, Ba, and Zn), and M3V2O8 (M328, M: Mg, Ca, Sr, Ba, and Zn). A113, M227, and M328 have characteristic crystal structures of chained, dimerized, and isolated VO4 tetrahedra, respectively.15–26 We measured the luminescence properties of these materials, including the η values, and evaluated the structural parameters of the materials in detail. Then, we investigated the correlation between the luminescence efficiency and structural properties of the vanadate phosphors. 2. Experimental Section Polycrystalline samples of vanadates A113 (A: Li, Na, K, Rb, and Cs), M227 (M: Mg, Ca, Sr, Ba, and Zn), and M328 (M: Mg, Ca, Sr, Ba, and Zn) were synthesized by a solid state reaction. The procedures for these syntheses have been reported previously.15–26 The purity and crystallinity of the polycrystalline samples were confirmed by X-ray powder diffraction analysis.

10.1021/jp910884c  2010 American Chemical Society Published on Web 03/01/2010

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Figure 2. Schematic crystal structures of A113 (A: Li, Na, K, Rb, and Cs).

Figure 4. (a) Schematic crystal structures of M227 (M: Zn, Mg, Ca, Sr, and Ba). (b) VO4 dimer transformation between M227 (M: Ba and Sr) and M227 (M: Mg and Zn). (c) Schematic crystal structure of ZnV2O6.

Figure 3. (a) PL spectra of A113 (A: Li, Na, K, Rb, and Cs). (b) PLE spectra of A113 (A: K, Rb, and Cs). PL spectra were measured at excitation of 350 nm; PLE spectra corresponded to the maximum emission wavelength.

All polycrystalline samples were a well-crystallized single phase without impurities. The photoluminescence (PL) and excitation (PLE) spectra, the η values, and the luminescent color properties of the samples were evaluated with a Hamamatsu Photonics C9920-02 spectrometer equipped with a xenon lamp as the excitation light source, as well as a monochromator, a highsensitivity back-illuminated multichannel charge-coupled device

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Figure 5. (a) PL spectra of M227 (M: Zn, Mg, Ca, Sr, and Ba). (b) PLE spectra of M227 (M: Ca, Sr, and Ba). PL spectra were measured at excitation of 350 nm; PLE spectra corresponded to the maximum emission wavelength.

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Figure 7. (a) PL and (b) PLE spectra of M328 (M: Zn, Mg, Ca, Sr, and Ba). PL spectra were measured at excitation of 350 nm; PLE spectra corresponded to the maximum emission wavelength.

(CCD) photodetector, and an integrating sphere. The CCD photodetector used in this study had higher reliability for measuring long-wavelength emissions (λ > 700 nm) compared with commonly used photomultiplier tubes in commercial spectrofluorometers. η (%) was calculated using the following equation: η ) Nem/Nabs × 100, where Nabs and Nem are the number of absorbed and emitted photons, respectively. To investigate the relation between the luminescence of these materials and their structural features, analyses of crystal structures were carried out on the basis of the structural parameters that were calculated using the refined atomic positions reported in the literature.15–26 The VESTA computer program was used to draw crystal structures and to calculate structural parameters such as bond lengths and angles.27

Figure 6. Schematic crystal structures of M328 (M: Zn, Mg, Ca, Sr, and Ba).

3. Results and Discussion 3.1. Luminescent Properties of Vanadate Phosphors with Chained, Dimerized, and Isolated VO4. The crystal structure of A113 (A: Li, Na, K, Rb, and Cs) has an orthorhombic pyroxene structure, as shown in Figure 2.15 The VO4 tetrahedra of A113 are one-dimensionally linked at two corner oxygen atoms. The one-dimensional VO4 chains are two-dimensionally arrayed, and the VO4 sheets and the A-cation layers are alternately stacked along the a-axis. Thus, the VO4 tetrahedra interact strongly with each other along the VO4 chains. The space group of A113 (A: Li, and Na) is Pbcm, while that of A113 (A: K, Rb, and Cs) is C2/c, due to a topological shift in

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TABLE 1: Luminescence Properties of A113 (A: K, Rb, and Cs), M227 (M: Ca, Sr, and Ba), and M328 (M: Zn, Mg, Ca, Sr, and Ba)a sample

Iem1/Iem2

Pem1 (nm)

Wem1 (nm)

Pem2 (nm)

Wem2 (nm)

K113 Rb113 Cs113 Ca227 Sr227 Ba227 Zn328 Mg328 Ca328 Sr328 Ba328

2.092 1.523 1.515 0.273 1.715 1.513 1.418 1.393 1.162 1.697 2.094

543 514 512 542 542 494 568 578 501 513 519

81.4 69.8 67.5 59.1 65.5 63.0 79.8 83.3 72.9 72.7 87.1

653 599 589 660 616 561 661 674 594 621 585

139.0 104.9 100.9 89.5 102.5 92.3 120.7 126.4 115.3 108.2 101.4

CIE (x,y) 0.362, 0.316, 0.306, 0.494, 0.393, 0.277, 0.432, 0.449, 0.328, 0.329, 0.303,

0.453 0.424 0.418 0.439 0.488 0.389 0.478 0.475 0.407 0.415 0.591

Emission color yellow green white white yellowish orange yellow green green yellow yellow white white yellowish green

a All properties were measured at room temperature. η, I, P, and W indicate the internal quantum efficiency, luminescence intensity, peak position, and peak width, respectively. These parameters were evaluated by using Gaussian fitting. Subscripts em1 and em2 represent emissions Em1 and Em2.

the structure of the VO4 chains.15,24 Figure 3 shows the PL and PLE spectra of A113. Luminescence was not observed for A113 (A: Li and Na); however, broadband emission was observed with increasing ionic radius of the A-cation, starting from K. In particular, Rb113 and Cs113 exhibited remarkably intense white luminescence.10 The origin of the luminescence of A113 phosphors is the CT transition in the VO4 tetrahedra, as discussed above (see Figure 1). The absorption bands in the PLE spectra correspond to the transitions from the ground state 1 A1 to the excited states 1T2 and 1T1, respectively. The PL emissions are attributed to the 3T2 f 1A1 and 3T1 f 1A1 transitions. η increased with increasing ionic radius of the A-cation: 0.05, 0.09, 4, 79, and 87% for A113 (A: Li, Na, K, Rb, and Cs), respectively. M227 (M: Ba and Sr) has a triclinic crystal structure (P1j), as shown in Figure 4a.20,21 Specifically, the VO4 tetrahedra are not isolated, but rather linked at one corner oxygen atom, forming VO4 dimers. With decreasing ionic radius of the A-cation, the distance between adjacent VO4 dimers decreases and Ca227 consists of edge-sharing VO5 square pyramids (Figure 4a).26 As the ionic radius of the A-cation decreases further, the VO4 dimers again appear with twisting of the VO4 tetrahedra (Figure 4b) to eliminate the structural distortion in M227 (M: Mg and Zn).18,23 Figure 5 shows the PL and PLE spectra of M227. M227 (M: Ba, Sr, and Ca) exhibited broadband emission between 400 and 800 nm, which corresponded to the CT transition in the VO4 tetrahedra. The absorption bands in the PLE spectra correspond to the transitions from the ground state 1A1 to the excited states 1T2 and 1T1. The clear double peaks observed in the PLE spectrum of only Ca227 at 290 and 350 nm are thought to be caused by the Ca227 structure differing from that of M227 (M: Ba and Sr). In calculations of the partial density of states (PDOS) of M, V, and O ions in M227 (M: Ba, Sr, and Ca) with the CASTEP program, only Ca227 exhibited splitting of the conduction band into two subbands, which consisted mainly of empty vanadium 3d orbitals. On the other hand, Ba227 and Sr227 exhibited a single peak. In addition, the formation of VO5 square pyramids was thought to reduce the luminescence intensity of Ca227, because high luminescence efficiency has never been observed for materials that are composed of square-pyramidal VO5. No luminescence was observed for MV2O6 (M: Ba, Sr, Ca, Mg, and Zn), which were prepared to compare with the similarly structured edge-shared VO5 materials (Figure 4c). In the case of VO4 dimers of M227 (M: Mg and Zn), no notable luminescence was observed. The η values of M227 reached only 25% with increasing ionic radius of the M-cation. The η values of M227 (M: Ba, Sr, Ca, Mg and Zn) were 25, 8, 0.4, 0.1, and 0.09%, respectively. The

observed luminescence of Ba227, Sr227, and Ca227 was green, yellow green, and yellowish orange, respectively. The crystal structure of M328 consists of isolated VO4 tetrahedra as shown in Figure 6.14,17,19,22 A topological shift in structure occurs due to decreasing ionic radius of the M-cation. The PL and PLE spectra for M328 are shown in Figure 7. Each compound exhibited broadband PL and PLE spectra corresponding to same CT transitions as those of the aforementioned A113 and M227 systems. In M328, all compounds exhibited visible luminescence, and their luminescence intensity roughly increased with decreasing ionic radius of the M-cation. The η values of M328 (M: Ba, Sr, Ca, Mg, and Zn) were 0.5, 3.7, 0.7, 6, and 52%, respectively. The color of the luminescence of these compounds changed with the ionic radius of the M-cation, from yellowish green (Ba328) to yellow (Mg328 and Zn328) via white (Sr328 and Ca328). The broadband emission spectra of the A113, M227, and M328 vanadate phosphor systems consisted of two broad peaks corresponding to the emissions Em1 (3T2 f 1A1) and Em2 (3T1 f 1A1). The CIE color coordinates of the luminescence,28 the peak positions, and the intensity ratio (IEm1/IEm2) of the prepared materials are listed in Table 1. The observed colors of luminescence are governed by peak position and intensity ratio. The emission colors of these materials ranged from green to yellowish orange via white, as shown in Figure 8d (0.277 < x < 0.494 and 0.389 < y < 0.488 on the CIE chromaticity diagram). In almost all these systems, the emission peak of Em1 at around 500-540 nm (green emission) had narrow width and strong intensity compared with the emission peak of Em2. This led to large y values and “green-shifted” luminescence from pure white (x ) 0.33 and y ) 0.33). In contrast, both Em1 and Em2 of Mg328 and Zn328 were red-shifted to around Pem1 ) 570 nm and Pem2 ) 670 nm. Accordingly, these materials exhibited yellow luminescence. The luminescence of M328 (M: Mg and Zn) was red-shifted compared with that of M328 (M: Ba, Sr, and Ca) due to their differing structural topology. This is thought to lower the energy levels of the intermediate triplet states (3T1 and 3T2) via additional energy loss in intersystem crossing (1T1, 1 T2 f 3T1, 3T2), although the detailed mechanism has not yet been elucidated. In the case of only Ca227, we observed that the color of luminescence was strongly dependent on the excitation wavelength. The intensity ratio of the emission peaks Em1 and Em2 typically does not depend on excitation wavelength, as shown in Figure 9a (Ba227). Interestingly, only the Em1 peak corresponding to green emission notably changed with increasing excitation wavelength, while the Em2 peak corresponding to red emission changed to a much smaller extent. Consequently, the color of luminescence of Ca227 shifted from

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Figure 9. Excitation wavelength dependence of PLE spectra of (a) Ba227 and (b) Ca227. (c) CIE chromaticity diagram showing color coordinates of luminescence of Ba227 and Ca227 under excitation at 290-350 nm.

Figure 8. PL emission spectra of (a) Cs113, (b) Ba227, and (c) Zn328. Thick line indicates emission spectra fitted with two Gaussian curves (dotted lines) corresponding to emission bands Em1 and Em2. (d) CIE chromaticity diagram showing color coordinates of the luminescence of A113 (A: K, Rb, and Cs), M227 (M: Ca, Sr, and Ba), and M328 (M: Zn, Mg, Ca, Sr, and Ba).

white to yellowish orange with increasing excitation wavelength (290 nm f 350 nm), whereas that of Ba227 was roughly constant in this excitation range (Figure 9c). For lighting applications employing vanadate phosphors, it will be necessary to find materials that exhibit intense luminescence with strong yellow-red components, as observed for M328 (M: Mg and Zn) and Ca227, to optimize the color rendering properties of LEDs for emission of pure white light.

3.2. Determinants of High Luminescence Quantum Efficiency in Vanadate Phosphors. The η values as a function of the ionic radius of the A(M)-cations of the vanadate phosphors observed in this study are summarized in Figure 10. The η values seemed to correlate with the ionic radius of the A(M)-cations. As discussed above, the luminescence of vanadate phosphors originates from the charge transfer transition in VO4 tetrahedra with Td symmetry (Figure 1). Basically, the excitation process (1A1 f 1T1, 1T2) is allowed, while the intersystem crossing (1T1, 1T2 f 3T1, 3T2) and luminescence process (3T1, 3 T2 f 1A1) are forbidden in the ideal Td symmetry due to the spin selection rule. However, the structure of the VO4 tetrahedron is distorted to some extent from that of an ideal tetrahedron, and thus, these forbidden processes are partially allowed due to the spin-orbit interaction.12 Therefore, the first essential factor that cannot be overlooked when discussing the variation in the η values is the distortion of the VO4 tetrahedron. Figure 11 shows the relation between the η values and the standard

Vanadate Phosphors with VO4 Tetrahedra

Figure 10. Relation between ionic radius of A(M) ions29 and η for A113 (A: Li, Na, K, Rb, and Cs), M227 (M: Zn, Mg, Ca, Sr, and Ba), and M328 (M: Zn, Mg, Ca, Sr, and Ba).

Figure 11. Variation in η as a function of the standard deviation (σ) of (a) four V-O bond lengths and (b) six O-V-O bond angles for A113, M227, and M328.

deviation (σ) of the four V-O bond lengths and the six O-V-O bond angles in the VO4 tetrahedra of A113, M227, and M328. These values indirectly indicate the strength of the VO4 tetrahedral distortion from the ideal Td symmetry. However, these σ values are not directly correlated with the η values, while the VO4 tetrahedra are distorted in all luminescent vanadates regardless of their crystal structures: the σ values of the V-O distance and O-V-O bond angle in luminescent vanadates are 0.02-0.09 Å and 0.3-4.0°, respectively. Therefore, these static values of tetrahedral distortion are not considered good indicators of η. The relations between η and the structural parameters of nearest-neighbor V-V bond length and A(M)-V bond length in A113, M227, and M328 are shown in Figure 12. In the A113 system, the adjacent VO4 tetrahedra strongly interacted with each other due to the chained VO4 linkages, while the V-V distance slightly increased by 3% with increasing η. Therefore, the A-V distance can be regarded as a dominant factor for the η variation in this system. The η values increased from nearly zero to a low value to a very large value of 87% as A-V bond length increased by 17%. In M227, η increased with decreasing V-V

J. Phys. Chem. C, Vol. 114, No. 11, 2010 5165 bond length and with increasing M-V bond length; both parameters of A-V bond length and M-V bond length varied widely, by -8 and 10%, respectively. In M328, the M-V bond length was similar in all compounds. On the other hand, the V-V bond lengths varied widely, by 12%, between Ba328 and Zn328. Thus, η was strongly enhanced by decreasing V-V bond length. To summarize these structural features, strong interaction between the adjacent V ions and weak interaction between the V and A(M) ions are crucial for increasing luminescence efficiency. Based on these results, we next discuss the key to high quantum efficiency in vanadate phosphors, in which the luminescence originates from the CT transition in VO4 tetrahedra. The origin of luminescent intensity of A113 has been speculated by Sayer as follows: the strong correlation of V ions would enhance exciton diffusion in the crystal, and the long exciton lifetime increases the encounter probability with lattice defects or impurities for the radiative transitions.9 (Here the exciton diffusion means a resonance transfer of excitation states.) We partly agree with this speculation about the importance of exciton lifetime on the basis of our qualitative analyses of quantum efficiency and structural parameters. However, we do not consider that the encounter probability between the excitons and lattice defects (impurity) are the determinants for the luminescent intensity. Here we would like to propose the other probable mechanism related to the exciton diffusion lifetime. Figure 13 shows a relative energy diagram of the CT transition in VO4 tetrahedra. An excited electron has two possible relaxation pathways, namely, the nonradiative recombination process (1T1, 1T2 f 1A1) or the radiative process (3T1, 3T2 f 1 A1) that proceeds via intersystem crossing (1T1, 1T2 f 3T1, 3T2). The excited electrons can move to the triplet states (3T1, 3T2) when they have moments in which the energy barrier of intersystem crossing (EIC) is lower than that of recombination (ERC). If there are some types of time-dependent distortion of the VO4 tetrahedron such as a dynamic Jahn-Teller effect owing to an excited 3d electron in the Td symmetry, the forbidden singlet-triplet transition could be further allowed (the energy barrier EIC is effectively lowered relative to ERC). In this case, the excited electrons could undergo intersystem crossing by overcoming the lowered EIC energy barrier if the lifetime of the excited singlet states (1T1, 1T2) were increased; a long lifetime of the excitation state could be realized through enhanced exciton diffusion in the crystals. Thus, we speculate this scenario as the key for understanding the luminescence quantum efficiency in the vanadate phosphors. Actually, the degree of dynamic Jahn-Teller distortion in the photoexcited VO43species has not been measured so far, however, the dynamic Jahn-Teller effect on physical properties of the other optical/ magnetic materials has already observed in the Jahn-Teller active center even at Td symmetry.30,31 In addition, the photoexcited VO4 tetrahedron seems to have a relative large distortion because the static VO4 tetrahedra with d electrons have never been observed: the vanadium ions with d electrons are normally stable in a pyramidal or an octahedral coordination. This fact also gave us the aforementioned idea. This hypothesis is supported by the experimental structural analyses. The vanadate phosphors with high η values had strong V-V and weak A(M)-V interactions. The strong interaction between V ions clearly affects exciton diffusion, leading to a longer lifetime of the excited state. The weak interaction between A(M)-V ions leads to suppression of the repulsion force between the excitons and A(M) ions, which would also be highly effective for increasing the exciton diffusion lifetime.

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Figure 12. Variation in η as a function of nearest-neighbor V-V bond length for (a) A113, (b) M227, and (c) M328. Variation in η as a function of A(M)-V bond length for (d) A113, (e) M227, and (f) M328.

interdimer distance is relatively large, whereas the intradimer distance is short (Figure 12b). On the other hand, the V-V distances in the isolated VO4 system of M328 are roughly averaged in three-dimensions, whereas they are not very much shorter than the V-V distance of the VO4 dimers in M227. Thus, the coherence length for exciton diffusion in the crystal is possibly an important factor for realizing high η. To further confirm this hypothesis, time-dependent measurements such as those of the dynamical structural distortion of VO4 tetrahedra and the lifetime of excitation states must be carried out. 4. Conclusion

Figure 13. Schematic relative energy diagram of one-electron CT transition in VO4 tetrahedron with Td symmetry in vanadate phosphors. ERC and EIC represent the energy barriers of the nonradiative recombination process (1T1,1T2 f 1A1) and intersystem crossing (1T1,1T2 f 3 T1,3T2), respectively.

Therefore, the present analysis considering structural features and the observed results of quantum efficiency are consistent with our proposed mechanism of the high η in vanadate phosphors. Comparing the chained (A113), dimerized (M227), and isolated (M328) VO4 systems, the chained VO4 structure of A113 was best for obtaining high η values. This is thought to be caused by the one-dimensional linkage of VO4 tetrahedra stimulating exciton diffusion. In the dimerized VO4 system, the

The η values and color properties of A113 (A: Li, Na, K, Rb, and Cs), M227 (M: Mg, Ca, Sr, Ba, and Zn), and M328 (M: Mg, Ca, Sr, Ba, and Zn) have been investigated. These vanadate phosphors exhibited broadband emission from 400 nm to over 800 nm due to the CT transition in the VO4 tetrahedra, and the colors of these luminescent materials ranged from green to yellow-orange via white, corresponding to 0.277 < x < 0.494 and 0.389 < y < 0.488 on the CIE chromaticity diagram. We found that the high luminescence quantum efficiency of vanadate phosphors with VO4 tetrahedra was strongly enhanced by the strong interaction between V ions and the weak interaction between V and A(M) ions in the crystal structures. We hypothesized that the long exciton diffusion lifetime induced by these structural properties enhanced luminescence, leading to high η values. These results indicate a promising approach

Vanadate Phosphors with VO4 Tetrahedra for finding new vanadate phosphor materials that exhibit intense luminescence. References and Notes (1) Xie, R. J.; Mitomo, M.; Uheda, K.; Xu, F. F.; Akimune, Y. J. Am. Ceram. Soc. 2002, 85, 1229. (2) Lopez, O. A.; McKittrick, J.; Shea, L. E. J. Lumin. 1997, 71, 1. (3) Chang, C.-K.; Chen, T.-M. Appl. Phys. Lett. 2007, 90, 161901. (4) Webster, G.; Drickamer, H. G. J. Chem. Phys. 1980, 72, 3740. (5) Xie, R.-J.; Hirosaki, N.; Kimura, N.; Sakuma, K.; Mitomo, M. Appl. Phys. Lett. 2007, 90, 191101. (6) Pardha Saradhi, M.; Varadaraju, U. V. Chem. Mater. 2006, 18, 5267. (7) Ding, W.; Wang, J.; Zhang, M.; Zhang, Q.; Su, Q. J. Solid State Chem. 2006, 179, 3582. (8) Gobrecht, H.; Heinsohn, G. Z. Phys. 1957, 147, 350. (9) Sayer, M. Phys. Status Solidi A 1970, 1, 269. (10) Nakajima, T.; Isobe, M.; Tsuchiya, T.; Ueda, Y.; Kumagai, T. Nat. Mater. 2008, 7, 735. (11) Nakajima, T.; Tsuchiya, T.; Kumagai, T. Appl. Surf. Sci. 2009, 255, 9787. (12) Ronde, H.; Blasse, G. J. Inorg. Nucl. Chem. 1978, 40, 215. (13) Park, K.-C.; Mho, S.-I. J. Lumin. 2007, 122, 95–123. (14) Benmokhtar, S.; El Jazouli, A.; Chaminade, J. P.; Gravereau, P.; Guillen, F.; de Waal, D. J. Solid State Chem. 2004, 177, 4175.

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