Preferential occupancy of Eu3+ and energy transfer in Eu3+ doped Sr2V2O7, Sr9Gd(VO4)7 and Sr2V2O7/Sr9Gd(VO4)7 phosphors

The vanadate-based phosphors Sr2V2O7:Eu3+ (SV:Eu3+), Sr9Gd(VO4)7:Eu3+ (SGV:Eu3+) and Sr9Gd(VO4)7/Sr2V2O7:Eu3+ (SGV/SV:Eu3+) were obtained by solid-state reaction. The bond-energy method was used to investigate the site occupancy preference of Eu3+ based on the bond valence model. By comparing the change of bond energy when the Eu3+ ions are incorporated into the different Sr, V or Gd sites, we observed that Eu3+ doped in SV, SGV or SV/SGV would preferentially occupy the smaller energy variation sites, i.e., Sr4, Gd and Gd sites, respectively. The crystal structures of SGV and SV, the photoluminescence properties of SGV:Eu3+, SV, SGV/SV and SGV/SV:Eu, as well as their possible energy transfer mechanisms are proposed. Interesting tunable colours (including warm-white emission) of SGV/SV:Eu3+ can be obtained through changing the concentration of Eu3+ or changing the relative quantities of SGV to SV by increasing the calcination temperature. Its excitation bands consist of two types of O2− → V5+ charge transfer (CT) bands with the peaks at about 325 and 350 nm respectively, as well as f–f transitions of Eu3+. The obtained warm-white emission consists of a broad photoluminescence band centred at about 530 nm, which originates from the O2− → V5+ CT of SV, and a sharp characteristic spectrum (5D0–7F2) at about 615 and 621 nm.


Introduction
Recently, vanadate-based phosphors have drawn increasing attention due to the self-activated emitting properties of [VO 4 ] 3À group, the sensitization from [VO 4 ] 3À to rare earth ions as well as their long wavelength excitation and the excellent chemical stabilities. [1][2][3][4][5][6][7][8] The vanadate group, namely, [VO 4 ] 3À , in which the central metal ion V is coordinated by four oxygen ligands in a tetragonal symmetry, exhibits broad and intense charge transfer (CT) absorption bands in the UV region and some of them can produce intense broadband CT emission spectra from 400 to more than 700 nm related to the local structure. [9][10][11] When excited by UV light, these vanadates or rare earth ions-doped materials have the capability to convert the ultraviolet emission into white light. 4,12,13 In general, the rst essential factor that determines the luminescence quantum efficiency of vanadate-based phosphors originating from O 2À / V 5+ CT transition is the distortion of the VO 4 tetrahedron. The excitation process of O 2À / V 5+ CT is always allowed; thus, most of the vanadates show self-activated properties, while the intersystem crossing ( 1 T 1 , 1 T 2 / 3 T 1 , 3 T 2 ) and luminescence process ( 3 T 1 , 3 T 2 / 1 A 1 ) are forbidden in the ideal T d symmetry due to the spin selection rule. 1 For example, in the crystal YVO 4 , O 2À / V 5+ CT luminescence process is forbidden and thus, the luminescence of O-V CT cannot be observed at room temperature because in this crystal, V atom is coordinated with four equal oxygens and shows ideal T d symmetry (all four Y-O bond lengths are 1.7Å). 14 However, in Eu 3+ -doped YVO 4 , the O 2À / V 5+ CT energy can effectively be transferred to Eu 3+ and shows intense red photoluminescence corresponding to the electric dipole transition, 5 D 0 / 7 F 2 , of Eu 3+ ions. 15 However, the structure of the VO 4 tetrahedron has, to some extent, a distorted T d symmetry as compared to that of an ideal tetrahedron; thus, these forbidden processes are partially allowed due to the spin-orbit interaction. 1,10 The vanadates with this type of structure can show intense O 2À / V 5+ CT emission. For example, AVO 3 (A: Rb and Cs) exhibits intense broadband emission from 400 nm to more than 700 nm under UV excitation. 5,9,16 M 2 V 2 O 7 (M ¼ Ca, Sr, Ba) with distorted T d symmetry around V atoms can emit strong O 2À / V 5+ CT luminescence. 17 Although only a few Eu 3+ doped with this type of vanadates, such as Ba 3 LiMgV 3 O 12 :Eu 3+ , 18 showed white light, while most rare earth ions-doped with this type of vanadates only produced red, yellow or green or blue-green light; other examples include M 2 V 2 O 7 (M ¼ Ca, Sr, Ba):Eu 3+ , 19 Li 2 Ca 2 ScV 3 -O 12 :Eu 3+ , 6 and Ba 2 Y 2/3 V 2 O 8 :Eu 3+ . 13 The vanadate phosphors exhibit two types of important advantages according to the symmetrical characteristic of VO 4 : rst, self-activated emission arising from O 2À / V 5+ CT with distorted T d symmetry and second, the efficient energy transfer from self-activated O 2À / V 5+ CT to Eu 3+ with T d symmetry. 1 It is very interesting and important to investigate the preferential occupancy of Eu 3+ and PL properties in the mixed phosphor. This is because the Sr 9 Gd(VO 4 ) 7 and Sr 2 V 2 O 7 vanadates show two different types of important advantages according to the symmetrical characteristic of VO 4 . Sr 2 V 2 O 7 shows self-activated emission arising from O 2À / V 5+ CT with distorted T d symmetry, but the energy transfer from O 2À / V 5+ CT to the Eu ions is not effective. However, Eu 3+ doped Sr 9 Gd(VO 4 ) 7 shows efficient energy transfer from self-activated O 2À / V 5+ CT to Eu 3+ , but Sr 9 Gd(VO 4 ) 7 could not produce self-activated emission due to the symmetrical characteristic of VO 4 with T d symmetry. Eu 3+ doped Sr 2 V 2 O 7 /Sr 2 Gd(VO 4 ) 7 possesses the two advantages of self-activated emission and efficient energy transfer from the host lattice to Eu 3+ . In order to investigate the structure and the photoluminescence of vanadate phosphors, we synthesized Sr 2 V 2 O 7 (SV), Sr 9 Gd(VO 4 ) 7 :Eu 3+ (SGV:Eu 3+ ) and Sr 9 Gd(VO 4 ) 7 / Sr 2 V 2 O 7 :Eu 3+ (SGV/SV:Eu 3+ ); in particular, Sr 9 Gd(VO 4 ) 7 /Sr 2 V 2 -O 7 :Eu 3+ can possess both the above advantages of vanadates. The occupying sites of Eu 3+ , photoluminescence properties and the relationship between O 2À / V 5+ CT energy and crystal structure are discussed. The photoluminescence and excitation mechanism as well as energy transfer phenomenon between the host lattice and Eu 3+ are investigated.

Materials and synthesis
The phosphor with nominal composition Sr 9 Gd(VO 4 ) 7 :5% Eu 3+ was synthesized using a high-temperature solid-state reaction method from a stoichiometric mixture of SrCO 3

Characterizations
Powder X-ray diffraction (XRD) measurements were recorded on a D/MAX 2500 instrument (Rigaku) with a Rint 2000 wide angle goniometer and Cu Ka1 radiation (l ¼ 1.54056 A) at 40 kV and 100 mA. The diffraction patterns were scanned over an angular (2q) range of 20À80 at intervals of 0.02 with a counting time of 0.6 s per step. Photoluminescence (PL) studies were conducted on a uorescence spectrophotometer (Photon Technology International) equipped with a 60 W Xe-arc lamp as the excitation light source. All the measurements were recorded at room temperature.

Theoretical method
Based on the chemical bond viewpoint, the dopants preferentially occupy the sites with smaller alterations of bond energy. 20 The variation of bond energy can be measured by the following expression DE0 where N M is the dopant content in the unit of mol. For the same host lattice, the N M is the same, so when we discuss the occupancy of any anion, we only analyse the DE Sr/V/Gd The total bond energy, P E Sr/V/Gd-O , in kcal of the crystals can be regarded as a bond-energy sum of all constituent chemical bonds. For example, in Sr 2 V 2 O 7 , there are four different sites, 21 that is, Sr1, Sr2, Sr3 and Sr4. Taking Sr1 site as an example, there are eight types of constituent Sr1-O, that is, Sr1-O1, Sr1-O2, Sr1-O6 (there are two Sr1-O6 bonds with different distance), Sr1-O8, Sr1-O9, Sr1-O11 and Sr1-O14. Thus, the bond energy, P E Sr1-O , can be calculated using the following formula: The P E M-O is the sum of bond energies of different dopants in the crystallographic frame. When the dopant ion is Eu 3+ , E M-O can be estimated through the following equation 20 where V Sr 2+ /V 5+ /Gd 3+ presents the valence state of Sr or V or Gd and V Eu 3+ is the dopant valence of Eu 3+ . This indicates that the valence state has inuence on the crystal bond energy if the valence state of the dopant is not equal to that of the original ions. The coefficient J is equal to the standard atomization energy, which can be estimated using the following formula: 22 where m is the number of cations in the formal molecule, z is the cation valence and E 0 a is molar atomization energy (kcal mol À1 ) of an oxide crystal M m O n at the standard state (normal pressure, 298 K). E 0 a may be expressed as where DH Q f is the standard heat of formation of M m O n (À592, À1550, À1816 and À1652 kJ mol À1 for SrO, V 2 O 5 , Gd 2 O 3 and Eu 2 O 3 , respectively), S is the heat of metal sublimation or, more generally, heat of atomization of M (164, 515, 397. 5  In eqn (5), d 0 is an empirically determined parameter, which is measured by processing all available crystallographic data; it is constant for a given atom pair. 23 The d 0 of Sr 2+ -O 2À , V 5+ -O 2À and Eu 3+ -O 2À are 2.118, 1.917, and 2.074 A, respectively. Furthermore, the d 0 value of Gd 3+ -O 2À can be estimated by the formula below: where r c and r a are contributions to d 0 from the cation and anion, respectively. The multiplier A is set to 0.8 for transition metal ions with d electrons or else it is set to 1.0. P, D and F are corrections required when cation contains non-bonding p, d and f electrons, respectively. The r c , r a and D values can be obtained in the ref. 23. P and F can be calculated using For lanthanide ions, the calculated d 0 values using (8) are larger 0.079 than those obtained in the ref. 23. Hence, we corrected the formula (8) to be as below: ðfor oxidation of lanthanideÞ (10) The structure of Sr 9 Gd(VO 4 ) 7 :Eu is similar to that of Sr 9 -Lu(VO 4 ) 7 , which is found to be isotypic with Ca 3 (VO 4 ) 2 (ref. 25) or Sr 3 (VO 4 ) 2 . 24 The above results indicate the formation of a pure Sr 9 Gd(VO 4 ) 7 :Eu. The XRD patterns of Sr 9 Gd(VO 4 ) 7 /Sr 2 -V 2 O 7 :Eu component is clearly shown in Fig. 1(c). The detailed crystal plane diffraction peaks ascribed to Sr 2 V 2 O 7 and Sr 9 -Gd(VO 4 ) 7 :Eu are labeled and distinguished using blue and red colors, respectively. Except the diffraction peaks arising from Sr 2 V 2 O 7 and Sr 9 Gd(VO 4 ) 7 crystals, no other peaks can be found, which indicates that we have obtained the component of "pure" Sr 9 Gd(VO 4 ) 7 /Sr 2 V 2 O 7 . From Fig. 1(c) and (e), we can judge that the mechanism of producing Sr 2 V 2 O 7 , Sr 9 Gd(VO 4 ) 7 /Sr 2 V 2 O 7 , and Sr 9 Gd(VO 4 ) 7 is as below:

Results and discussion
From mechanism (12), it can be shown that there are some Gd 2 O 3 and SrO except for the main products of Sr 9 Gd(VO 4 ) 7 / Sr 2 V 2 O 7 ; however, it cannot be observed in Fig. 1(c) Fig. 2 shows the PL excitation and emission spectra of pure Sr 2 V 2 O 7 at room temperature. Upon monitoring the wavelength at 526 nm, Sr 2 V 2 O 7 shows a broad excitation band with peak at 355 nm spanning from 200 to 400 nm, which can be matched well with the excitation band of UV-LED chips. The excitation spectrum originates from O 2À / V 5+ CT transition, which can be deconvoluted into two peaks at 330 (3.76 eV) and 356 nm (3.48 eV) corresponding to the 1 A 1 / 1 T 2 and 1 A 1 / 1 T 1 transition of VO 4 3À group, respectively, as shown in Fig. 2(a) and (c). The values are listed in Table 2. In Fig. 2(a), red dotted lines indicate excitation spectra tted with two Gaussian curves (green dotted lines) corresponding to two excitation bands: Ex 1 and Ex 2 . The energy difference between the peaks Ex 1 and Ex 2 is 0.28 eV, which is ascribed to the energy difference between 1 T 2 and 1 T 1 as reported in many ref. 6, 18 and 26. The corresponding excitation and emission mechanisms are shown in Fig. 2(c). Under the excitation of 355 nm UVlight irradiation, Sr 2 V 2 O 7 shows a strong green emission ( Fig. 2 The (x, y) chromaticity coordinate of Sr 2 V 2 O 7 is (0.308, 0.427) in the green region. The emission spectrum can be further tted by two Gaussian components with two peaks at Em 1 (490 nm, i.e. 2.53 eV) and Em 2 (550 nm, i.e. 2.25 eV), which are labeled using green color of Fig. 2(b). The energy gap between the two peaks is 0.28 eV, which is ascribed to the energy difference between 3 T 1 and 3 T 2 as reported in many ref. 6, 18 and 26. Fig. 3 shows the PL properties of Eu 3+ in SV, which are different in the ref. 2 and 19. The emission spectrum (excited at 355 nm) is shown in Fig. 3(b). The emission spectrum contains a broadband emission in the 400-590 nm wavelength region with a maximum at about 530 nm and a sharp peak at 614 nm. The broad peak has a 30 nm redshi compared with the reported value of 500 nm, which is very large, but the breadth and peak of the broad band emission is similar to that of SV. This corresponds to the charge transfer from the O 2À to V 5+ localized within the tetrahedrally coordinated VO 4 3À group. The sharp peak at 614 nm originates from the 5 D 0 / 7 F 2 transition in Eu 3+ dopant. In Fig. 3(a), the excitation spectrum monitored at 614 nm has a broadband in the 250-400 nm wavelength region with the peak at 355 nm, which is close to the excitation spectrum monitored at 530 nm. This indicates that the broad excitation band arises due to the charge transfer from oxygen 2p orbital of O 2À to an empty d orbital of V 5+ , and not to the orbital of Eu 3+ .
Coordination atoms and their bond lengths of four sites of Sr and four sites of V are listed in columns 2 and 3 of Table 1 Fig. 4(a) and (b) show the typical excitation and emission spectra of SGV:Eu. The excitation spectrum of SGV:Eu obtained by monitoring the 5 D 0 / 7 F 2 at 617 nm is shown in Fig. 4(a). It consists of a broad excitation band with peak at 327 nm spanning from 200 to 350 nm, which can be matched well with the UV-LED chips, along with some dominated sharp lines in the wavelength region of 350 to 500 nm, which arise due to the characteristic f-f transition of Eu 3+ at about 398 and 469 nm. The excitation is the typical O 2À / V 5+ CT transition spectrum of [VO 4 ] 3À , which can be separated into two peaks at 300 and 327 nm corresponding to the 1 A 1 / 1 T 2 and 1 A 1 / 1 T 1 transition of [VO 4 ] 3À group, respectively. This also indicates that the energy transfer can be taken place efficiently from VO 4 3À to Eu 3+ ions in SGV:Eu. The mechanism of energy transfer and luminescence of Eu 3+ are shown in Fig. 4(c), which is similar to the energy transfer from [VO 4 ] 3À to Eu 3+ ions in YVO 4 :Eu. 15,27 Comparing the O-V CT energy in pure sample SV (355 nm), the CT energy of [VO 4 ] 3À in pure SGV (327 nm) is much higher. This is because the crystal eld or environmental factor (h e ) around V atoms in the two samples is different. 28 Fig. 4(b) shows the emission spectra of SGV:Eu excited at 327 nm. The SGV:Eu phosphor shows bright red color. The (x, y) chromaticity coordinate of SGV is (0.547, 0.333) in the red region. Fig. 4(d) shows a graphic of the CIE 1931 chromaticity coordinate of pure SGV phosphors excited at 327 nm. As shown in Fig. 4(b), the dominant red emission bands of 615 and 620 nm are attributed to the electric dipole transition 5 D 0 / 7 F 2 , indicating that Eu 3+ ions are located at the sites of noninversion symmetry. The emission peaks at about 573, 595, 650, and 700-705 nm are derived from the transition of 5 D 0 / 7 F 0 , 5 D 0 / 7 F 1 , 5 D 0 / 7 F 3 , and 5 D 0 / 7 F 4 , respectively, which are much weaker than the intensity of 5 D 0 / 7 F 2 .
Consequently, 5 D 0 / 7 F 2 red emission (615 and 620 nm) presents the most prominent intensity in the emission spectrum. In SGV:Eu, the structure of SGV is isotypical with that of Sr 9 Lu(VO 4 ) 7 , so Sr 9 Gd(VO 4 ) 7 can be analyzed according to the structure in the reference. In Sr 9 Gd(VO 4 ) 7 , there are three  Table 3.
All calculated E  Table 3. The result indicates that the value of bond energy variation is in the order Gd1 < Sr3 < Sr1 < Sr2 < V2 < V3 < V1. According to the bond energy method, Eu 3+ ions should preferentially occupy the sites    absorption range in the UV region. Eu 3+ -doped SGV exhibits efficient energy transfer from [VO 4 ] 3À to Eu 3+ and emits strong red light and SV can emit strong green light. According to the calculated DE Sr/Gd/V Eu-O , in the "pure" SGV/SV system, the result indicates that the smallest value of bond energy variation would be at the Gd site. Consequently, we designed Eu 3+ -doped "pure" SGV/SV system expecting that Eu 3+ can enter the site of SGV and emit strong red light, while SV can retain its strong green light and thus, the composition of red and green will produce white light or exhibit excellent luminescence properties.  The excitation and emission spectra of "pure" SGV/SV are shown in Fig. 5. The peaks, full wave at half maximum (FWHM), Ex 1 and Ex 2 as well as Em 1 and Em 2 of excitation and emission spectra for "pure" SGV/SV are listed in Table 2.
In contrast with the values of pure SV, all the parameters are almost the same, which demonstrate that the emission with the peak at 525 nm comes from the SV in SGV/SV.
The typical excitation and emission of SGV/SV:5% Eu 3+ , represented by SGV/SV:5% Eu 3+ , monitored with different wavelengths and excited at different wavelengths are shown in Fig. 6(a) and (b). Similar excitation spectra with two strong broad bands (at about 320 and 350 nm) can be observed in SGV/ SV:Ln 3+ (Ln 3+ ¼ Sm 3+ , Dy 3+ , or Tm 3+ ) as shown in Fig. S1. † It therefore can be demonstrated that the two broad excitation spectra of SGV/SV:Eu 3+ arise from the O / V charge transfer of the host lattice, i.e., SGV/SV and not from the CT transition of O 2À / Eu 3+ . Under 315 nm UV light irradiation, SGV/SV:5% Eu 3+ shows warm white light emission, consisting of a strong broad band (400-650 nm) with a maximum at 530 nm and some sharp f-f transitions of Eu 3+ ions with the peaks at 595, 615 and  621, 700 and 705 nm attributed to, 5 D 0 / 7 F 1 , 5 D 0 / 7 F 2 and 5 D 0 / 7 F 4 , respectively. The f-f transition peak position of Eu 3+ in SGV/SV:5% Eu 3+ are similar to those in SGV:5% Eu 3+ , which demonstrates that the Eu 3+ ions enter the crystal of SGV. However, the relative strengths at 595, 621 and 700 nm of SGV/ SV:5% Eu 3+ are changed, which can be expressed using I( 5 D 0 / 7 F 1 ) : I( 5 D 0 / 7 F 2 ) : I( 5 D 0 / 7 F 4 ), that is, 2.1 : 6.57 : 1. Moreover, comparing with the f-f transition of Eu 3+ in pure SGV ( Fig. 4(b)), whose relative value is 1.74 : 4.48 : 1, the red light was demonstrated to be enhanced in SGV/SV:5% Eu 3+ compared with the pure SGV:5% Eu 3+ .
The excitation spectrum ( Fig. 6(b), blue line) recorded by monitoring the emission of 621 nm (the strongest emission line in the PL spectrum shown in Fig. 6(e)) contains two visible broad absorption bands at 315 and 350 nm as well as some dominated sharp lines in the wavelength region of 350 to 500 nm due to the characteristic f-f transition of Eu 3+ at about 398 and 469 nm. Under 350 nm UV radiation excitation, the SGV/SV:Eu exhibits green emission, and the obtained emission spectrum consists of a broad band with the peak at 530 nm due to the CT transition of O 2À / V 5+ of SV and very weak f-f transition lines at 614 nm within the Eu 3+ electron conguration. Thus, SV in SGV/SV can retain its excitation energy for strong green emission and its energy transfer efficiency to Eu 3+ is lower. Monitored at 530 nm, the excitation spectrum of SGV/ SV:Eu 3+ sample displays a broad absorption band with the peak at 350 nm, as shown in Fig. 6(c), which is similar to the peak of pure SV. This asserts that the broad excitation band in SGV/ SV:Eu 3+ originates from the CT transition of O 2À / V 5+ of SV. Another broad band at 315 nm in SGV/SV:Eu 3+ must arise from the CT transition of O 2À / V 5+ , which has larger blue-shi compared with the O 2À / V 5+ CT transition of pure SGV at 327 nm ( Fig. 4(a)). This is due to the change of crystal environment around V atoms and the existence of oxygen deciency. 29 The SGV:Eu 3+ was obtained at 950 C, and the SGV/ SV:Eu was obtained at 750 C. With increasing calcination temperature, the crystallization of SGV increases, which changes the lattice constant and the oxygen deciency around V atoms. The change of the lattice constant is small, which generally makes a 1-5 nm shi in CT band, while 5-15 nm blue-shi primarily arises due to of oxygen deciency. 29 3.2.4. The dependence of photoluminescence of SGV/ SV:Eu 3+ on the concentration of Eu 3+ . Changing the concentration of activator doped in host lattice is a feasible route to realize color-tunable emission; a white emission can be obtained through mixing the green and red light sources at a suitable ratio. [30][31][32] In our case, the effects of concentration of Eu 3+ on the PL excitation and emission spectra of SGV/SV:Eu 3+ were investigated. Fig. 7(a) and (b) show the variation of PL spectra and emission intensity of Eu 3+ in SGV/SV:Eu 3+ samples with the increase of Eu 3+ -doping concentrations from 0 to 20 mol% (l ex ¼ 320 nm), respectively. The changing curve of relative intensity of f-f transition emission of Eu 3+ at 621 nm (labelled using red circles) and the V-O CT green emission at 530 nm (labelled using green circles) followed the change in concentration of Eu 3+ as shown in Fig. 7(c). Although the concentration of Eu 3+ is changed, the emission intensity of V-O CT changed slightly, which indicates that the intensity of V-O CT emission originating from SV in SGV/SV is independent of the concentration of Eu 3+ . This further conrmed that the Eu ions enter the sites of SGV in SGV/SV, while no (or a little) Eu ions can be doped into the sites of SV in SGV/SV. This is because the radius and properties of Eu 3+ ions are similar to those of Gd 3+ in SGV; thus, they will occupy the sites of Gd 3+ ions on priority. The experimental analysis is consistent with the theoretical result. The excitation spectrum monitoring at Eu 3+ 5 D 0 / 7 F 2 at 621 nm clearly shows two broad bands: 200-345 nm (centered at 315 nm) and 345-400 nm (centred at 350 nm), which occur due to the O 2À / V 5+ CT transition of SGV and SV in SGV/SV, respectively. Furthermore, the excitation intensity at 320 nm is stronger than that at 350 nm, which demonstrates that the O 2À / V 5+ CT transition energy can be efficiently transferred to the Eu 3+ ion. The energy transfer and luminescence mechanism of SGV/SV:Eu is shown in Fig. 7(e). Moreover, the emission intensity of Eu 3+ rst increases with increasing Eu 3+ from 0 to 15 mol%, and then decreases at 20 mol%. Thus, the emitting color of SGV/SV:Eu 3+ samples can be tuned through changing the concentration of Eu 3+ . The results can also be conrmed by their CIE chromaticity coordinates shown in Fig. 7(d). The CIE chromaticity coordinates (x, y) for SGV/ SV:xEu 3+ phosphors excited at 320 nm are listed in Table 4.
Except that the emission of "pure" SGV/SV is at green region, all other SGV/SV:xEu 3+ phosphors (x ¼ 0.005, 0.01, 0.05, 0.1, and 0.15) exhibit warm-white-light emissions. Therefore, we can realize white light in a single component. This component consists of two types of host lattices and one type of rare earth ion activator (Eu 3+ ), which can achieve the perfect union between the red f-f transition emission of Eu 3+ and the O 2À / V 5+ CT transition emission from the two different vanadate host lattices with different [VO 4 ] 3À symmetries under the UV light excitation. This is a very promising novel method with a wide range of adaptability to obtain white or other color lights.
2. The bond-energy method is used to investigate the site occupancy preference of Eu 3+ based on the bond valence model. By comparing the change in bond energy when the Eu 3+ ions are incorporated into different Sr or V or Gd sites, we observed that Eu 3+ in SV, SGV or SV/SGV would preferentially occupy the smaller energy variation sites Sr4, Gd and Gd sites, respectively. 3. PL properties of Sr 2 V 2 O 7 :Eu 3+ (SV:Eu 3+ ) and Sr 9 -Gd(VO 4 ) 7 :Eu 3+ (SGV:Eu 3+ ) and their products Sr 9 Gd(VO 4 ) 7 /Sr 2 -V 2 O 7 :xEu 3+ (SGV/SV:xEu 3+ ) were investigated, which shows their excitation and emission characteristics. Their excitation wavelengths ranging from 220 to 400 nm t well with the characteristic emission of UV light-emitting diode (LED) chips. Two broad charge transfer bands arising from O 2À / V 5+ of [VO 4 ] 3À with the peaks at about 325 and 350 nm coexist in SGV/SV:Eu 3+ , which indicates that the phosphors can be efficiently excited in the UV region.
4. Eu 3+ ions primarily enter the sites of Gd 3+ of SGV. The photoluminescence properties of SGV/SV:Eu 3+ component shows two prominent properties: rst is that V-O CT energy of SGV can be efficiently transferred to Eu 3+ and strong red photoluminescence can occur coming from the 5 D 0 / 7 F 2 transition of Eu 3+ ; second is that the intensity of V-O CT transition emission originating from SV is not inuenced by the concentration of Eu 3+ and retains green light emission.

Conflicts of interest
There are no conicts to declare.