Luminescence in external dopant-free scandium-phosphorus vanadate solid solution: a spectroscopic and theoretical investigation

Manipulating the high valence secondary P5+/V5+ ions in the external dopant-free Sc(Px,V1−x)O4 (0.0 ≤ x ≤ 1.0) solid solution enables resulting in the spectral tuning, enhanced photoluminescence (PL) intensity, and improved thermally induced PL quenching stability.


Introduction
Due to the merit of the flexible spectral wavelength selection for the device designer (e.g., the phosphor-converted white LED designer), spectrally tunable inorganic solid solutions, formed by the modification of the host isostructural ions that feature the same valence yet different ion radii at the same coordination number (CN), are now receiving growing attention, but almost all of the previously reported solid solutions cannot exhibit their spectral tuning when they lack a rare-earth (RE) and non-RE ion (e.g., Eu 2+ , Ce 3+ , Mn 2+ , Bi 3+ etc.) as the luminescent center, or the activators coupled by arrangement of these RE and/or non-RE ions. [1][2][3][4][5][6][7] Besides, some solid solutions, such as (La,Gd)Sr 2 AlO 5 :Ce 3+ , 1 Ca 1Àx Li x Al 1Àx Si 1+x N 3 :Eu 2+ , 2 La 2 Mg (1Àw) Zn w TiO 6 :Bi 3+ ,Mn 4+ , 4 and (Ca 1Àx Sr x ) 16 Si 17 N 34 :Eu 2+ , 6 are found to show unexpected photoluminescence (PL) properties, which include, for instance, the enhanced PL intensity and/or the improved thermally induced PL quenching stability. However, RE-and non-RE-free inorganic solid solutions, which exhibit spectral tuning, enhanced PL intensity, and improved thermal PL stability simultaneously, have still not been reported so far. Note that the luminescent activators mentioned here are those ions that do not belong to any type of cation that constitutes the crystal host itself.
LnBO 4 (Ln = trivalent lanthanide or Sc 3+ ; B = P 5+ , V 5+ , and Nb 5+ ) is a type of important crystal system in the big family of inorganic oxides and possesses advantages of excellent thermal, chemical and mechanical stability, and optical properties. Relying on the Ln and B cations and the coupled arrangement of Ln and B cations, LnBO 4 features various crystal systems, including zircon (e.g., ScVO 4 , 8 ScPO 4 , 9 and YVO 4 , [10][11][12], monazite (e.g., LaPO 4 13 ), fergusonite (e.g., GdNbO 4 14 ), and wolframite (e.g., ScNbO 4 15 ). As a result, diverse LnBO 4 -related applications, such as laser host materials, 16 high-pressure mercury lamps, 17 color-television tubes, 18 infrared light detectors, 19 photocatalysis, 20 and ultrasonic generators, 21 have been discovered. Among the LnBO 4 crystals, zircon-type ScVO 4 and ScPO 4 crystals are two host materials particularly desirable for single trivalent RE (e.g., Eu 3+ /Tb 3+ , [22][23][24] Dy 3+ , 23 Ce 3+ /Er 3+ 23,24 ) and non-RE (e.g., Bi 3+ 26-28 ) dopants, or multiple dopants coupled by these RE and non-RE ions (e.g., Eu 3+ -Tb 3+ /Sm 3+ /Tm 3+ , [22][23][24] Eu 3+ -Bi 3+ 29 ). Accordingly, a variety of emission colors, such as green from Tb 3+ , 22 red from Eu 3+ , 23 blue from Tm 3+ , 27 and reddish/red from Sm 3+ 24 or Bi 3+ , 28 along with tunable colors from a combination of these dopants' emissions, 22,27,30 can be achieved using the ScVO 4 and ScPO 4 hosts. The crystal structures of ScBO 4 (B = V, P), as shown in Fig. 1a, are relatively simple, containing only one type of Sc and B site coordinated respectively by eight and four oxygen atoms, but the Sc-O bonds have two different lengths. 26,[31][32][33] Since ScVO 4 and ScPO 4 are all crystallized in a tetragonal space group of I4 1 /amd and the [ScO 8 ] and [BO 4 ] polyhedral structures share the same oxygen atoms and/or oxygen edges, the formation of the Sc(P x ,V 1Àx )O 4 (0 r x r 1) solid solutions becomes, therefore, possible. According to the ICSD files no. 78074 (i.e., for ScVO 4 ) and no. 74483 (i.e., for ScPO 4 ), we show the average VÁ Á ÁOÁ Á ÁScÁ Á ÁOÁ Á ÁV and PÁ Á ÁOÁ Á ÁScÁ Á ÁOÁ Á ÁP bond lengths, from which we can understand well the difference between the two isostructural crystals. Besides, in principle, since the number of Sc ions in the asexpected solid solutions is fixed, a regular change of the crystal lattice cell with the larger V 5+ ions (i.e., 0.495 Å at four oxygen coordination numbers (CN)) being gradually replaced by the smaller P 5+ (i.e., 0.31 Å at CN = 4) 34-36 can therefore be expected further. In this case, when taken into account the emission positions of bulk ScVO 4 (i.e., 465 nm, 25,33 20.7 Â 10 3 cm À3 (blue-green) 32 ) and ScPO 4 (i.e., 430 nm (blue) under a moist atmosphere, 32 or B210 nm (UV, 5.6-5.9 eV) under dry conditions 37,38 ), varying the V/P ratios in the as-expected Sc(P x ,V 1Àx )O 4 solid solutions may lead to spectral tuning. Meanwhile, due to the lattice microenvironment modification that results from the radii matching between the V 5+ and P 5+ ions, there may exist some unexpected PL properties. Furthermore, several experimental and theoretical studies [39][40][41][42][43][44][45] have confirmed existence of the oxygen vacancies/defects in the ScVO 4 crystals by means of e.g., reduction of a ScVO 4 precursor under an H 2 /N 2 flow, 39 three-step liquid phase co-precipitation reaction, 40 and two-, 41 three-42 and four-step 43 solid state reactions in air, and the vacancies/defects play a significant role in adjusting the mechanical, optical, and magnetic properties of the ScVO 4 crystals. 40,42 In this case, the substitution of V 5+ ions with P 5+ ions would affect the presence of the ScVO 4 -related vacancies/defects, typically in V-rich intermediate solid solution compounds. In this perspective, we believe that the vacancies/defects are also important in discussing the PL observations that we cannot predict but deserve to be taken into account seriously. Unfortunately, all of the aspects we observed above are still not discussed or noticed in the previous studies, which are also the major motivation of why we carry out this work.
In this work, we have prepared and characterized structurally and spectroscopically external dopant-free scandium-phosphorus vanadate solid solutions, i.e., Sc(P x ,V 1Àx )O 4 (0.0 r x r 1.0). We show that the PL properties of ScP x V 1Àx O 4 (0.0 r x r 0.9) are very peculiar since: (1) emission is observed at room temperature, which is not expected in zircon orthovanadates, and (2) the emission position is found to first switch to longer wavelengths as x increases up to 0.3 and then tune back to shorter wavelengths as x increases further. This unusual behavior is investigated in detail based on Rietveld XRD refinement, bandgap energies and environmental factor calculations based respectively upon density functional theory (DFT) and dielectric theory of electronegativity, as well as UV-vis diffuse reflectance and temperature-dependent PL spectral measurements. The origin of this anomaly can allow gaining better insight into the key parameters governing the spectral tunability of virgin (or doped) zircon crystal systems.

Experimental details
Based upon the nominal chemical composition of Sc(P x ,V 1Àx )O 4 (0.0 r x r 1.0, with an interval x value of 0.1), we ground the raw reagents of Sc 2 O 3 (99.9%), NH 4 H 2 PO 4 (99.99%), and NH 4 VO 3 (99.95%) in an agate mortar and then fired the mixtures at 1100 1C for 2 h in an alumina tube furnace under air atmosphere. After cooling down to room temperature, the products were ground again in the same agate mortar for the belowmentioned characterization.
The powder X-ray diffraction (XRD) patterns were collected from a Rigaku D/max-IIIA X-ray diffractometer using Cu Ka radiation (l = 1.5405 Å, 1.21 min À1 ) and a scanning rate of 1.21 min À1 in the 2y range of 10-901. To understand better the dependence relationship between the lattice structural variation and the P/V substitution ratio, we analyzed further the raw XRD data by using a FullProf Suite program. The excitation and emission spectra at room and high temperature were recorded on a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. During the spectral collection, we kept the emission and excitation slits to 2.0 nm and 2.5 nm, respectively. The internal quantum efficiency (IQE) and external quantum efficiency (EQE) values were measured using a home-This journal is © The Royal Society of Chemistry 2020 Mater. Adv., 2020, 1, 2467--2482 | 2469 built PL quantum yield (QY) collection system which used a 266 nm UV laser (Jewel Laser series) as the excitation source and equipped with an integrating sphere connected to an optical spectrometer. The UV-vis diffuse reflectance spectra of the solid solutions were recorded on a Hitachi U-4100 UV-vis-NIR spectrophotometer by using BaSO 4 as a reference. This journal is © The Royal Society of Chemistry 2020

Theoretical details
Our first-principle calculations were performed within the framework of DFT using the Vienna ab initio simulation package (VASP). 46 A kinetic energy cutoff of 400 eV was used for the plane wave basis set expansion and 2p Â 0.1 Å À1 k-spacing had been chosen to sample the Brillouin zone. Lattice dynamical properties were calculated with projector-augmented plane-wave (PAW) potentials 47 with the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) parametrization. 48 The total energy and forces were converged to smaller than 10 À7 eV and 10 À2 eV Å À1 during the structural relaxation. To obtain an accurate bandgap, we also used the Heyd-Scuseria-Ernzerhof (HSE) 49 screened hybrid functional. We adopted virtual crystal approximation (VCA) to describe the V-P alloying.
A convenient manner to obtain information on the degree of covalency of a chemical bond in a crystal lattice is through calculating the environmental factor he(X) experienced by a cation site X of interest. The methodology derived from the dielectric theory of electronegativity 50-52 that had been applied successfully to a wide variety of physical problems in the years 60-70 before being extended to phosphors at the end of the 90's [53][54][55][56] and more recently in ref. 57. Following these models, we have the formula: where CN is the coordination number of the cation; X, L represent ligands (i.e., oxygen in our case); fc (X-L) and a (X-L) represent respectively the fractional covalency and the volume polarization of each individual chemical bond separating cation X from its nearby ligands L in binary units of type X m L n to which the initial host lattice must first be decomposed; Q L is the effective charge carried by ligand L in each given X m L n unit. Q L is calculated as (n/m)ÁQ X , where Q X is the effective charge of cation X. This charge was taken as the bond valence sum of atom X in its polyhedron as calculated using the facilities provided in VESTA software. 58 The necessary values of the bond valence parameters were obtained from ref. 59 i.e., 1.849 for Sc 3+ , 1.803 for V 5+ and 1.604 for P 5+ . The procedure for decomposing a crystal system and calculating the sum of individual chemical bonds was described in detail in ref. 53

Analysis of structural phase-purity
The XRD patterns of Sc(P x ,V 1Àx )O 4 (0.0 r x r 1.0) (Fig. 2a) confirm the successful formation of continuous solid solutions that are crystallized in the zircon-type structure with a space group of I4 1 /amd. As expected, the substitution of larger V 5+ ions with smaller P 5+ ions causes the shift of the diffraction position to a higher angle (Fig. 2b), revealing the shrinkage of the crystal lattice cell. The d hkl values corresponding to the plane spacing (020) reflection are compiled in Table 1 and Fig. 2c(i), which are evaluated by the Bragg equation of 2d sin y = l (where l is the X-ray wavelength, and y is the diffraction angle). 25,26 All the raw XRD data are refined further using the Rietveld refining method, and four typical refined XRD patterns are shown in Fig. 3, i.e., the Sc(P x ,V 1Àx )O 4 (x = 0.2 (Fig. 3a), 0.4 (Fig. 3b), 0.7 (Fig. 3c), and 0.9 (Fig. 3d)). The refined lattice parameters a(b)c and cell volumes (V) are compiled in Table 1, which, together with the refined finial reliability factors, reveal that our refined results are desirable. The P 5+ content dependent lattice parameters a(b)c and V values can be linearly fitted ( Fig. 2c(ii-iv)), revealing the regular variation of the crystal structure as the V 5+ ions are replaced by the P 5+ ions. Based on our refined parameters, we have re-drawn the crystal structure (Fig. 1b). Obviously, it is the same as that of Fig. 1a. Moreover, with Fig. 1c, we can also know how the VÁ Á ÁOÁ Á ÁScÁ Á ÁOÁ Á ÁV, V/PÁ Á ÁOÁ Á ÁScÁ Á ÁOÁ Á ÁV/P and PÁ Á ÁOÁ Á ÁScÁ Á ÁOÁ Á ÁP bond lengths change in the Sc(P x ,V 1Àx )O 4 (0.0 r x r 1.0) solid solutions with a substitution of larger V 5+ ions with smaller P 5+ ions.  Table 1, the tendency of which is consistent with that of the Rietveld refined parameters. The electronic bandgap (E g ) energies of Sc(P x ,V 1Àx )O 4 (0.0 r x r 1.0) and the corresponding DOS position maxima for Sc(3d), V(3d), P(3p) and O(2p) states are given in eV in Tables 2 and 3, respectively. Our first-principles calculations reveal that a gradual substitution of V 5+ ions with P 5+ ions leads to a gradual increase of E g energies (Table 2). Typically, the tendency of the E g energies experiences a relatively flat increase with a P(x) content of less or equal to 0.2. But after that the E g energy shows a fairly rapid growth as the P(x) content increases further. Although two DFT methods (i.e., PBE, and HSE06) have been adopted to calculate the E g energies and the E g values derived from the HSE06 functional (Fig. 4b, curve 2) are larger than those from the PBE functional (Fig. 4b, curve 1), the variation tendency of E g is on the whole the same. Besides, the VCA simulation matches well the tendency reported in our previous work 26 that used supercells to simulate the V-P alloying. To understand better this variation relationship and the differences of E g values     achieved by different DFT calculations, we show in Fig. 4b all the E g energies. In addition to the above DFT calculations, we also recorded the UV-vis diffuse reflectance spectra of Sc(P x ,V 1Àx )O 4 (0.0 r x r 1.0) at room temperature, and treated them using the Kubelka-Munk method through which the absorption coefficient a is estimated from the reflectance coefficient R using a = (1 À R) 2 /2R, 7,60,61 as shown in Fig. 4c. Obviously, the samples with 0 r x r 0.5 present the strongest absorption in the UV-NUV region, and some of them also exhibit a certain strong absorption in the visible region (e.g., violet-blue), which contributes to color the powders' body in yellow. In comparison, the samples with x 4 0.5 only show reduced absorption in the visible region and thus appear as white body powders.

Density functional theory (DFT) calculations and reflectivity spectra
The reflectivity spectra corresponding to samples with 0 r x r 0.5 can be decomposed into a sum of 4 Gaussian curves, as representatively illustrated in the inset of Fig. 4c for Sc(P 0.2 ,V 0.8 )O 4 . The energy positions of the first three absorption bands (i.e., Abs0, Abs1, and Abs2 in order of increasing energy) are listed in Table 2. The fourth component was found in average at 5.22 AE 0.15 eV. In the zircon-type vanadate system, the lower absorbing band Abs0 reveals the presence of defects associated with oxygen vacancies that form in concomitance with vanadium atoms in valence state +4 or +3 located in the V 5+ or Y 3+ crystal sites. [62][63][64] Following the methodology introduced in ref. 65, we can qualitatively access the defect amounts by calculating the intensity ratio R = Abs0/(Abs0 + Abs1 + Abs2). This was made possible here by integrating the area of the corresponding Gaussian curves obtained after spectral decomposition. The values listed in Table 2 demonstrate the presence of defects in all phosphovanadates, with the amount being smaller in the end member ScVO 4 of the solid solution.
Finally, indicative E g values were determined by plotting the relationship of a(hn) 2 = A(hn À E g ) where a, A and hv represent the Kubelka-Munk absorption, the absorption constant and the photon energy, respectively. The experimental E g data ( Table 2, and the curve 3 of Fig. 4b) follow the trends obtained  The emission occurs from the close-lying 3 T 1,2 triplets and has a charge transfer (CT) character. 66,69 It occurs at 483 nm (2.57 eV) in ScVO 4 at 77 K. 68 Here we observe that the Sc(P x ,V 1Àx )O 4 (x o 1) solid solutions glow in the green-blue spectral region when excited by UV light at room temperature. Since the ScPO 4 does not glow and following earlier reports dedicated to YVO 4 , 70-72 we ascribe this unusual room temperature emission to the tetrahedral tetroxo (VO 4 ) 3À complex groups perturbed by the previously evidenced lattice defects. The excitation spectra collected in correspondence with this emission are shown in Fig. 4d. A band with increasing contribution as x is lowered appears in the near-UV region. We have successfully achieved the spectral decomposition of the low energy part of the excitation spectra shown in Fig. 4d (expressing in energy) into a sum of 3 Gaussian-shaped bands. The third, higher-lying, excitation band is positioned at 5.09 AE 0.04 eV (243 AE 2 nm) and is about 4 times broader (E1.06 eV) compared  Interesting to note is that Exc1 is not the lower-lying absorption in the Sc(P x ,V 1Àx )O 4 system. Despite the monotonic variation of the crystal structure and electronic properties of Sc(P x ,V 1Àx )O 4 as x increases from 0 to 1, we observe an irregular behavior of the (VO 4 ) 3À emission excited at 260 nm (i.e., corresponding to Abs2 or Exc2) that consists of a red shift of the position from 495 nm to 524 nm as x increases to 0.2 followed by a blue-shift back to 457 nm for 0.2 o x r 0.9 (Fig. 5a-c). Concomitantly, we observe a continuous increase of the emission intensity with a maximum in the Sc(P 0.3 ,V 0.7 )O 4 sample and then a rapid decrease of the emitted intensity as x is increased further (Fig. 5d). Moreover, it is shown in Table 2 and Fig. 5b (curve 2) that the variation tendency of EQE values under 266 nm laser excitation basically matches that of the relative emission intensity (Fig. 5d), and it is obvious that the Sc(P 0.3 ,V 0.7 )O 4 sample has a maximum EQE of 42.3%. This PL behavior can be ascribed to the consequence of a complex compromise between the absorption rate at 260 nm (see Fig. 4c) and radiationless losses due to reabsorption by lattice defects contributing to band Abs0 and concentration quenching. In zircon vanadates, the concentration quenching involves the migration of the excitation energy among the isolated vanadate groups followed by radiationless losses of this excitation energy by energy transfer to sinks. As demonstrated by G. Blasse in the solid solution Y(P,V)O 4 , 73 the energy migration and the related emission quenching by the concentration effect start operating for (VO 4 ) 3À doping rates of E25%, which can be expected to explain the PL intensity variation of the Sc(P,V)O 4 in the same manner.
In addition to the room temperature spectra, we also have investigated the temperature-dependent emission spectra of Sc(P x ,V 1Àx )O 4 solid solution up to 300 1C, with the aim to collect information on the thermal quenching process and connect these data to the room temperature behaviors. We show the temperature-dependent emission spectra in Fig. 6a for the ScP 0.4 V 0.6 O 4 sample, which is taken as a representative of the thermal evolution of emission spectra and intensity for all other samples. We also show in Table 2 the temperature T 50% at which the emission intensity has lost 50% of its initial room temperature emission intensity in all compounds. The T 50% values, as exemplarily depicted by the fitted T 50% process of the ScP 0.4 V 0.6 O 4 sample (Fig. 6b), were obtained by using the integrated emission intensity through the Boltzmann sigmoidal equation: 74,75 IðTÞ ¼ Table 2 Electronic band gap energies (E g ), absorption (i.e., Abs0, Abs1, and Abs2) and excitation (i.e., Exc1 and Exc2) values, emission peak positions (l em ), relative emission intensity at room temperature (I emr ), CIE chromaticity coordinates, IQE and EQE values, as well as the thermal quenching temperatures (T 50% ), and activation energy (DE a ) of Sc(P x ,V 1Àx )O 4 (0.0 r x r 1.0) solid solutions. All energies are in eV

Room temperature
High temperature Previous Here Previous Here Abs0 Abs1 Abs2 R/% Exc1 Exc2 l em I emr /% CIE  where I(T) is the emission intensity value at a given temperature; A 1 and A 2 are the initial value (left horizontal asymptote) and final value (right horizontal asymptote), respectively; T 0 and dT denote the center of the sigmoid and the change in T associated with the most significant change in I(t) values, respectively; and T is the Kelvin temperature. The temperature-dependent intensity variation of the ScP 0.4 V 0.6 O 4 sample was reproduced with a reliability above 99% using a single energy barrier model (Fig. 6c) where I 0 and I T are the integrated emission intensity at room and higher temperatures, respectively; k is the Boltzmann constant; and DE a is the energy separating the emitting and quenching states (i.e., also referenced to as the activation energy). All DE a values are also given in Table 2, and they follow the variation tendency analogous to the tendency of T 50% values (Fig. 6d), which jointly reveal that an appropriate amount of P content has improved the thermally induced PL quenching of the Sc(P x ,V 1Àx )O 4 solid solutions.

Discussion
As known from the archival literature, 77 Fig. 7a, where the decrease of D(V) values with the increase of P 5+ (x) content reveals a closing structural rigidity.
On the above grounds, we further discuss the evolution of the optical behavior of the (VO 4 ) 3À groups going from isolated to concentrated systems. As specified earlier and represented in To conveniently and qualitatively understand these evolutions, we plot a single coordinate configurational diagram based on the potential energy of the (VO 4 ) 3À luminescent unit (Fig. 7c(i)). In general, the average distance between the central metal ion and its surrounding anions is taken for this purpose. Within this model, the emission occurs from the thermalized close-lying 3 T 1,2 excited states to the 1 A 1 ground state after that internal radiationless relaxation from the absorbing 1 T 1 state has occurred. Thermal quenching of the emission occurs by cross-over of 3 T 1,2 to the 1 A 1 ground state, and it depends critically on the equilibrium distance Dr of the ground and excited states. Thereby, we can understand that the decrease of the lattice stiffness as the vanadate amount is increased releases the constraints experienced by the V-O bond upon optical excitation, increases the Dr, and results in smaller values of DE a and of the emission energy ( Fig. 7c(i), from a black curve to a red 3 T 1,2 curve). These effects are reinforced by the downward shift of the excited 1 T 1 and 3 T 1,2 states and by the lowering of the bonding force between the V and its nearby O atoms that results in a smaller force constant of the 1 A 1 ground state potential curve (Fig. 7c( Fig. 7c(ii) (from solid to dotted 3 T 1,2 curve). We infer that this modification from the V-O bond is related to a smaller amount of oxygen vacancies in ScVO 4 as compared to ScP 0.9 V 0.1 O 4 (see Fig. 4c(i) and Table 2). In this regard, we show in Table 4  This work is not presented here but under progress and will be the subject of a forthcoming paper soon.

Conclusion and perspectives
In summary, we used a conventional high temperature solidstate reaction method to synthesize a type of external dopantfree Sc(P x ,V 1Àx )O 4 (0.0 r x r 1) solid solution and reveal that they are all crystallized in the zircon-type structure with a space group of I4 1 /amd as confirmed by the powder XRD patterns and corresponding Rietveld refinement results. In addition, as expected, the XRD positions with the substitution of larger V 5+ by smaller P 5+ ions follow the Vegard's law nicely, where the shrinkage of the crystal lattice parameters a(b)c, cell volumes (V) and average Sc-O lengths is observed. Our PL findings show that beyond ScPO 4 , other compounds, i.e., Sc(P x ,V 1Àx )O 4 (0.0 r x r 0.9), exhibit that the emission position initially shifts from 495 nm to 524 nm followed by a subsequent blueshift back to 457 nm as the x is gradually increased from 0.0 to 0.9. Meanwhile, an enhancement of B40% of room temperature emission intensity, together with an optimal improved thermally induced PL quenching stability, referenced by the emission intensity of bulk ScVO 4 , is observed in the intermediate ScP 0.3 V 0.7 O 4 and ScP 0.4 V 0.6 O 4 samples, respectively. The DFT calculations reveal the P 5+ (x) content dependent electronic band-gap (E g ) energies in the Sc(P x ,V 1Àx )O 4 (0.0 r x r 1.0) solid solutions, and the variation tendency of the E g values matches with those experimentally derived from the diffuse reflectance spectra. Through a combination of the DFT and dielectric electronegativity calculations and corresponding discussions, the underlying reason for the observed color tuning and the improved PL properties is mostly due to the interplay of the electronic bandgap energy adjustment, the bond covalency regulation, the gradual closing structural rigidity caused by the lattice microenvironment modification, and the contribution of lattice defects like oxygen vacancies. Based on the potential energy of the luminescent centre (i.e., (VO 4 ) 3À unit) and relevant analysis and discussion, a convenient yet qualitative singlecoordinate configurational diagram is constructed further to discuss the observed PL evolutions. Retrospecting this work, it is a little pity that the external dopant-free Sc(P x ,V 1Àx )O 4 (0.0 r x r 1) emission-tunable solid solutions do not belong to the class of phosphor materials particularly desirable for phosphor-converted white LEDs because of the following reasons: on one hand, the weakness of their excitation ranges less than the mainstream commercial UV LED chips with emission ranges of 375-410 nm and, on the other hand, their relatively low EQE (l ex = 266 nm laser, 5.6-42.3%) values compared to those of RE and/or non-RE doped phosphor materials such as Sr x Ba 2Àx SiO 4 :Eu 2+ (l ex = 405 nm laser, x = 43-73%, quantum yield (QY) = 88-90%), 2 (Y,Lu)VO 4 :0.02Bi 3+ (l ex = 330 nm xenon lamp, EQE = 75% for YVO 4 :Bi 3+ , and 68% for LuVO 4 :Bi 3+ ), 25 ScVO 4 :0.01Bi 3+ (l ex = 330 nm, and 380 nm xenon lamp, EQE = 56%, and 57%, respectively), 28 Table 4 Average lengths of Sc-O and V/P-O bonds, bond valence sum (BVS) of Sc and V/P atoms, and environmental factor he(X) experienced by the Sc and V/P sites in the ScVO 4 and ScPO 4 crystals, where these data are extracted from and calculated basing on the archival literature (i.e., ScVO 4Àx ), 43 a series of standard ICSD cards, and the end members of the as-obtained solid solutions (i.e., ScVO 4