Structural phase transition and its consequences on optical behavior of LaV_{1-x}Nb_xO_4
SStructural phase transition and its consequences on optical behavior of LaV − x Nb x O Hemanshu Dua, ∗ Rishabh Shukla, ∗ and R. S. Dhaka † Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India (Dated: February 5, 2021)We present the structural, electronic, vibrational, and photoluminescence properties of polycrys-talline LaV − x Nb x O ( x = 0–0.2) samples at room temperature. The x-ray absorption measure-ments reveal that the La ions exist in a trivalent oxidation state, while V and Nb cations possess 5+oxidation state in tetrahedral coordination. The substitution of Nb at the V site show fascinatingstructural and optical behavior due to their isoelectronic character and larger ionic radii of Nb as compared to the V . The Rietveld refinement of x-ray diffraction patterns demonstrate thatthe x = 0 sample exist in a monoclinic (P2 /n) phase, whereas for the x >
0, both monoclinicand scheelite-tetragonal (I4 /a) phases co-exist in a certain proportion. Interestingly, a monotonousenhancement in the Raman spectral intensity with Nb substitution is correlated with the substi-tution induced increase in the scheelite-tetragonal phase. Moreover, the Fourier-transform infrared(FTIR) spectra indicate that the Nb substitution give origin to some additional IR modes owing tothe deformation of the VO − tetrahedra and mixing of monoclinic and tetragonal phases. The pho-toluminescence measurements on these samples exhibit broadband spectra and their deconvolutiondesignate the availability of more than one electron-hole pairs recombination center. INTRODUCTION
Orthovanadates are an eminent class of compoundswith technological and fundamental significance, whichhave been extensively utilized as luminescent materi-als, polarizers, catalysts, biological sensors, battery elec-trodes, alternatives in green technologies, etc. [1–5].These compounds exhibit phenomenon of temperatureand pressure-dependent structural phase transformation[6, 7], which was debated for a long time among the re-searchers for its order type [8]. In recent years, rare earthorthovanadates having a general formula of RVO (R =rare-earth ion) have gained tremendous popularity fortheir useful applications in the area of solar cells [9, 10],thin film phosphors [11], photocatalysis [12, 13], etc. Forenhancement in the optical performance of the ortho-vanadate compounds, researchers predominantly followthree paths, which can be defined as the substitutionof metal cations [14], coupling with the other metal ox-ides [15], and synthesis of these compounds with variousnovel routes [16–18]. Primarily, the RVO materials areknown to crystallize into two polymorphs, namely mon-oclinic (m-) monazite-type (space group: P2 /n, Z = 4)and tetragonal (t-) zircon-type (space group: I4 /amd, Z = 4). The space group P2 /n is defined as a non-conventional setting of the standard P2 /c space group( /c [19]. In a monazitestructure, R cations forms an edge-sharing nonahedra(RO ) with a distorted VO tetrahedra along the c-axishaving four dissimilar V–O bonds [19], while a tetragonalstructure possesses an edge-sharing dodecahedra (RO )and undistorted VO tetrahedral chains parallel to thec-axis with four identical V–O bonds [20]. It has beenobserved that the larger size of lanthanide ions (Ln )favor monoclinic structure over tetragonal one due to a higher oxygen coordination number [13, 20]. Hence, theLa cations having the largest ionic radii in the Ln series generally crystallizes into the thermodynamicallystable monoclinic structure and a metastable tetragonalstructure [17, 21], while other lanthanide orthovanadatespredominantly crystallize into the zircon-type tetragonalstructure [22].Interestingly, the LaVO can also be crystallized intoa stable tetragonal structure in the form of nanocrys-tals/nanowires via solution method but stabilizing thetetragonal structure is always been a challenging task[16, 22, 23]. A combination of the experimentslike temperature-dependent photoconductivity, emissionmeasurements, and different empirical models were em-ployed to explain the complete energy level diagramof lanthanide-doped LaVO compounds [24], which canbe utilized to explain their luminescence properties andwould help in the preparation of new luminescent mate-rials. This is well known that the luminescence proper-ties of lanthanide (Ln) doped rare-earth orthovanadatesdepend on the location of the 4 f energy levels of theLn dopants with respect to the valence and conductionband of the parent compound [25]. The t-LaVO exhibitsprominent photoluminescence properties as compared toits m-counterpart because the t-LaVO has four sigmabonds of angle 153 ◦ , which leads to the efficient trans-fer of energy whereas, in m-LaVO the bond angles aresmaller than t-LaVO , which results in the less effectivetransfer of energy [26]. The first-principles calculationsfor LaVO polymorph performed using plane-wave pseu-dopotential method suggest that the m-LaVO has anindirect bandgap of 3.5 eV, while the t-LaVO has a di-rect bandgap of 3 eV [27]. Thus, the phase transforma-tion from m- to t-structure leads to a significant increasein the photoluminescent properties of LaVO , which areattributed to the structural difference of m- and t-phases.In recent years, intensive research is being carried out a r X i v : . [ c ond - m a t . m t r l - s c i ] F e b in order to ameliorate the optical properties of LaVO via substituting M + (Li), M (Mg, Sr, etc.), M (Eu,Sm, etc.), and M (Sn, Zr, etc.) ions at La site [11, 28–34], however, enhancement in the performance with thesubstitution at V site still has room to explore for theresearchers, where only a report of Mn cationic sub-stitution induced catalytic properties are present in theliterature [35]. Therefore, in order to enhance the opticalperformance of the LaVO compound with B-site sub-stitution, we have chosen the isoelectronic Nb cationshaving a larger ionic radius of 0.48 ˚A as compared toV (0.355 ˚A) ions [21]. Moreover, the end memberLaNbO is well known for its multifunctional applicationsin phosphors and scintillators [36, 37], great tunability[38], an efficient blue luminescence material when excitedwith UV/X-ray source [39]. Also, the LaNbO manifestsa second-order temperature-dependent structural phasetransformation in a temperature range of 500 ± o C froma fergusonite-monoclinic (f-m) phase (space group: I2/a, /a, compound consist of the NbO tetrahedraand LaO dodecahedra, which reflect the twelve trian-gular lattices [41]. The s-t structure have all 4 × Nb-Oequal bond-lengths including two sets of 4 × La-O bonds,whereas in the f-m phase two sets of Nb-O bonds appearwith four pairs of La-O bond-lengths [41]. This scheelitetetragonal (s-t) phase is known as a close-packed (denser)polymorph with a volume reduction of nearly 10% withrespect to the standard zircon-type tetragonal phase [6].Moreover, a fergusonite-scheelite transformation is ob-tained from the cyclic rotation of axes with a transfor-mation matrix and the long-axis (unique b-axis) in thefergusonite phase become the c-axis of the scheelite phasewith a gradual change of β from 94 o to 90 o [41, 42]. Thesetetragonal phases in the orthovanadates were known topossess the remarkable optical properties and hence be-ing explored for their practical applications [43]. To thebest of our knowledge, the B-site cationic substitution in-duced enhancement in the luminescence properties of themonoclinic LaVO were not reported in the literature.Therefore, we investigate the structural, elec-tronic, vibrational, and photoluminescence properties ofLaV − x Nb x O ( x = 0–0.2). The Rietveld refinement ofx-ray diffraction patterns demonstrates the monoclinic(P2 /n) phase ( x = 0), and appearance of scheelite-tetragonal (I4 /a) phase for the x > ascompared to V results in the structural phase transfor-mation from a monoclinic to scheelite-tetragonal. More-over, an enhancement in the Raman spectral intensity isrelated to the increase in the tetragonal-phase with theNb substitution at V site. Furthermore, additional in- frared modes in the Fourier-transform infrared (FTIR)spectra were observed due to the Nb induced structuraltransformation. In the room temperature PL measure-ments, we found that the strongly overlapped spectracan be deconvoluted into six peaks, where each emissionprocess involves the separate energy levels. EXPERIMENTAL
The LaV − x Nb x O ( x = 0–0.2) samples were preparedby solid-state reaction method by mixing V O (99.6%,Sigma), Nb O (99.99%, Sigma) and La O (99.99%,Sigma) in the stoichiometric amount as initials, whereLa O was pre-dried at 900 ◦ C for 6 hrs to remove mois-ture. The mixture was homogeneously ground for 8 hrs,followed by calcination at 1000 o C for 17 hrs, then theobtained mixture was reground and sintered at 1200 o Cfor 13 hrs. To improve the crystallinity of prepared sam-ples, final sintering was done at 1250 o C for 13 hrs. Wehave also sintered the Nb substituted samples ( x = 0.1and 0.2) at 1450 o C for 13 hrs to explore the sinteringtemperature-induced phase transformation. The powderx-ray diffraction (XRD) data of prepared polycrystallineLaNb x V − x O ( x = 0–0.2) samples were recorded us-ing CuK α radiation ( λ = 1.5406 ˚A) from Rigaku Ul-tima IV, Ri x-ray diffractometer. The Rietveld refine-ment of recorded XRD patterns were accomplished usingFullProf software with background fitted using linear in-terpolation between the data points. The x-ray absorp-tion measurements were recorded at room temperaturein the transmission mode at La-L, V-K, and Nb-K edgesusing the scanning EXAFS beamline (BL-09), Indus-2,RRCAT, Indore, India. The energy-dispersive x-ray anal-ysis (EDX) was performed with the Zeiss EVO 18 at anoperating voltage of 20 keV inside a vacuum chamber.The Raman spectra were recorded with the RenishawinVia confocal Raman microscope using a 532 nm laserwith grating 2400 lines/mm, 20X objective, and a powerof 0.05 mW. The Fourier-transform infrared (FTIR) spec-troscopy of the samples was performed in the range 400-4000 cm − using the Thermo Electron Scie spectropho-tometer. The photoluminescent properties were investi-gated using the Horiba scientific spectrophotometer atan excitation wavelength of 325 nm. Finally, the diffusedreflectance spectroscopic measurements were conductedusing a Shimadzu UV-2450 spectrophotometer at a slitwidth of 5 mm. RESULTS AND DISCUSSION
Fig. 1 shows the Rietveld refined room temperaturepowder x-ray diffraction (XRD) patterns of polycrys-talline LaV − x Nb x O ( x = 0–0.2) samples. We ob-serve that the measured XRD patterns are well fitted FIG. 1. The Rietveld refined x-ray diffraction patterns of LaV − x Nb x O (a) x = 0, (b) x = 0.05, (c) x = 0.1, (d) x =0.15 and (e) x = 0.2 samples. Open red circles, black solid line, and blue solid line exhibit the experimental, simulated, anddifference between experimental and simulated spectra, respectively; green and magenta vertical markers present the Braggpositions corresponding to the P2 /n and I4 /a space groups, respectively. (f, g) Schematic unit cell diagrams for the monoclinicmonazite (P2 /n) and scheelite tetragonal (I4 /a) phases of LaVO compound prepared using vesta software [44], respectively,these phases are utilized to perform the Rietveld refinement of recorded x-ray diffraction patterns. using (i) a monoclinic space group (P2 /n) for the x =0 sample, and (ii) a combination of monoclinic (P2 /n)and scheelite-tetragonal space group (I4 /a, x = 0.05–0.2 samples. We found that the Nb substi-tution induced structural transformation prevail in thesamples for x ≥ (0.48 ˚A) ions as compared to V ions (0.355 ˚A) in a tetrahedral coordination [21]. Notethat the phenomenon of structural phase transformationfrom monoclinic (P2 /n) to tetragonal (I4 /amd) phaseis well investigated in LaVO with the cationic substi-tution of other lanthanides (Ln ) via reducing the av-erage ionic radii of the atoms present at the La site,as the structural transformation into tetragonal phaselead to the remarkable enhancement in the photolumi-nescence properties [26, 29]. Interestingly, in our case the substitution of Nb cations at V site lead to the struc-tural phase transformation from monoclinic (P2 /n) toa scheelite-tetragonal (I4 /a) phase. Since the end mem-bers, LaVO and LaNbO , exist in the monoclinic mon-azite (P2 /n) and fergusonite monoclinic (I2/a) phasein the ambient conditions, respectively, it was expectedthat the cationic substitution of Nb in LaVO will inducea structural transformation from P2 /n to I2/a spacegroup [19, 41, 45]. However, there are several reports onthe orthovanadate compounds, which explain a pressure-induced phase transformation between the various crystalsymmetries, e.g., zircon-tetragonal, scheelite-tetragonal,fergusonite-monoclinic, and BaWO -II, etc [6, 7, 46–49].This structural transformation from one phase to anotheris correlated to the occupation of f − electrons of the lan-thanide cations and to their ionic radii [7]. Interestingly, TABLE I. The Rietveld refined lattice parameters of polycrystalline LaV − x Nb x O ( x = 0–0.2) samples and with temperature-induced metastable scheelite-tetragonal phase for the x = 0.1 and 0.2 samples. x χ Space Group a (˚A) b (˚A) c (˚A) β ( ◦ ) Volume (˚A ) sinteringtemperature0 1.09 P2 /n 7.0413(6) 7.2759(6) 6.7172(6) 104.850(6) 332.64(5) 1250 o C0.05 1.05 P2 /n - 94% 7.0416(3) 7.2777(4) 6.7235(4) 104.911(5) 332.96(3) 1250 o CI4 /a - 6% 5.3312(12) 5.3312(12) 11.712(5) 90 332.88(18)0.1 1.03 P2 /n - 88% 7.0409(3) 7.2769(4) 6.7230(4) 104.896(5) 332.88(3) 1250 o CI4 /a - 12% 5.3323(5) 5.3323(5) 11.7115(16) 90 333.00(6)0.15 1.1 P2 /n - 84% 7.0431(4) 7.2781(4) 6.7261(5) 104.932(6) 333.14(3) 1250 o CI4 /a - 16% 5.3313(4) 5.3313(4) 11.7165(14) 90 333.01(5)0.2 1.14 P2 /n - 72% 7.0395(4) 7.2760(5) 6.7262(5) 104.921(6) 332.89(4) 1250 o CI4 /a - 28% 5.3312(3) 5.3312(3) 11.7128(11) 90 332.89(4)0.1 1.44 I4 /a 5.3248(2) 5.3248(2) 11.7311(6) 90 332.61(2) 1450 o C0.2 1.47 I4 /a 5.3289(2) 5.3289(2) 11.7478(7) 90 333.60(3) 1450 o C a competition between the scheelite to fergusonite andscheelite to P2 /n phase transition exists in the largerR cation compounds [50]. Therefore, it is very chal-lenging to synthesize the pure scheelite phase of RVO compounds since the transition pressure is too high for acomplete structural transformation [51]. Here, we founda phase transformation from the fergusonite to scheelitephase during the sample synthesis and at room tempera-ture we obtained a signature of the scheelite phase in therecorded x-ray diffraction patterns of the final product[see Fig. 1(b–e)]. We observe that the peak at 2 θ -valueof 17.83 o (110) diminish in intensity, whereas the peakat 27.77 o (120) splits into two with the Nb substitution.Moreover, we observed that a few peaks emerge at 2 θ -values of 28.08 o , 33.63 o , 46.08 o , 56.81 o , and 58.25 o withthe Nb substitution, which corresponds to the scheelite-tetragonal phase with the (112), (004), (024), (312), and(224) planes, respectively [52]. Note that, a fergusonite toscheelite transformation is a second-order and obtainedfrom the cyclic rotation of axes using a transformationmatrix and the long-axis (unique b-axis) in fergusonitephase becomes the c-axis of the scheelite phase. Thisphase transformation of the unit cell is well accompaniedby the movement of the atoms (change in the Wyckoffpositions), i.e., the La and V(Nb) cations move with thesame magnitude, while the O atoms move in such a waythat reduces the bond-length distortion index [48, 53].In short, the fergusonite phase is a compressed and lessdistorted version of the scheelite phase, with the β valueof 94 o as compared to 90 o for scheelite phase. There isa very effective way to visualize these structural phasetransformations using group-subgroup relations by mak-ing a B¨aernighausen tree and applying the translation-gleiche and klassengleiche transformations [53]. Since itis well known that the t-LaVO possess the superior opti-cal properties as compared to the phases with monoclinic symmetry [43], hence Nb substitution induced emergenceof scheelite-tetragonal phase lead to an enhancement ofluminescence properties in the LaV − x Nb x O samples.The Rietveld refined unit cell parameters are presentedin table-I, where the lattice parameter values for the x =0 sample are in good agreement with the previous reports[19, 45]. Moreover, the lattice parameter values for the x = 0.05 sample increases with the Nb substitution anda scheelite phase also emerges. For the samples x > et al. havesynthesized the x = 0.1 sample to study its microwavedielectric properties and obtained a composite phase ofscheelite and monoclinic; however, they did not extractthe relative phase fraction and lattice parameters [45].It has been reported that the metastable structure oftetragonal LaVO is commonly stabilized through a ’softchemical’ process, e.g., the hydrothermal method wasused to stabilize the tetragonal phase of LaVO [17, 34].Moreover, there are few studies related to the hydrostaticpressure-dependent structural phase transformation fromm- to t-LaVO phase [48, 49]. Interestingly, in order toobtain a stable tetragonal phase, which shows superiorluminescent properties, we use high-temperature sinter-ing of LaV − x Nb x O ( x = 0.1, 0.2) samples at 1450 o Cfor 13 hrs and found a reversible transformation into ametastable scheelite-tetragonal phase [see Figs. 2(a, b)].However, this metastable phase is again transformed tothe initial stable phase (a mixture of monoclinic andscheelite-tetragonal phases). The presence of a scheelite-tetragonal phase in the x = 0.1, 0.2 samples at roomtemperature in a certain fraction act as a nucleation cen-ter to transform this into a complete scheelite tetrago-nal phase. The Rietveld refinement of this metastablephase in LaV − x Nb x O ( x = 0.1, 0.2) samples is per- FIG. 2. The Rietveld refined x-ray diffraction patterns ofmetastable LaV − x Nb x O (a) x = 0.1, and (b) x = 0.2samples sintered at 1450 o C using a scheelite-tetragonal phase(space group, I4 /a, /a space group, inset in(a) shows the schematic unit cell diagram for the scheelite-tetragonal phase utilized to fit the recorded x-ray patterns. formed using the space group I4 /a ( x = 0.1, 0.2 sam-ples manifests that a scheelite-tetragonal phase can bestabilized at higher temperatures. The energy dispersivex-ray measurements performed at room temperature (notshown here) confirm the stoichiometric compositional ra-tio of constituent elements and their homogeneity in allthe prepared LaV − x Nb x O ( x = 0–0.2) samples.In Fig. 3(a) we present the room temperature Ramanspectra of LaV − x Nb x O ( x = 0–0.2) samples, which areshifted vertically for a clear presentation, and the left andright insets in this graph highlights the enhancement inthe Raman spectral intensity with the Nb substitutionand marked with the red arrows. The recorded spectrawere deconvoluted and fitted using the Lorentzian peakshape function in Fig. 3(b-f), with twenty individual Ra-man modes marked with S0–S19. It was found that 14 Raman peaks are between 100–600 cm − and 6 peaks arein the range of 750–900 cm − , which have been listed inTable II. In the LaVO monazite structure, all the atomspossess 4 e Wyckoff position and the symmetry decom-position of the zone center phonons are obtained usingthe point group symmetry of 2/m [48]. According to thegroup theory calculations, m-LaVO consists of 72 vi-brational modes (18B u + 18A u + 18A g + 18B g ), whichincludes 3 acoustic modes (A u + 2B u ), 33 infrared-activemodes (16B u + 17A u ) and 36 Raman active modes (18A g + 18B g ) [54, 55]. Here, we are using the Mulliken sym-bols, where notations ‘A’ and ‘B’ represent that the vi-brations are symmetric and antisymmetric with respectto the principal axis of symmetry, respectively. And, thesubscripts ‘g’ and ‘u’ represent that the vibrations aresymmetric and antisymmetric with respect to a center ofsymmetry, respectively. One can differentiate A g and B g modes experimentally using the polarized Raman mea-surements. For the m-LaVO compound, there have beenfew reports on the Raman spectra [27, 48, 49, 56], andwe found a good agreement between our observed dataand those in the references [48] and [56]. In RVO (R islanthanide ion), the Raman modes can be classified as in-ternal and external modes. The RVO can be consideredas composed of two sublattices, R and VO units wherethe internal modes are due to vibration of VO unit only,while external modes are due to the vibrations of bothR and VO units. In an ideal situation the modes dueto the strong V-O bond in VO − tetrahedron were mea-sured in an aqueous solution of Na VO , which can becharacterized as ν (A ) = 870 cm − , ν (E) = 345 cm − , ν (F ) = 825 cm − , and ν (F ) = 480 cm − and denotesthe symmetric stretching, symmetric bending, antisym-metric stretching, and antisymmetric bending of the VO tetrahedron, respectively [48]. The vibrational spectraof LaVO can be divided into three groups, (i) the low-frequency ( <
240 cm − ) region, (ii) the middle-frequency(270–450 cm − ) region, and (iii) the high-frequency re-gion (850–970 cm − ), which are predominantly due tothe translation of La atoms, the bending vibration of O–V–O bonds, and stretching vibration of O–V–O bonds,respectively [27]. The higher frequency stretching andbending modes are due to the shortest V–O bonds. Inthe m-LaVO , the Raman spectra are complex due tothe presence of distorted VO tetrahedron having fourdifferent V–O bond lengths. However, some modes arerelated to the tetrahedral symmetry like most intense A g ( ν ) breathing mode at 859 cm − , A g ( ν ) bending modeat 374 cm − , and the antisymmetric B g ( ν ) mode at439 cm − [48]. Also, Errandonea et al. found that nomode possesses pure ν characteristics, there exists al-ways a mixture of ν and ν modes. We have comparedthe peak positions of individual Raman modes from thereported experimental [48, 49, 56] and theoretical [27, 48]values in the table-II with our experimental data.Further, we have analyzed the variation in the inte- ! " $ " % & ’ ( ) * +& ,! " $ - &&&&’.-/&0&1+12 :(;(!&<= >"&’.; ?9 - &&&&’ & <1 9 8 7 6 2 B 5 4 3<91 9998 97 96 92 9B 95 <94<93 & ! " ! " ’(- :(;(!&<= >"&’.; ?9 - 311421411521 &&&&’>-/&0&1+81 ( ) & ’ ( ) * +& ,! " $ - &’C*- 914B681 D E F G & ’ . ; ? - <94 92+196+296+197+297+1 D E F G & ’ . ; ? - &’C*-B+4B+6B+12+B2+8 ( ) & ’ ( ) * +& ,! " $ - <911+811+941+9B1+96 ( ) & < H < &’C*- 8+19+B9+8 ( ) & < H < ( ) & < H < &’C*-1+9B1+981+14 ( ) & < H < AI $ " I! & ’ . ; ? - &’C*- <91 FIG. 3. (a) The room temperature Raman spectra of LaV − x Nb x O ( x = 0–0.2) samples, left and right inset present a closerview of the as-recorded Raman modes in the lower (250-470 cm − ) and higher (760-910 cm − ) regions, respectively. (b-f) Solidlines present the fitted individual modes of the recorded Raman spectra for LaV − x Nb x O ( x = 0–0.2) samples, respectively,using the Lorentzian peak function and solid thick black line presents the total fit of the measured spectra. Insets in (b)presents the variation of the relative area of S8/S10 and S9/S10 Raman modes with x on the left and right-axis, respectively,(c) shows the dependence of relative area of S8/S9 and S16/S18 Raman modes with x on the left and right-axis, respectively,(d-e) display the variation of the integrated area and FWHM values for the S11 (372 cm − ) and S18 (855 cm − ) individualRaman modes with x plotted on the left and right-axis, respectively, and (f) presents the variation of the peak position of theS11 (372 cm − ) individual Raman mode with x . grated area of the selected Raman modes and their full-width-at-half-maximum (FWHM) with the Nb concen-tration ( x = 0–0.2). We observe that the S0 (A g ) in-dividual mode (126 cm − ) is only present for the x =0 sample and completely disappears for the Nb substi-tuted samples ( x > − tetrahedra and also can be seen in the FTIR spec-tra of the Nb substituted samples. To see the variationin the intensity of S8 and S9 Raman modes with x , weplot their relative integrated area ratios as compared tothe intense S10 mode on the left and right axis of the in-set in Fig. 3(b). We can see that overall the relative area TABLE II. The experimentally observed frequencies ( ω obs )of the individual Raman modes in polycrystalline monaziteLaVO sample at room temperature. The peak positions ofthese modes are compared with the reported theoretical ( ω th )values in the refs. [27] & [48] and previously obtained experi-mental values ( ω exp ) in the refs. [48, 49, 56].Peak ω obs ω th [27] ω th [48] ω exp [48] ω exp [56] ω exp [49]cm − cm − cm − cm − cm − cm − S0 126 A g (126) B g (127) 127 127 124.2S1 143 B g (143) A g (143) 146 147 143.8S2 160 B g (170) B g (158) 160 158 156.6S3 189 – A g (188) 189 189 187.3S4 208 A g (203) B g (204) 209 208 204.7S5 237 A g (232) A g (230) 235 238 242.1S6 250 – A g (252) 252 251 260.4S7 307 B g (315) B g (316) 309 309 306.2S8 328 A g (334) A g (336) 326 329 326.3S9 347 – A g (355) 349 349 345.8S10 373 B g (378) A g (380) 373 374 370.7S11 397 B g (394) B g (389) 397 398 394.8S12 423 – A g (423) 426 – 420.7S13 439 – B g (427) 439 440 436.4S14 768 – A g (784) 768 770 766.5S15 791 – B g (799) 790 794 792.1S16 819 – A g (806) 819 819 817.5S17 841 – A g (836) 843 – 840.9S18 858 A g (865) B g (861) 855 859 856.5S19 881 A g (883) B g (892) 882 – – ratio of S8/S10 decreases, while S9/S10 increases withincreasing the x , see the inset in Fig. 3(b). Moreover,we show the relative integrated area ratio of the Ramanmodes S8 and S9 on the left axis of the inset in Fig. 3(b),which is found to decrease with x . It can be seen thatthe intensity of the S8 mode, which was lower than theS9 mode for x = 0 sample, become higher for the x = 0.2sample, see Fig. 3(b–f). Similarly, the variation of therelative area ratio of S16 Raman mode with respect tothe most-intense S18 mode is shown with x on the rightaxis of the inset in Fig. 3(b), which exhibits an increas-ing trend. These results exhibit that the Nb substitutionenhances the intensity of the selected Raman modes sig-nificantly with x . Further, we analyze the behavior ofthe most intense Raman modes in the lower and higherwavenumber region, i.e., S10 and S18. A variation of theintegrated area and FWHM values of S10 (A g ) mode with x is shown on the left and right-axis, respectively [see in-set of Fig. 3(d)]. We observe that the integrated area andFWHM values of A g bending mode increase monotoni-cally with the x . Also, we present the behavior of thepeak position of S10 mode [see inset of Fig. 3(f)] with x and found a monotonous decrease in the peak position,which indicates that the A g bending mode frequency de-creases with deformation of VO tetrahedra owing to theNb substitution. Interestingly, for the S18 Raman mode(B g ) we found that the area and FWHM values exhibit a jump in the increment from the x = 0 to 0.05 sample,and then a monotonous increase in the values with theNb concentration. However, we found that the positionof the S18 mode does not change despite the Nb substitu-tion in the system (not shown), i.e., breathing vibrationsassociated to this mode are unaffected due to tetrahedraldistortions. In addition to that, we have also investigatedthe peak position of the other fitted components with theNb substitution, and found that there is not much shiftin the peak position of the S7, S8, S11, S12, S13, S15,and S20 Raman modes, whereas S9, S16, and S17 (allA g ) Raman modes exhibit a monotonous change with x (not shown here). The most intense S10 (A g ) and S18(B g ) Raman modes manifest a significant change in theintegrate area/FWHM with the Nb substitution in theVO tetrahedra. These results further motivate us to in-vestigate the local structure of these samples using x-rayabsorption spectroscopy (XAS). ! " $ %& ’ () * + , - * .///010/01// 23!4! *( ("56*+(7- +$- :()5( *8$ ; < *8$7; = ! " $ %& ’ () * + , - * >1.//>1=//>19//>1/// 23!4! *( ("56*+(7- ?@*A:()5( +B- *?@ ; *8$7 /CD ?@ /C9 ; = ) E ) , * ) E ) , * >1/0/>1///>D10/ ! " $ %& ’ () * + , - * *7 ; *8$7; =* +@- < *G*7*A:()5( ) E ) , * >C0>C//C0/C/ HI * J K + K - J D.=9/ "$)&$%*)&L4$ B(*+M- *(N24C*O&4*"(L&)P$% +)- /CD ?@ /C9 ; = FIG. 4. The room temperature x-ray absorprtion spectrarecorded in the transmission mode for, (a) LaVO and ref-erence La O samples at La L -edge, inset shows a compari-son of the peak position in the first-order derivatives, (b) thecombined La L and V K-edges of LaVO and V O sam-ples where left inset highlights the pre-edge peak and rightinset exhibit the comparison of maximum obtained in thefirst-order derivative, (c) the Nb K-edge of LaV . Nb . O and Nb O samples where the inset shows a closer view ofthe maxima in the first-order derivative, (d) the measuredextended x-ray absorption feature spectra with curve fittinganalysis for LaV . Nb . O sample at Nb K-edge. Therefore, the XAS measurements were performed forthe La L-, Nb K-, and V K-edges at room tempera-ture for the x = 0 and 0.2 samples. We have calibratedeach spectrum with the reference metal foils utilizing themaxima in the first-order derivative about the inflectionnear the edge-jump. The normalized x-ray absorptionspectra for LaVO and reference La O samples mea-sured at the La L -edge are presented in Fig. 4(a) wherethe inset shows the derivative about the first inflectionnear the edge jump and a solid line along the maxima(5907.9 eV) affirm that La cations exist in the trivalentoxidation state analogous to the La O sample. As thephoton energies of the V K-edge (5465 eV) and La L -edge (5483 eV) are close to each other, an absorptionmeasurement at one of them will eventually result in thestrongly overlapped absorption spectra corresponding toboth the edges of the LaVO sample. In Fig. 4(b) wepresent the normalized absorption spectra at these (VK- and La L ) edges for the LaVO sample as well asV K-edge of V O as reference. A pre-edge peak (near5469.8 eV) in Fig. 4(b) appears due to the tetrahedralcoordination of V ions in the monoclinic structure andconfirms the d configuration consistent with the earlierreports, as the intensity of the pre-edge feature is high-est for the d compounds and monotonously decreases tozero for d configuration [57, 58]. This pre-edge peak inthe V K-edge emerges due to the electric dipole transi-tion to the p component in the p − d hybridized orbitalsand some smaller contributions arise from the quadrupo-lar transitions within the same orbitals [58]. Note thatthis pre-edge feature is solely emerging due to the V K-edge and unaffected by the contributions from the LaL -edge. A comparison of the measured spectra with ref-erence V O sample at the pre-edge peak (near 5469 eV)in the left inset and the first-order derivative in the rightinset of Fig. 4(b) portray a 5+ oxidation state of V ionssimilar to the reference V O sample. Further, we haverecorded the absorption spectrum of the Nb K-edge forthe LaV . Nb . O sample having the highest Nb con-centration and compared the maxima in the first-orderderivative (around 18991 eV) with the reference Nb O sample in Fig. 4(c), which reveals that the Nb ions inour sample exist in the 5+ oxidation state. Moreover, wehave performed the curve fitting of the Nb K-edge ex-tended x-ray absorption fine structure (EXAFS) for the x = 0.2 sample using the Artemis program [59] up to theradial distance of 4.1 ˚A, as shown in Fig. 4(d). Note that,the fitted spectrum is not corrected with back-scatteredand central phase shifts. In the curve-fitting procedurethe atomic scattering paths were generated with the helpof results obtained from the Rietveld refinement of XRDpattern for the x = 0.2 sample, which shows both mono-clinic (72%) and scheelite-tetragonal (28%) phases. Ouranalysis manifest that in the monoclinic phase thereare four unequal V/Nb–O bond lengths available in agroup of two sets, i.e., 3 × × × × ! " $ ! " % & ’ ( ) * +& ,! " $ - .///0///1///2/// 3(4 - & ! " ! " FIG. 5. The Fourier transform infrared (FTIR) spectra ofLaV − x Nb x O ( x = 0–0.2) samples measured at room tem-perature, inset presents a closer view of the recorded spectrain the range of 400-930 cm − and peaks are marked with theblack vertical arrows; peaks marked with the red arrows inthe inset are emerged due to the Nb substitution. panel are vertically shifted for clear presentation, whilea closer view of the as-recorded spectra in the range of350-930 cm − is presented in the inset of Fig. 5. All thewell-resolved individual peaks are marked with the blackvertical arrows, whereas the additional peaks emergingwith Nb substitution are marked with the red arrows.The obtained spectra are similar to the other orthovana-date compounds, i.e., RVO (R = Y, Ce-Yb) [32] andalso with the compounds which possess the isolated tetra-hedral VO − groups, e.g., multi-metal orthovanadates[60, 61]. In the monoclinic structure, the VO − groupsare available at the point symmetry positions. Note thatthe stretching ( ν & ν ) and bending ( ν & ν ) vibra-tions in the VO − tetrahedra are associated with theO-V-O bonds, which exhibit the characteristic IR peaksfor the different orthovanadate compounds in the rangeof 700-1100 cm − and 400-700 cm − , respectively. In therecorded spectra of the x = 0 sample, we observe the well-resolved strong and sharp bands between 400–700 cm − at 429, 478 cm − , and strong wide bands between 700-1100 cm − , where a peak at 799 cm − is associated withthe other overlapped peaks near 815, 831, 847 cm − .A few additional peaks near 982, 1021, 1052 cm − arealso observed in the spectra, which form a wide bandin the IR spectra [see Fig. 5]. These peaks are simi-lar as compared to earlier reports for the monoclinic-LaVO sample [62]. The small feature near 1021 cm − in these samples correspond to the impurity feature fromthe V O [60]. The modes in the range of 700–900 cm − are on account of VO − vibrations. For example, thepeak at 799 cm − quantifies the anti-symmetric stretch-ing vibrations of VO − . Also, a strong and broadbandat 813 cm − with shoulders at 942, 880, and 668 cm − can be observed for the LaVO sample [62]. Moreover,the peaks at wavenumbers 1400, 1434, 1550, 1574, 1622,2654, and 3621 cm − are observed in Fig. 5 where thesmall peaks in the range of 1600–2500 cm − are due tomoisture, and a strong band at 1622 cm − correspondsto the bending vibrations of adsorbed H O molecules[63]. The modes near 2654 cm − are related to the CH stretching vibration and the peak near 3621 cm − is at-tributed to the O–H stretching arising from the water ab-sorption [63]. The position of these peaks do not changedespite Nb substitution, which validates their origin insupport with the literature [62, 63]. We can see thatthese two bands persist in all the samples, since our sam-ples are sintered at such a high temperature, i.e., 1250 o C,therefore these modes are expected to be associated withthe surface absorbed impurities of carbon and water. Itis little difficult to assign each spectral component to aspecific type of vibrational mode and especially in thestretching range of vibrations. We observe the widebandspectra owing to the strong overlapping of the IR lines,which appears with the symmetry of monoclinic LaVO and consistent with the theoretical predictions of theirorigin. The addition of Nb cations will distort the VO tetrahedra and that results in the observed changes in theIR spectra of substituted samples. In the present case,all the substituted samples are in a mixed phase of mon-oclinic and scheelite-tetragonal in a certain proportion,as summarized in Table I. Here, we found the emergenceof three new peaks for the Nb substituted samples at678, 730, and 758 cm − , which were absent for the x = 0sample and marked by red arrows in the inset of Fig. 5.These additional peaks in the IR spectra appear due tothe additional symmetry modes of the tetragonal phasein the Nb substituted samples and have their contribu-tion from the LaNbO phase [64]. The intensity of thesepeaks enhance monotonically with the Nb substitution.The IR mode at 478 cm − diminish in intensity with theNb substitution. Also, the peaks at 1400, 1434, 1623positions disappear completely when the Nb concentra-tion increased. These results reveal that the IR bandscorresponding to VO − vibrations are influenced by thecrystal structure and morphological evolution.Moreover, the room temperature photoluminescence(PL) spectra of LaV − x Nb x O ( x = 0–0.2) samples wererecorded in a spectral range of 300-750 nm using an exci-tation wavelength of λ ex = 325 nm [see Fig. 6(a)]. We ob-serve a broad asymmetric and strongly overlapped peakcentered near 540 nm with one small peak at a lowerwavelength side near 415 nm [see Fig. 6]. The strongemission in the PL spectra manifests of rapid recombi-nation of electron-hole pairs [12]. The observed spectraare complex in nature having several strongly overlappedcomponents, i.e., emerge due to the transitions from amore complicated energy scheme. The observed spec-trum for the x = 0 sample is analogous to the earlierreports, where several groups have observed the stronglyoverlapped bands centered near 540 nm [62, 65, 66]; how- ! " ! " !" !$ % & ’ (& ) % ’ * ! " + $ , -& % ’ ) !. !/ !( !< FIG. 6. (a) A comparison of the photoluminescence (PL)spectra of LaV − x Nb x O ( x = 0–0.2) samples measured atroom temperature using 325 nm excitation wavelength, (b–f)deconvoluted and fitted PL spectra with six Gaussian peaksfor the x = 0–0.2 samples, respectively. ever, a small kink near 415 nm is not observed in the liter-ature. The broad peak shape is related to the several d - f transitions of the lanthanum orthovanadate compound[68]. We have deconvoluted and fitted the measured PLspectra using six Gaussian peak shapes as shown in theFigs. 6(b–f) for the x = 0–0.2 samples, respectively. Theposition of these four Gaussian peaks (marked by arrows)are taken from the related reference from the literature[25, 65, 66], while two peaks (marked by vertical lines)in the respective ends are inserted to fit the data withthe minimum number of Gaussian peaks. The presenceof six peaks in the deconvoluted PL spectra indicate theavailability of more than one recombination centers forthe electron-hole pairs, as the emission and excitation inthese samples are all governed by the VO − tetrahedronhaving the T d symmetry. Interestingly, the V molecu-lar orbitals consist of a ground state A and four excitedstates named as T , T , T , and T [67]. Among thesestates, the transitions related to the absorption process,i.e, T , T ← A are allowed, whereas the emissionprocess transitions, i.e., T , T → A are forbidden forthe ideal T d symmetry in terms of the spin-selection rule.These transitions are allowed partially by the spin-orbitcoupling, since it depends on the central-field potentialalong with the spin and orbital angular momentum [67].Interestingly, the tetrahedral distortion in VO − causeddue to cationic substitution enhances the spin-orbit in-0 !" ! ’ ( ) * + , - . ! / " )1,2030 )6,2030 )<,2030 &4" )=,2030 )>,2030 >=7<9=? ) ; , )A, FIG. 7. (a-e) The Kubelka-Munk plots for the polycrys-talline LaV − x Nb x O ( x = 0–0.2) samples obtained fromthe diffused reflectance spectroscopy data, and (f) the opticalbandgap as function of x calculated from the x-axis interceptalong the optical absorption edge in the Tauc plot. teraction and hence the intensity of the forbidden transi-tions is observed significantly higher in these compounds.These transitions are related to bands observed near the570 and 635 nm [62]. Moreover, in m-LaVO two bandsat 550–650 nm and 650–700 nm are attributed to the D → F and D → F transitions, respectively [69].The first is broadband which displays transitions of ex-cited 5 d states to the F / ground state, and last is smallband-related to D → F transition [68, 69]. The peaknear 460 nm is related to the electron transitions between T → A states [65]. Furthermore, in the La V . O x compounds, doping of V O decreases the electron-holepair recombination. However, the impurities of the V O were present in the LaVO compound, which can be usedto predict the band-edge positions theoretically and theelectron-hole transfer from one semiconductor to anothercould limit the electron-hole recombination and lead tothe enhancement of the photoluminescence properties inthe system [70]. We observe that for the x = 0.1 sam-ple, the intensity of the emission spectra increases andFWHM decreases, indicating the reduction in the recom-bination centers. However, for the x = 0.2 sample, thePL intensity decreases and FWHM increases, which sug-gests the increase in the recombination centers for theelectron-hole pairs. We found that the peak near 415 nmcorresponds to the 2.98 eV of photon energy is relatedto the electron transition in V O [71]. We observe amonotonous increase in the integrated area and FWHMvalues of this peak [marked by arrows in Figs. 6(b–f)]with an increase in the Nb substitution. Finally, in order to find the bandgap we performthe diffuse reflectance measurements of all the preparedLaV − x Nb x O ( x = 0–0.2) samples sintered at 1250 o C.The recorded diffuse reflectance spectra were utlized toestimate the Kubelka-Munk (K-M) function [F(R ∞ )],which can be defined in terms of diffuse semi-infinite re-flectance, i.e., R ∞ = R sample /R standard as [72], F ( R ∞ ) = (1 − R ∞ ) / (2 R ∞ )Here, we have used TiO as a standard sample in ourmeasurements. The bandgap of samples can be estimatedfrom the absorption threshold energy (given by the x-axisintercept) in the Tauc plot, which can be written as( αhν ) = k ( hν − E g ) η where α , ν , k , E g , and η are the absorption coefficient,frequency, absorption constant, bandgap energy, and ex-ponent of the equation [73]. The value of this exponent η depends upon the type of the transition/bandgap inthe material and can have values of 1/2, 2, 3/2, and 3for the direct allowed, indirect allowed, direct forbidden,and indirect forbidden transitions, respectively [73]. Inthe case of diffuse reflectance spectra the α in Tauc plotcan be replaced by the K-M function to estimate thebandgap of the material [74]. Since, our material hasan indirect bandgap so the value of η is 2 and thereforewe have plotted the [F(R ∞ ) × h ν ] / versus E for eachsample in Figs. 7(a-e). The x-axis intercept of the linearfitting (presented as a solid black line in each graph ofFigs. 7(a-e)] along the optical absorption edge for sam-ples LaV − x Nb x O ( x = 0 – 0.2) gives an estimation ofoptical bandgap. We present the variation of the opticalbandgap of these samples with the Nb concentration ( x )in Fig. 7(f). The bandgap value of the parent m-LaVO ( x = 0) compound is about 3.4 eV, which is consistentwith the reported experimental value of 3.5 ± x = 0.1 sample to 3.22 eV and then slightly increasesfor the samples with x > CONCLUSIONS
In conclusion, we have successfully prepared polycrys-talline LaV − x Nb x O ( x = 0–0.2) samples by solid-statereaction method including the pure scheelite-tetragonalphase. The room temperature x-ray absorption measure-ments reveal that the La cation exists in a trivalent oxi-dation state, while V and Nb cations have their 5+ oxida-tion state in a tetrahedral coordination. The substitutionof Nb ions in place of V ions reveals a structural phasetransformation due to its larger size as compared to the1V . The Rietveld refinement of room temperature x-raydiffraction patterns demonstrate that the x = 0 sampleexist in a monoclinic (m) phase, whereas for the x = 0.05–0.2, both monoclinic and scheelite-tetragonal (s-t) phasesco-exist in a certain fraction. Interestingly, a monotonousenhancement in the Raman spectral intensity with Nbsubstitution is correlated with the substitution inducedincrease in the s-t phase. The room temperature photo-luminescence measurements on these samples exhibit abroad spectra, the deconvolution of these spectra usingsix Gaussian peaks designate the availability of more thanone electron-hole pairs recombination center. Moreover,the FTIR spectra indicate that the Nb substitution givesorigin to some additional infrared active modes owing tothe deformation of the VO − tetrahedra. ACKNOWLEDGMENTS
HD and RS are grateful to MHRD and DST-INSPIRE,respectively for their financial support through fellow-ship. Authors thank FIST (DST, govt. of India) UFOscheme of IIT Delhi for providing Raman facility atphysics department. We also acknowledge the CentralResearch Facility (CRF) for EDX, and Nanoscale Re-search Facility (NRF) for PL, FTIR, and UV-Vis facili-ties. We thank Ravi Kumar and S. N. Jha for help andsupport during XAS measurements at RRCAT, India.RSD acknowledges the financial support from SERB-DST through Early Career Research (ECR) Award(project reference no. ECR/2015/000159). ∗ These authors have contributed equally to this work † [email protected][1] Y. Zhao, M. Shao, S. Liu, Z. Zhang, and H. Lin,Hydrothermal synthesis of lanthanide orthovanadate:EuVO particles and their fluorescence application,Cryst. Eng. Comm. , 8033 (2012).[2] G. Wang, Q. Peng, and Y. Li, Lanthanide-dopednanocrystals: synthesis, optical-magnetic properties, andapplications, Acc. Chem. 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