Structural phase transition and its consequences on optical behavior of LaV${}_{1-x}$Nb${}_x$O${}_4$

Abstract

We present the structural, electronic, vibrational, and photoluminescence properties of polycrystalline LaV${}_{1-x}$Nb${}_x$O${}_4$ ($x=$ 0--0.2) samples at room temperature. The substitution of Nb at the V site shows the fascinating structural and optical behavior due to their isoelectronic character and larger ionic radii of Nb${}^{5+}$ as compared to the V${}^{5+}$. The Rietveld refinement of x-ray diffraction patterns demonstrate that the $x=$ 0 sample exist in a monoclinic (P2${}_1$/n) phase, whereas for the $x>$ 0, both monoclinic and scheelite-tetragonal (I4${}_1$/a) phases co-exist in a certain proportion. Interestingly, a monotonous enhancement in the Raman spectral intensity with Nb substitution is correlated with the substitution induced increase in the scheelite-tetragonal phase. The x-ray absorption measurements reveal that the La ions exist in a trivalent oxidation state, while V and Nb cations possess 5+ oxidation state in tetrahedral coordination. Moreover, the Fourier-transform infrared (FTIR) spectra indicate that the Nb substitution give origin to some additional IR modes owing to the deformation of the VO${}_4$${}^{3-}$ tetrahedra and mixing of monoclinic and tetragonal phases. The photoluminescence measurements on these samples exhibit broadband spectra and their deconvolution designate the availability of more than one electron-hole pairs recombination center.

submitted