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 LaV1−xNbxO4 (x = 0-0.2) samples at room temperature. 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. The substitution of Nb at the V site show 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 (P21/n) phase, whereas for the x > 0, both monoclinic and scheelite-tetragonal (I41/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. 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 VO4 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.