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We present the structural, electronic, vibrational, and photoluminescenceproperties of polycrystalline LaV$_{1-x}$Nb$_x$O$_4$ ($x =$ 0--0.2) samples atroom temperature. The substitution of Nb at the V site shows the fascinatingstructural and optical behavior due to their isoelectronic character and largerionic radii of Nb$^{5+}$ as compared to the V$^{5+}$. The Rietveld refinementof x-ray diffraction patterns demonstrate that the $x=$ 0 sample exist in amonoclinic (P2$_1$/n) phase, whereas for the $x >$ 0, both monoclinic andscheelite-tetragonal (I4$_1$/a) phases co-exist in a certain proportion.Interestingly, a monotonous enhancement in the Raman spectral intensity with Nbsubstitution is correlated with the substitution induced increase in thescheelite-tetragonal phase. The x-ray absorption measurements reveal that theLa ions exist in a trivalent oxidation state, while V and Nb cations possess 5+oxidation state in tetrahedral coordination. Moreover, the Fourier-transforminfrared (FTIR) spectra indicate that the Nb substitution give origin to someadditional IR modes owing to the deformation of the VO$_4$$^{3-}$ tetrahedraand mixing of monoclinic and tetragonal phases. The photoluminescencemeasurements on these samples exhibit broadband spectra and their deconvolutiondesignate the availability of more than one electron-hole pairs recombinationcenter.

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