  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.
