  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 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 (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. 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.
