  We investigate the structural, vibrational, morphological, and electronic
properties of Nb substituted orthovanadate LaV$_{1-x}$Nb$_x$O$_4$ samples
prepared by the solid-state reaction method. The x-ray diffraction (XRD)
analysis reveals the presence of three crystal structures [monoclinic monazite
($m-m$) type for the $x=$ 0, two-phase equilibrium of monoclinic monazite
($m-m$) and tetragonal scheelite ($t-s$) type for the 0.2$\leq$$x$$\leq$0.8,
and monoclinic fergusonite ($m-f$) type for the $x=$ 1 samples] with an
increase in Nb$^{5+}$ concentration. The Raman spectroscopy and x-ray
photoelectron spectroscopy (XPS) were employed to study the vibrational and
electronic properties of all the samples, respectively. In order to choose an
excitation wavelength that does not cause undesirable fluorescence and has
observable intensities of all the vibrational modes, the Raman spectra are
collected using 532 nm, 633 nm, and 785 nm laser lines. With increasing the
Nb$^{5+}$ concentration, new Raman modes associated with Nb-bonds are clearly
visible and the intensity of V-bonds assigned modes is decreasing. The XPS
analysis shows the unchanged 3+ oxidation state of La ion where the intensity
of the V 2$p$ core-level decreases while the Nb 3$d$ core-level increases with
$x$. The equal spin-orbit energy splitting of the states is confirmed by the
average energy difference (across La core-level spectra for all the samples)
for state I as well as bonding and anti-bonding of state II. Interesting, the
relative intensity of La 3$d$ state I and state II show systematic change with
Nb doping altering the metal ligand overlap. We discuss and provide insight
into the evolution of the structural, morphological, and chemical features with
Nb substitution in LaV$_{1-x}$Nb$_x$O$_4$ samples.
