Structural, vibrational and electronic properties of Nb substituted orthovanadates LaV1-xNbxO4

We investigate the structural, vibrational, morphological, and electronic properties of Nb substituted orthova-nadate LaV1-xNbxO4 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 = x = 0.8, and monoclinic fergusonite (m -f) type for the x = 1 samples] with an increase in Nb5+ 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 Nb5+ 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 2p core-level decreases while the Nb 3d 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 3d 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 LaV1-xNbxO4 samples.

Automated summary

We investigated the structural, vibrational, morphological, and electronic properties of Nb substituted orthova-nadate LaV1-xNbxO4 samples prepared by the solid-state reaction method. The x-ray diffraction (XRD) analysis revealed the presence of three crystal structures with an increase in Nb5+ concentration. Raman spectroscopy and x-ray photoelectron spectroscopy (XPS) were employed to study the vibrational and electronic properties of all the samples, respectively. Our study provided insights into the evolution of the structural, morphological, and chemical features with Nb substitution in LaV1-xNbxO4 samples.