Vibrational spectra and optical properties of Fe1-xCrxVO4 solid solutions: With a group theory analysis.
The present manuscript reports vibrational spectra and optical studies of polycrystalline Fe1-xCrxVO4 solid solutions through FT-IR spectroscopy augmented with a group theory (G.T.) analysis and UV-Visible DRS spectroscopy. Full set of IR and Raman modes are determined by G. T. for various crystal symmetries in FeVO4-CrVO4 solid solutions where Triclinic, Monoclinic and Orthorhombic structures evolve with increasing Cr concentration. Experimentally obtained vibrational modes support the structural phase transitions and confirm formation of continuous solid solutions in Fe1-xCrxVO4. The Diffuse Reflectance Spectra (DRS) of Fe1-xCrxVO4 depicts the electronic structure and different optical transitions due to absorption of photon energy. The d-d transitions are manifested for all compounds in terms of crystal field stabilization energy (CFSE) caused by distorted lattice sites. The band gap energy of Fe1-xCrxVO4 is calculated using Tauc formula. It shows a red shift initially within triclinic structure then blue shift with the increase of Cr concentration. Urbach energy (Eu) tails in the spectra show the electronic structural disorder in Fe1-xCrxVO4 due to impurity energy levels of Cr ions within band gap region. It is observed that Eu decreases with the doping concentration due to the increase in crystal symmetry corresponding to the structural phase transitions in Fe1-xCrxVO4.