Characterization of LaCrO4 and NdCrO4 by XRD, Raman Spectroscopy, and ab Initio Molecular Orbital Calculations

Abstract

Abstract Single phase LnCrO4 (Ln = La-Nd) compounds were attempted to synthesize by the pyrolysis of precursors prepared from LnIII-CrVI mixed solutions. Only LaCrO4 and NdCrO4 were obtained as a single phase. LaCrO4 was monazite type (monoclinic, P21/n), as reported, and the cell parameters were refined to be a = 0.70369, b = 0.72348, c = 0.66918 nm and β = 104.950° by an X-ray Rietveld method. NdCrO4 was zircon type (tetragonal, I41/amd) and the cell parameters were a = 0.73107 and c = 0.66999 nm. It was found that CrO43- tetrahedra in LaCrO4 have C1 symmetry and the four Cr-O bond lengths are different, whereas those in NdCrO4 have D2d symmetry and all Cr-O bond lengths are 0.1702 nm. Detailed Raman spectra of LaCrO4 and NdCrO4 were measured and assigned to the vibrational modes of the CrO43- tetrahedra. Molecular orbital calculations indicated that the charge density on the chromium ions in the tetrahedra is much less than the nominal 5+ due to the migratory electrons from oxygen atoms, and nearly the same irrespective of the symmetry of tetrahedron, suggesting that the Cr-O bonds are strongly covalent in nature.