Characterization of LaCrO4 and NdCrO4 by XRD, Raman spectroscopy, and ab initio molecular orbital calculations

Single phase LnCrO(4) (Ln = La-Nd) compounds were attempted to synthesize by the pyrolysis of precursors prepared from Ln(III)-Cr-VI mixed solutions. Only LaCrO4 and NdCrO4 were obtained as a single phase. LaCrO4 was monazite type (monoclinic, P2(1)/n), as reported, and the cell parameters were refined to be a = 0.70369, b = 0.72348, c = 0.66918 nm and beta = 104.950 degrees by an X-ray Rietveld method. NdCrO4 was zircon type (tetragonal, I4(1)/amd) and the eel parameters were a = 0.73107 and c = 0.66999 nm. It was found that CrO43- tetrahedra in LaCrO4 have C-1 symmetry and the four Cr-O bond lengths are different, whereas those in NdCrO4 have D-2d 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 CrO(4)(3-)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.