Synthesis and Structure of Novel CrV–CrVI Mixed Valence Compounds, Nd1−xCaxCrO4 (x=0.02–0.20)

Abstract

Single phase Nd1−xCaxCrO4 (x=0–0.20) oxides were synthesized by the pyrolysis of precursors prepared from NdIII–CaII–CrVI mixed solutions. Nd1−xCaxCrO4 having x≥0.25 was not obtained as a single phase. All Nd1−xCaxCrO4 were zircon type (tetragonal, I41/amd), and the composition was almost stoichiometric without any essential defects, which was determined by chemical analyses. The lattice constants and atomic positions was refined by the X-ray Rietveld method. The calculated densities of Nd1−xCaxCrO4 (x=0–0.20) were in good agreement with the ones measured by the picnometry. XPS and Raman spectra indicated that Nd1−xCaxCrO4 (x=0.02–0.20) are mixed valence oxides containing two types of tetrahedra, CrVO3−4 and CrVIO2−4, having D2d symmetry in the structure, and this compensates the decrease of positive charges introduced by CaII ions. Though two types of tetrahedra were not distinguishable by XRD, lattice constants a and c decreased almost linearly with x. The values for x=0.02–0.20, however, were not on the line expected by Vegard's law between NdCrO4 and CaCrO4 but larger. The calculated O–Cr–O bond angles, however, did not change monotonously as lattice constants and other crystallographic parameters such as Cr–O bond length did, indicating that CrO4 tetrahedra in Nd1−xCaxCrO4 (x=0.02–0.20) are more elongated than in NdCrO4 and CaCrO4. It was deduced that the limit of x (about 0.25) may be determined by the difference in geometry between CrIVO2−4 and CrVO3−4 tetrahedra.