Synthesis and structure of novel CrV-CrVI mixed valence compounds, Nd1-xCaxCrO4 (x=0.02-0.20)

Single phase Nd1-xCaxCrO4 (x = 0-0.20) oxides were synthesized by the pyrolysis of precursors prepared from Nd-III-Ca-II-Cr-VI mixed solutions. Nd1-xCaxCrO4 having x greater than or equal to 0.25 was not obtained as a single phase. All Nd1-xCaxCrO4 were zircon type (tetragonal, I4(1)/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, (CrO43-)-O-V and (CrO42-)-O-VI) having D-2d symmetry in the structure, and this compensates the decrease of positive charges introduced by Ca-II 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 (CrO42-)-O-IV and (CrO43-)-O-V tetrahedra, (C) 2001 Academic Press.