Abstract Single phase Nd 1− x Ca x CrO 4 ( x=0–0.20) oxides were synthesized by the pyrolysis of precursors prepared from Nd III–Ca II–Cr VI mixed solutions. Nd 1− x Ca x CrO 4 having x≥0.25 was not obtained as a single phase. All Nd 1− x Ca x CrO 4 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 Nd 1− x Ca x CrO 4 ( x=0–0.20) were in good agreement with the ones measured by the picnometry. XPS and Raman spectra indicated that Nd 1− x Ca x CrO 4 ( x=0.02–0.20) are mixed valence oxides containing two types of tetrahedra, Cr VO 3− 4 and Cr VIO 2− 4, 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 NdCrO 4 and CaCrO 4 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 CrO 4 tetrahedra in Nd 1− x Ca x CrO 4 ( x=0.02–0.20) are more elongated than in NdCrO 4 and CaCrO 4. It was deduced that the limit of x (about 0.25) may be determined by the difference in geometry between Cr IVO 2− 4 and Cr VO 3− 4 tetrahedra.