COMPETITION OF COVALENCY BETWEEN CRIII-O AND TAV-O BONDS IN THE PEROVSKITES CA2CRTAO6 AND SR2CRTAO6

According to the Rietveld refinement, the perovskites Ca2CrTaO6 and Sr2CrTaO6 are found to have space groups P2(1)/n (monoclinic, a = 5.4199(1) angstrom, b = 5.4936(1) angstrom, c = 7.7108(2) angstrom, and beta = 90.02(1)degrees) and Fm3mBAR (cubic, a = 7.8837(1) angstrom), respectively, with partial disordering of Cr and Ta ions. Structural analysis indicated that the Cr-O bond distance is shorter than the Ta-O distance in both compounds. XANES for Ta L(III) edge and EPR spectroscopic analyses and magnetic susceptibility measurements were performed to obtain information on the covalency between B cations (B = Cr, Ta) and oxygen ions. The variation of crystal structure and magnetic properties in the two compounds could be explained by the size and the covalent bonding nature between metal and oxygen ions. (C) 1994 Academic Press, Inc.