Competition of covalency between Cr^III-O and Ta^v-O bonds in the perovskites Ca₂CrTaO₆ and Sr₂CrTaO₆

According to the Rietveld refinement, the perovskites Ca₂CrTaO₆ and Sr₂CrTaO₆ are found to have space groups P2₁/n (monoclinic, a = 5.4199(1) Å, b = 5.4936(1) Å, c = 7.7108(2) Å, and β = 90.02(1)°) and Fm3̄m (cubic, a = 7.8837(1) Å), 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 anti 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.