Crystal structure and the role of covalency in eight-layered hexagonal baacrtaoe

According to the Rietveld refinement, the perovskite-related Ba2CrTa06 was found to have a P63/mmc space group with lattice constants of a = 5.7383(l) and c =18.7832(2) Å. The Cr and Ta ions are completely ordered; the face-shared octahedra are occupied by Cr ions and the corner-shared octahedra by Ta ions. The increased covalent bonding nature of (Cr-O-Cr) and/or (Cr-Cr) bonds via π(eg7r—pπ—egπ) and cr(a1g—a1g) would be the main driving force to stabilize the 8-layer hexagonal structure. The existence of such covalent bondings was confirmed by the ESR spectroscopic analysis and magnetic susceptibility measurement. The XANES spectra for Ta Lm-edge indicated that such a covalency induces a relatively weak covalency of competing (Ta-O) bond.