CRYSTAL-STRUCTURE AND THE ROLE OF COVALENCY IN 8-LAYERED HEXAGONAL BA2CRTAO6

According to the Rietveld refinement, the perovskite-related Ba2CrTaO6 was found to have a P6(3)/mmc space group with lattice constants of a = 5.7383(1) and c = 18.7832(2) angstrom. 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 pi(e(gpi)-p(pi)-e(gpi)) and sigma(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 L(III)-edge indicated that such a covalency induces a relatively weak covalency of competing (Ta-O) bond.