Unusual magnetic property of Ba₂CrTaO₆ and phase transformation under high pressure

Under atmospheric pressure conditions, Ba₂CrTaO₆ crystallizes in the eight-layered hexagonal structure (Ba-H) with a = 5.7383(1) Å and c = 18.7832(2) Å. Magnetic susceptibility calculations indicate that the (Cr-Cr) metal bonding exists via σ(a_1g-a_1g) bonding through the shared-face of the octahedra in the hexagonal structure (Ba-H), in that the temperature dependencies of the observed magnetic susceptibilities could be explained successfully in the whole temperature domain from 10 to 350 K by a model for a localized d²-like electronic configuration rather than a d³-one in octahedral symmetry. E.P.R. measurements also showed the presence of strong (Cr-Cr) covalent bonding in the hexagonal phase. Such a magnetic interpretation is new for the system with face-shared octahedra. At a high pressure (80 kbar) and high temperature (900°C), Ba-H undergoes a structural phase transformation to the cubic perovskite structure with a = 8.0518(1) Å. The magnetic susceptibilities for the high pressure phase (Ba-C) follow the Curie-Weiss law very well above 10 K, which is consistent with the magnetic behavior of isolated Cr^III(d³) ions.