Polymorphism in NaSbO3: structure and bonding in metal oxides.

A new polymorph of NaSbO(3) has been synthesized at 10.5 GPa and 1150 degrees C in a uniaxial split sphere anvil type press (USSA-2000) and recovered back to ambient conditions. The high-pressure form of NaSbO(3) adopts an orthorhombically distorted perovskite structure, isostructural with CaTiO(3), GdFeO(3), and NaTaO(3). The space group is Pnma, and the unit cell dimensions are a = 5.43835(6) A, b = 7.66195(8) A, c = 5.38201(5) A. It is a white insulator with an optical band gap of 3.4 eV. This compound represents the first ternary perovskite prepared containing Sb(5+) on the octahedral site. The octahedral tilting distortion in this compound is much larger than expected from ionic radii considerations. The distortion is driven by a second-order Jahn-Teller distortion originating on oxygen that can be traced back to strong Sb-O covalent bonding. A conflict arises between the strong covalent bonding interactions at oxygen that favor a large octahedral tilting distortion and the repulsive Na-O interactions that oppose excessive octahedral tilting. This conflict destabilizes the perovskite topology, thereby stabilizing the ilmenite polymorph under ambient conditions. Analysis of ionic and covalent bonding explains why ASbO(3) and ABiO(3) compositions frequently adopt structures that violate Pauling's rules.