Crystal structure of double oxides of the perovskite type

Abstract

The cell dimensions of a number of double oxides belonging to the perovskite type have been accurately determined from examination of high-angle lines on x-ray powder photographs. The structures found fall into groups, as follows: Cubic (ideal perovskite type) . This includes SrTiO 3 , SrSnO 3 , SrZrO 3 , BaSnO 3 , BaZrO 3 , BaThO 3 ; also BaTiO 3 above 120°C. Tetragonal . This includes the usual form of BaTiO 3 at room temperature, PbTiO 3 , and PbZrO 3 . The unit cell is derived from the cubic by simple extension or compression along one tetrad axis; and, like the cubic, it contains the formula number of atoms. Orthorhombic . This includes CaTiO 3 (the mineral perovskite), CaSnO 3 , CaZrO 3 , and CdTiO 3 (fired above 1100° C.). The structure is derived from the cubic by a shear in the (010) plane and a slight extension or compression along the b -axis, giving a monoclinic pseudo-cell with a and c edges equal. The lattice is thus actually orthorhombic, and should be referred to new a and c axes lying at approximately 45° to the old in the same plane. There is an observed doubling of the cell edges, attributable to changes in some of the atomic parameters. Rhombohedral . BaTiO 3 can be prepared in this form, though the conditions are not yet fully established. The pseudo-cell is obtained by a very slight compression of the cubic cell along the cube diagonal; the true cell is a multiple of this Steric considerations, based on Goldschmidt's ionic radii, are used to account for the occurrence of the different structure modifications.