CORRIGENDUM: Crystal structure of double oxides of the perovskite type

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 SrTiO3, SrSnO3, SrZrO3, BaSnO3, BaZrO3, BaThO3; also BaTiO3 above 120?C. Tetragonal. This includes the usual form of BaTiO3 at room temperature, PbTiO3, and PbZrO3. 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 CaTiO3 (the mineral perovskite), CaSnO3, CaZrO3, and CdTiO3 (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. BaTiO3 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.