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:

1. Cubic (ideal perovskite type). This includes SrTiO₃, SrSnO₃, SrZrO₃, BaSnO₃, BaZrO₃, BaThO₃; also BaTiO₃ above 120°C.

2. 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.

3. 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.

4. 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.