Crystal structure and microtwinning of natural orthorhombic perovskite CaTiO3

The crystal structure of the orthorhombic modification of natural perovskite, CaTiO3 has been studied by single crystal X-ray technique. The unit cell parameters are a=5.378(1), b=5.444(1), and c=7.637(3) A; sp. gr. Pbnm; Z=4. The structure is refined in the anisotropic approximation using the experimental set of the data obtained on a four-component twinned single crystal with the {010} and {112} twinning planes up to R=0.0435 (1736 reflections, MoKα-radiation). The symmetry lower in comparison with that of the cubic modification (sp. gr. Pm3m, a=3.87 A) is explained by the reduction of the Ca-O distances from 2.73 to <2.51> A with a decrease in the coordination number of Ca atoms from 12 to 8 due to a lower temperature of crystal formation. Possible ways of twinning of the centrosymmetric orthorhombic modification are considered