Space Group and Structure for the Perovskite Ca0.5Sr0.5TiO3

Powder diffraction patterns from Ca1−xSrxTiO3, at x=0.5, show superlattice peaks indicative of both R-(q=12 [111]*p) and M-(q=12 [110]*p) point octahedral tilting, though the metric is pseudo-tetragonal and indeed very nearly cubic. In a previous study [C. J. Ball, B. D. Begg, D. J. Cookson, G. J. Thorogood, and E. R. Vance, J. Solid State Chem. 139, 238–247 (1998)] this pseudo-tetragonal structure was assigned to space group Cmcm (orthorhombic) rather than to space group Pnma, the accepted space group for the structure of CaTiO3. These two space groups are, however, very difficult to distinguish by powder diffraction techniques. Electron diffraction has been used to obtain diffraction patterns from single domain regions and by such means it has been established unequivocally that the space group at room temperature is not Cmcm but Pnma.