(Ca_0.37Sr_0.63)TiO₃ perovskite - An example of an unusual class of tilted perovskites

The structure of Ca_0.37Sr_0.63TiO₃ perovskite has been carefully investigated using electron and neutron diffraction. Electron diffraction supports the supercell and Pbcm space group previously proposed, while high resolution neutron powder diffraction data provide the basis for a new structure refinement. The distortions of the resultant structure relative to the ideal parent perovskite structure type have been decomposed into normal modes. It is found that the primary modes of distortion are octahedral tilting modes associated with the R point (k = [1/2, 1/2, 1/2]^*) and the ξ = 1/4 point on the T line of symmetry (k = [1/2, 1/2, ξ]^*). There is a smaller secondary, probably induced, mode of distortion involving antiferroelectric displacements of the Ti cations associated with the ξ = 1/4 point on the Δ line of symmetry (k = [0,0,ξ]^*). This compound provides one of the very few currently known examples of a tilted perovskite structure that shows tilting modes associated with the ubiquitous modulation wavevectors for a value of ξ different from 0 and 1/2.