Strain analysis of phase transitions in (Ca,Sr)TiO3 perovskites

A single Landau free energy expansion is used to describe phase transitions in perovskites, from a cubic parent structure to tetragonal and orthorhomb ic structures with space groups related to the M-3 and R-25- points of the Pm (3) over barm reciprocal lattice. This expansion permits relationships b etween symmetry-adapted forms of the spontaneous strain and individual orde r parameter components to be predicted. Data from the literature for (Ca,Sr )TiO3 perovskites are analyzed in the light of these predictions. Shear str ains for I4/mcm, Pnma, and Cmcm structures tend to conform to the predicted pattern. The Pm (3) over barm <-> I4/mcm transition has nearly tricritical character as a function of temperature in CaTiO3 and more nearly second-or der character as a function of composition at the Sr-rich end of the solid solution. Coupling with the volume strain appears to be both temperature an d composition dependent, which may be a general feature of phase transition s in perovskites. Renormalization of fourth-order terms by changing the vol ume coupling coefficients could be responsible for the unusual order parame ter evolution shown by CaTiO3 and for changes in thermodynamic character of the phase transitions as a function of composition. The pattern of strain variations also correlates closely with patterns of variations in heal capa city from the literature, suggesting revisions to the subsolidus phase diag ram for the (Ca,Sr)TiO3 solid solution above room temperature.