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3.2 c. Strain analysis of phase transitions in (Ca,Sr)TiO3 perovskites (M.A. Carpenter/ Cambridge, UK, in collaboration with A.I. Becerro and F. Seifert)

Many minerals - both chemically complex or simple (such as quartz) - may undergo rapid, unquenchable (=spontaneous) phase transitions upon changes of temperature or pressure, where the atomic bonds remain essentially intact, but symmetry is changed due to tilting or rotation of building blocks (e.g. SiO4-tetrahedra, TiO6-octahedra). Such displacive phase transitions may have marked influences on the physical and chemical properties of minerals. The energetics of displacive phase transitions can be conveniently described by a Landau free energy expansion. This expansion permits relationships between symmetry-adapted forms of the spontaneous strain and individual order parameter components to be predicted. We have applied this approach to perovskites in the system CaTiO3-SrTiO3, which show - as a function of composition and/or temperature - the transition from a high-temperature cubic structure (Pm3m, with untilted TiO6 octahedra) to a tetragonal (I4/mcm) and eventually two orthorhombic (Cmcm and Pnma) perovskite structures. The lattice constants at room temperature (cf. Annual Report 1999) have been analysed in the light of the predictions of a single Landau free energy expansion. Shear strains for I4/mcm, Cmcm and Pnma structures tend to conform to the predicted pattern. The Pm3m - I4/mcm transition has nearly tricritical character as a function of temperature in CaTiO3 and more nearly second order 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 and composition dependent, which may be a general feature of phase transitions in perovskites. The pattern of strain variations also correlates closely with patterns of variations in heat capacity as determined by DSC measurements.

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