Previous page     Contents     Next page

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.

Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
Tel: +49-(0) 921 55 3700 / 3766, Fax: +49-(0) 921 55 3769, E-mail: bayerisches.geoinstitut(at)uni-bayreuth.de