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Elastic anomalies accompanying phase transitions in (Ca,Sr)TiO₃ perovskites: Part III. Experimental investigation of polycrystalline samples

¹ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K.
² Department of Geosciences, SUNY at Stony Brook, Stony Brook, New York 11794, U.S.A.

Correspondence: ^* E-mail: mc43{at}esc.cam.ac.uk

Bulk and shear moduli of polycrystalline samples of perovskites with different compositions across the CaTiO₃-SrTiO₃ solid solution have been measured at ambient conditions and in-situ at high pressures by pulse-echo ultrasonic methods. The samples were prepared as dense pellets by hot pressing synthetic powders at ~7.5 GPa and ~1000 °C. Any variations of bulk modulus due to phase transitions are small, but significant anomalies have been observed in the shear modulus at ambient conditions. These are associated with a sequence of symmetry changes Pmm I4/mcm Pbcm Pnma with increasing CaTiO₃ content. Comparison with variations in elastic properties predicted using Landau theory suggests that a substantial part of the elastic softening observed in tetragonal samples could be due to anelastic contributions from transformation twin walls. This additional softening does not occur in orthorhombic samples, and the transition from tetragonal to orthorhombic symmetry results in a stiffening of the shear modulus. No overt evidence was found for a phase transition I4/mcm Pnma at high pressures in Ca_0.35Sr_0.65TiO₃ but small changes in the trends of both bulk and shear moduli in the range 2.5–3 GPa could be due either to a different transition or a change in compression mechanism. A Pmm I4/mcm transition at ~2 GPa in Ca_0.05Sr_0.95TiO₃ shows the same form of softening as observed for the transition as a function of composition. A simple model of twin wall contributions to the compliance of tetragonal samples failed to match the observed variations that, alternatively, seem to follow G q₄ where G is the change in shear modulus and q₄ the driving order parameter for the Pmm I4/mcm transition. Analogous elastic behavior is expected to occur in (Mg,Fe)SiO₃ and CaSiO₃ perovskites at high pressures and temperatures.

Key Words: Elastic constants • phase transitions • CaTiO₃-SrTiO₃ solid solution • anelasticity

This article has been cited by other articles:

				M. A. Carpenter Elastic anomalies accompanying phase transitions in (Ca,Sr)TiO3 perovskites: Part I. Landau theory and a calibration for SrTiO3 American Mineralogist, February 1, 2007; 92(2-3): 309 - 327. [Abstract] [Full Text] [PDF]

				M. A. Carpenter Elastic anomalies accompanying phase transitions in (Ca,Sr)TiO3 perovskites: Part II. Calibration for the effects of composition and pressure American Mineralogist, February 1, 2007; 92(2-3): 328 - 343. [Abstract] [Full Text] [PDF]