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Phase transitions in perovskites near the tricritical point: an experimental study of KMn_1–xCa_xF₃ and SrTiO₃

¹ Departamento de Física de la Materia Condensada, Universidad de Sevilla, PO Box 1065, E-41080 Sevilla, Spain
² Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

^* E-mail: mcgallar{at}cica.es

We present experimental data for the Pm3m–I4/mcm phase transitions in the perovskite crystals KMn_1–xCa_xF₃ and SrTiO₃. Comparison of calorimetric data (latent heat and specific heat) with order parameter data (measured with X-ray rocking methods) indicates that these transitions follow mean-field behaviour, and may be described using Landau potentials where the free energy expansion includes terms up to Q⁶. This potential is characteristic of transitions close to the tricritical point. Comparison of the behaviour of SrTiO₃ and KMnF₃ indicates that KMnF₃ is closer to the tricritical point; a small amount of substitution of Ca for Mn causes the transition to cross the tricritical point from first order to second order behaviour.

KEYWORDS: phase transition, perovskite, tricritical point, Landau theory