Finite strain Landau theory of high pressure phase transformations

Abstract

The properties of materials near structural phase transitions are often successfullydescribed in the framework of Landau theory. While the focus is usually on phasetransitions, which are induced by temperature changes approaching a critical temperatureTc, here we will discuss structural phase transformations driven by high hydrostatic pressure,as they are of major importance for understanding processes in the interior of the earth.Since at very high pressures the deformations of a material are generally very large, oneneeds to apply a fully nonlinear description taking physical as well as geometricalnonlinearities (finite strains) into account. In particular it is necessary to retuneconventional Landau theory to describe such phase transitions. In Tröster et al (2002Phys. Rev. Lett. 88 55503) we constructed a Landau-type free energy based on an orderparameter part, an order parameter–(finite)strain coupling and a nonlinear elasticterm. This model provides an excellent and efficient framework for the systematicstudy of phase transformations for a wide range of materials up to ultrahighpressures.We illustrate the model with the specific example ofBaCr(Si4O10), showing that it fully accounts for the elastic softening which is observed near the pressureinduced phase transformation.