A model of elastic softening and second order phase transitions in anisotropic phases, with application to stishovite and post-stishovite

This paper presents a Landau framework for describing instantaneous and time-dependent elastic softening in anisotropic solid solutions. As with previous Landau models, the framework minimizes a thermodynamic potential by varying isochemical parameters (q) that describe changes in structural, ordering or electronic spin state. Unlike previous models, no coupling terms are used; strain is not a parameter involved in the energy minimization, but is instead a property of the material. The framework satisfies all the connections between elastic and thermodynamic properties required by Maxwell's relations. The stishovite to post-stishovite transition is used to illustrate the validity of the formulation. It is shown that around the transition, the stishovite-post stishovite phase is auxetic in many directions and has negative linear compressibility along the long axis of the cell.