THE GINZBURG INTERVAL IN SOFT-MODE PHASE-TRANSITIONS - CONSEQUENCES OF THE RIGID UNIT MODE PICTURE

In soft-mode structural phase transitions the Ginzburg temperature int erval in which fluctuations and the interactions between them become i mportant is often observed to be small on the scale of the transition temperature. We consider the size of the Ginzburg interval (GI) in fra mework and 'cogwheel' structures using the concept of 'rigid unit mode s'. Such materials, as well as being very displacive, i.e. close to th e soft-mode limit, have an extremely anisotropic phonon spectrum. Mode lling these two properties with a suitable effective Hamiltonian for t he degrees of freedom driving the transition we find that the GI can r ange from very small to large, depending on the balance between displa civeness and anisotropy. For the two perovskites SrTiO3 and LaAlO3 and the 'cogwheel' structure K2SeO4, we obtain values of the model parame ters describing displaciveness and anisotropy from experimentally meas ured phonon dispersions and find, for the size of the GI, quantitative agreement with experiment. We also estimate typical values for the mo del parameters and the size of the GI for framework silicates, using q uartz and cristobalite as examples. Finally, we use computer simulatio ns to confirm the results of our theoretical analysis over a wider ran ge of model parameters.