Structural order parameters of the transition - a soft-Ising-system

Abstract

Order parameter and spontaneous strain of the coelastic Brillouin zone boundary transition at K were obtained from Rietveld refinement of high-resolution neutron powder diffraction data. The main aspect of the transition is that of zone boundary ordering of an Ising pseudo-spin variable, describing two distinct distorted states , where the absolute value has some displaced character and is also temperature dependent. For Rietveld refinement the system was thus described by an Ising-like occupational parameter and displaced atomic shifts from the aristotype structure, which were derived from a constrained split model. The thermodynamic order parameter, representing the average state of order in the system, is the product of the occupational and displaced variables. An empirical power law leads to a scaling exponent , whereas the total symmetric spontaneous strain of the unit cell follows a critical scaling . Due to a small hysteresis derived from thermo-analytical measurements Landau first-order behaviour could be verified with a discontinuous shift of the polyphosphate chains in [010] of pm. Below about 840 K deviations of the order parameter behaviour to a less pronounced temperature dependence indicate a change in local dynamics.