Structural order parameters of the RbPO3-HT-RbPO3-H transition - a soft-Ising-system

Order parameter and spontaneous strain of the coelastic Brillouin zone boundary transition RbPO3-HT (Bbmm) --> RbPO3-H (Pbnm) at T-c = 915 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 +/-Phi, where the absolute value \Phi\ has some displaced character and is also temperature dependent. For Rietveld refinement the system was thus described by an Ising-like occupational parameter Q(OD) 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 Q similar to (T - T-c)(beta) leads to a scaling exponent beta = 0.17, whereas the total symmetric spontaneous strain epsilon of the unit cell follows a critical scaling epsilon similar to (T - T-c)(2 beta). 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 Delta y approximate to 10 +/- 0.5 pm. Below about 840 K deviations of the order parameter behaviour to a less pronounced temperature dependence indicate a change in local dynamics.