Holst, C.; Schmahl, Wolfgang W.; Knight, K.S.; Fuess, H.
Structural order parameters of the RbPO3-HT-RbPO3-H transition - A soft-Ising-system.
In: Journal of Physics Condensed Matter, Vol. 10, No. 39: pp. 8799-8813
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Order parameter and spontaneous strain of the coelastic Brillouin zone boundary transition RbPO3-HT (Bbmm)→ RbPO3-H (Pbnm) at Tc = 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 ±φ, where the absolute value |φ| has some displaced character and is also temperature dependent. For Rietveld refinement the system was thus described by an Ising-like occupational parameter QOD 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 ∼ (T - Tc)β leads to a scaling exponent β = 0.17, whereas the total symmetric spontaneous strain ε of the unit cell follows a critical scaling ε ∼ (T - Tc)2β. 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  of Δy ≈ 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.