Displacive phase transition in anorthoclase; the "plateau effect" and the effect of T1-T2 ordering on the transition temperature

Abstract

The effects of compositional changes and non symmetry-breaking AI-Si ordering on the displacive phase transition in anorthoclase have been studied using X-ray powder diffraction. The results were analyzed using a model based on Landau theory. The observed transition temperature, T;, was found to be independent of orthoclase content in the range 0 0.02, T; decreases linearly with increasing Xo,' This plateau behavior is explained by the finite volume of the strain field around each K+ ion: Chemical mixing behavior is observed only for average K+-K+ distances of <20 A. Increasing AI-Si order between the TI and T2 sites causes T; to increase; this is unlike Tlo-Tlm ordering, which prevents the transition to monoclinic symmetry altogether. The range of T; values reported in the literature is shown to be consistent with different degrees of TI- T2 order.