Modelling phase changes in the potassium titanyl phosphate system
Potassium titanyl phosphate (KTiOPO4 , KTP) is the principal member of a group of isomorphous compounds, some of which have important non-linear optical properties. These structures have the acentric space group symmetry Pna21 and are ferroelectric. KTP itself undergoes a first-order, displacive isosymmetric phase change at 5.8 GPa at ambient temperature, as well as a second-order, displacive, order–disorder change to a paraelectric phase, with space group Pnan, under ambient pressure at 934 °C. Eight of these materials have been modelled using the static lattice modelling techniques in the GULP code. The materials modelled are: potassium, thallium and rubidium titanyl phosphates; potassium stannyl phosphate; potassium vanadyl phosphate; and potassium, rubidium and caesium titanyl arsenates. The modelling of the effects of pressure (based on the ambient pressure structures only) reproduces the isosymmetric phase change in KTP, and predicts similar pressure-driven phase changes for seven of the eight structures; only CsTiOAsO4 fails to demonstrate this phase change. Certain of the high pressure structures modelled are rotationally twinned relative to their ambient pressure progenitor structures, in a manner in accord with suggestions in the literature. It did not prove possible to model the temperature-driven phase change using a static, ordered model; this problem is ascribed to disorder in the cation positions in the high-temperature phase.