Structural study of the ferroelectric to paraelectric phase transition in TlTiOPO4

The crystal structures of thallium titanyl phosphate, TlTiOPO4, have been determined at room temperature, at 500 °C, and at 650 °C, by using time-of-flight neutron single crystal diffraction. The structures below and above the ferroelectric to paraelectric phase transition (Tc = 583 °C) are compared: the data suggest a displacive second order phase transition which is dominated by thallium ion motion, but is also accompanied by subtle displacements of the framework atoms, during which, one of the two distinct TiO6 octahedra regularizes to 1̄ symmetry.