STRUCTURAL STUDY OF THE FERROELECTRIC TO PARAELECTRIC PHASE-TRANSITION IN TLTIOPO(4)

The crystal structures of thallium titanyl phosphate TlTiOPO4, have be en determined at room temperature, at 500 degrees C, and at 650 degree s C, by using time-of-flight neutron single crystal diffraction. The s tructures below and above the ferroelectric to paraelectric phase tran sition (T-c = 583 degrees C) are compared: the data suggest a displaci ve second order phase transition which is dominated by thallium ion mo tion, but is also accompanied by subtle displacements of the framework atoms, during which, one of the two distinct TiO6 octahedra regulariz es to (1) over bar symmetry.