Order parameter behaviour in the relaxor ferroelastic lead scandium tantalate

Symmetry properties and structural phase transitions of the relaxor ferroelectric PbSc_0.5Ta_0.5O₃ are described using Landau theory. Symmetry reductions in disordered material occur via a ferroelastic (Pm3m-R3m) and a ferroelectric transition (R3m-R3m). Ordering involves the zone boundary instabilities Pm3m(Z=1)-Fm3m(Z=2) (R point), R3m(Z=1)-R3m(Z=2) (Z point) and R3m(Z=1)-R3m(Z=2) (Z point). The Landau potential includes the coupling between the three primary order parameters describing the cation ordering, the ferroelastic and the ferroelectric lattice distortions. Relaxor behaviour is related to the kinetic properties of order parameters with intermediate conservation lengths. Kanzig domains are related to inhomogenous solutions of the kinetic rate law.