Order-disorder behaviour in the transition of PbTiO 3

Single-crystal neutron-diffraction studies of PbTiO3, in its tetragonal (ferroelectric) phase at room temperature, at Tc - 60 K and at Tc - 20 K, and in its cubic phase at Tc+ 2 K, Tc + 50 K and Tc + 100 K, give evidence that the phase transition, at T, = 763 K, is order-disorder in character. In particular, the Pb atoms can be described as being disordered over six sites above Tc, each displaced -0.2 1 along the ⟨001⟩ directions, and then ordering onto one of these sites below Tc.