Crystal structures of the relaxor oxide Pb2(ScTa)O6 in the paraelectric and ferroelectric states

摘要

The crystal structure of the relaxor ferroelectric Pb2ScTaO6 has been refined from high-resolution neutron time-of-flight powder diffraction data recorded at various temperatures from 4 to 400 K. Upon warming, Pb2ScTaO6 undergoes a first-order transition at 295 K from the rhombohedral ferroelectric state into the cubic paraelectric state. At 4.2 K, in the ferroelectric state, this compound adopts R3 space-group symmetry, with a = 8.15231 (7) Å and α = 89.8488 (3)°. At 400 K, in the paraelectric state, this compound adopts Fm3̄m space-group symmetry, with a = 8.15345 (3) Å. In the ferroelectric state, the Pb2+ coordination polyhedra are quite asymmetric, clearly indicating the presence of a stereoactive electron lone pair. The Sc3+ and Ta5+ ions are also shifted away from the centers of their respective octahedra, each toward an octahedral face. The large displacement parameters associated with both the Pb and the O ions, in the 400 K structure reveal significant local shifts of these ions from their ideal sites in the paraelectric state. Thus, the paraelectric to ferroelectric transition is driven by long-range cooperative ordering of the cation displacements. Synchrotron X-ray powder diffraction measurements are used to estimate the domain size of the Sc3+/Ta5+ ordering and the ferroelectric cation displacements as 88 nm and 10 nm, respectively.