Crystal structures of the relaxor oxide Pb 2 (ScTa)O 6 in the paraelectric and ferroelectric states

The crystal structure of the relaxor ferroelectric Pb 2 ScTaO 6 has been refined from high‐resolution neutron time‐of‐flight powder diffraction data recorded at various temperatures from 4 to 400 K. Upon warming, Pb 2 ScTaO 6 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 R 3 space‐group symmetry, with a = 8.15231 (7) Å and α = 89.8488 (3)°. At 400 K, in the paraelectric state, this compound adopts F m m space‐group symmetry, with a = 8.15345 (3) Å. In the ferroelectric state, the Pb 2+ coordination polyhedra are quite asymmetric, clearly indicating the presence of a stereoactive electron lone pair. The Sc 3+ and Ta 5+ 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 Sc 3+ /Ta 5+ ordering and the ferroelectric cation displacements as 88 nm and 10 nm, respectively.