Journal of Applied Crystallography

Volume 35, Part 2 (April 2002)

research papers

 

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

P. M. Woodward and K. Z. Baba-Kishi

Abstract: 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 = 89.8488 (3)°. At 400 K, in the paraelectric state, this compound adopts Fmm 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.

Keywords: relaxor oxide Pb₂(ScTa)O₆; paraelectric state; ferroelectric state; phase transitions; low-temperature diffraction; high-temperature diffraction; neutron diffraction; perovskites.

Copyright © International Union of Crystallography
IUCr Webmaster