Crystal structure of the partially ordered relaxor ferroelectric Pb 2ScTaO6 in the paraelectric and ferroelectric states

The crystal structure of the relaxor ferroelectric Pb₂ScTaO₆ has been refined from high-resolution neutron time-of-flight powder diffraction data at 400 and 4.2 K. At 400 K, in the paraelectric state, the structure is cubic, space group Fm3m, with lattice parameter 8.15513(4) W. This structure is derived from the simple cubic perovskite structure by partial ordering of the octahedral cations (Ta, Sa). At 4.2 K, in the ferroelectric state, the structure is rhombohedral, space group R3, with lattice parameters a=8.15441(7) Å and α=89.8485(3)°. This structure represents a previously unknown distortion of the perovskite structure. It differs from the paraelectric crystal structure by cooperative ion shifts. The coordination polyhedra observed in the ferroelectric state, clearly indicate the presence of a stereochemically active electron lone pair on lead. The large displacement parameters associated with both lead and oxygen ions, in the paraelectric structure, suggest significant local shifts of these ions (~0.2 Å) from their ideal sites