The structural evolution of PbSc0.5Nb0.5O3 under pressure was studied by in situ powder neutron diffraction. Rietveld refinements of the data showed that the continuous phase transition detected by X-ray diffraction at pc = 4.1GPa was associated with long-range ordering of antiphase octahedral tilts and local ordering of ferroic Pb displacements. Similarly to PbSc0.5Ta0.5O3, antiphase octahedral tilting even exists below the critical pressure in a regime in which the structure retains a cubic metric. In contrast to PbSc0.5Ta0.5O3, in which the Pb atomic displacement parameters form ellipsoids elongated along the cubic <111> directions, the atomic displacement parameters of Pb in PbSc0.5Nb0.5O3 formed flattened discs parallel to the cubic {111}-planes. This indicated that in PbSc0.5Ta0.5O3 the Pb displacements were along the cubic <111> directions, whereas in PbSc0.5Nb0.5O3 the local Pb displacements were randomly distributed along the cubic <110> directions. The latter could be explained by the abundance of under-bonded oxygen atoms associated with the chemical B-site disorder.

High-Pressure Powder Neutron Diffraction Study on Lead Scandium Niobate. B.J.Maier, R.J.Angel, B.Mihailova, W.G.Marshall, M.Gospodinov, U.Bismayer: Journal of Physics - Condensed Matter, 2011, 23[3], 035902