Structural state of relaxor ferroelectrics PbSc0.5Ta0.5O3 and PbSc0.5Nb0.5O3 at high pressures up to 30 GPa
The pressure-induced structural changes in perovskite-type (ABO(3)) Pb-based relaxor ferroelectrics are studied on the basis of in situ single-crystal synchrotron x-ray diffraction and Raman scattering experiments on PbSc0.5Ta0.5O3 and PbSc0.5Nb0.5O3 conducted under hydrostatic conditions up to 30 GPa. Complementary density functional theory calculations have been performed to compare the stability of various atomic configurations for both compounds at high pressures. By combining the experimental and theoretical results, the following sequence of structural transformations is proposed. At a characteristic pressure p(1)* the mesoscopic polar order is violated and a local antipolar order of Pb atoms as well as quasidynamical long-range order of antiphase octahedral tilts is developed. These structural changes facilitate the occurrence of a continuous phase transition at p(c1) > p(1)* from cubic to a nonpolar rhombohedral structure comprising antiphase octahedral tilts of equal magnitude (a a a). At a characteristic pressure p(2)* > p(c1) the octahedral tilts around the cubic , , and  directions become different from each other on the mesoscopic scale. The latter precedes a second phase transition at p(c2), which involves long-range order of Pb antipolar displacements along cubic [uv0] directions and a compatible mixed tilt system (a(+)b(-)b(-)) or long-range ordered antiphase tilts with unequal magnitudes (a(0)b(-)b(-)) without Pb displacement ordering. The phase-transition pattern at p(c2) depends on the fine-scale degree of chemical B-site order in the structure.