Diffuse scattering from the lead-based relaxor ferroelectric PbMg_1/3Ta_2/3O₃

The relaxor ferroelectric PbMg_1/3Ta_2/3O₃ was studied by single-crystal neutron and synchrotron X-ray diffraction, and its detailed atomic structure modelled in terms of static Pb displacements that lead to the formation of polar nanoregions. Similar to the other members of the Pb-based relaxor family like PbMg_1/3Nb_2/3O₃ or PbZn_1/3Nb_2/3O₃ the diffuse scattering in the [H00]/[0K0] scattering plane has a butterfly shape around the h00 Bragg reflections and is orthogonal to the scattering vector for hh0 peaks. In the [HH0]/[00L] plane the diffuse scattering is elongated along the <112> directions and is orthogonal to the scattering vector for hhh reflections. It is found that a model consisting of correlated Pb displacements along the <111> directions reproduces adequately the main features of the diffuse scattering in PbMg_1/3Ta_2/3O₃ when the correlation lengths between the Pb-ion displacement vectors are longest along the <111> and shortest along the <11> and <10> directions.