A structural model for the relaxor PbMg_1/3Nb_2/3O₃ at 5 K

The perovskite structure of the relaxor PbMg_1/3Nb_2/3O₃ (PMN) is studied using X-ray and neutron powder diffraction data. The static diffuse scattering (SDS) observed in the diffraction patterns at low temperatures is interpreted using a two-phase Rietveld analysis. The structural model is based on a long-range structure with an average cubic symmetry, and a short-range order due to atomic shifts involved by the formation of polar regions. The two-phase model provides a good improvement in reliability factors. The correlation length and the coexistence of different phases at low temperatures are discussed.