The perovskite structure of the relaxor PbMg,,,Nb,,,O, (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 thecoexistence of different phases at low temperatures are discussed.

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