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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For pt. I see ibid., vol. 5, p.1261 (1993). An EXAFS study of lead magnesium niobate was performed at the Nb K edge and at the Pb LIII edge between 300 and 4.5 K. Two Nb-O bond lengths of 1.95 and 2.16 AA are observed and there is no significant modification of the local environment of the Nb atoms with temperature. The distribution of these two bonds in both polar nanodomains and 1-1 Nb-Mg ordered nanodomains is discussed in relation to results obtained previously by other techniques. It is shown that the ordered nanodomains could occupy about one third of the whole volume. The results obtained for the Nb-Nb and Nb-Mg distances confirm the structural results and particularly point to an increasing correlation of the Nb shift at their site, when the temperature is lowered to below 300 K. The weak EXAFS signal obtained at the Pb LIII edge and the small backscattered contribution of Pb at the Nb K edge reveal a strong static disorder of the Pb positions.