RIETVELD REFINEMENT OF THE INCOMMENSURATE STRUCTURE OF THE ELPASOLITE(ORDERED PEROVSKITE) PB2MGTEO6

Lead magnesium tellurate, Pb2MgTeO6, undergoes two structural phase tr ansitions at about 194 K and 142 K. The structures of the cubic and of the lower-temperature phase have been refined by the Rietveld method at three different temperatures (350, 220 and 6 K). The relative simpl icity of the structure has allowed the incommensurately modulated stru cture for powder samples to be characterized. The modulated displaceme nts of O atoms give rise to a large spread of bond lengths. Pb atoms a re disordered and their average positions are away from the threefold axis. The structural resolution is compared to that from the analysis of the symmetry-adapted coordinates of the optic modes becoming totall y symmetric in the lower-temperature phase.