Rietveld refinement of the incommensurate structure of the elpasolite (ordered perovskite) Pb2MgTeO6

Lead magnesium tellurate, Pb2MgTeO6, undergoes two structural phase transitions 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 simplicity of the structure has allowed the incommensurately modulated structure for powder samples to be characterized. The modulated displacements of O atoms give rise to a large spread of bond lengths. Pb atoms are 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 totally symmetric in the lower-temperature phase.