RESUMO
The structure of thallium selenate, Tl2SeO4, in a paraelastic phase (above 661 K) has been analysed by Rietveld analysis of the X-ray powder diffraction pattern. Atomic parameters based on the isomorphic K2SO4 crystal in the paraelastic phase were used as the starting model. The structure was determined in the hexagonal space group P6(3)/mmc, with a = 6.2916 (2) A and c = 8.1964 (2) A. From the Rietveld refinement it was found that two orientations are possible for the SeO4 tetrahedra, in which one of their apices points randomly up and down with respect to [001]. One Tl atom lies at the origin with 3m symmetry, the other Tl and one of the O atoms occupy sites with 3m symmetry, the Se atom is at a site with 6m2 symmetry and the remaining O atom is at a site with m symmetry. Furthermore, it was also found that the Tl atoms display anomalously large positional disorder along [001] in the paraelastic phase.
RESUMO
The structure of trithallium hydrogen bis(sulfate), Tl(3)H(SO(4))(2), in the super-ionic phase has been analyzed by Rietveld analysis of the X-ray powder diffraction pattern. Atomic parameters based on the isotypic Rb(3)H(SeO(4))(2) crystal in space group R3m in the super-ionic phase were used as the starting model, because it has been shown from the comparison of thermal and electric properties in Tl(3)H(SO(4))(2) and M(3)H(SO(4))(2) type crystals (M = Rb, Cs or NH(4)) that the room-temperature Tl(3)H(SO(4))(2) phase is isostructural with the high-temperature R3m-symmetry M(3)H(SO(4))(2) crystals. The structure was determined in the trigonal space group R3m and the Rietveld refinement shows that an hydrogen-bond O-H...O separation is slightly shortened compared with O-H...O separations in isotypic M(3)H(SeO(4))(2) crystals. In addition, it was found that the distortion of the SO(4) tetrahedra in Tl(3)H(SO(4))(2) is less than that in isotypic crystals.
