Studies on some MG6· LR6compounds. I. Mechanism of transition in cobalt fluosilicate hexahydrate

Abstract

AbstractVariations in the structures of seven fluosilicate hexahydrates of bivalent metals, belonging to the apparently isomorphous series of compounds MG6· LR6, have been summarised. Four of these seven crystals undergo phase transition on lowering the temperature. Of these, the fluosilicate hexahydrates of Mg, Mn and Fe have Laue group [unk]m: the corresponding Co-compound is an unique case, with a disordered structure of Laue group [unk], suffering a transition. A geometrical relationship between the three orientations of the monoclinic cell of the twinned low temperature phase and the hexagonal cell of its room temperature structure has been established. The mechanism of transition has been described in terms of rotations, through different angles of the metal-water and fluosilicate octahedra in neighboring columns. Angles of rotations for the different octahedra, that reduce the symmetry from rhombohedral to monoclinic, have been calculated and a rough model of the structure of the low temperature phase has been refined using intensity data measured for a few single spots appearing in the Weissenberg photographs of the twinned crystal at low temperature. The structure thus obtained is found to be similar to that of the lower symmetry phase of magnesium fluosilicate hexahydrate, the higher temperature structure of which is quite different from that of the cobalt compound.