Twinning, disorder and phase transition in ferrous Perchlorate hexahydrate crystals

Abstract

Abstract The crystal structure of ferrous Perchlorate hexahydrate resembles that of lithium Perchlorate trihydrate (space group P63 mc) as far as the perchlorate-water arrangement is concerned. The distribution of metal atoms among twice their number of available sites gives rise to orthorhombic symmetry (space group Pmn21}. Crystals of this compound grow as threecomponent twins, in which the perchlorate-water arrangement is continuous throughout a crystal, the discontinuity at the twin boundaries being only in the distribution of metal atoms. With some specimens, certain spots in the Xray photographs are found to be extended into diffuse streaks, indicating disorder in the arrangement of metal atoms. Oscillation photographs of some other specimens suggested the existence of a disordered trigonal isomorph. While the specimens with disorder remain unaffected, those with no disorder undergo a phase transition when cooled below 237 K. After transition the crystal loses its partially continuous nature and consists of three discrete orthorhombic components. Contrary to earlier suggestions, the transition does not seem to be associated with an ordering of Perchlorate tetrahedra, and there is no isotypy of the low temperature phase with the monoclinic copper Perchlorate hexahydrate. A slight change in the mutual orientations of the Perchlorate tetrahedra, brought about by a change in the hydrogenbond configuration, is sufficient to cause the structural change observed.