X-ray Study of M II(ClO4)2.6H2O (M II = Zn, Ni): Twinning, Disorder and Phase Transitions

Abstract

X-ray studies of zinc and nickel perchlorate hexahydrate, Zn(ClO4)2.6H2O and Ni(ClO4)2.6H2O, respectively, at different temperatures have been carried out to correlate the structural changes with phase transitions in the compounds. The crystals are pseudohexagonal (P63 mc), exhibiting a three-component orthorhombic twinning (Pmn21). At high temperatures a slight deviation of the b/a ratio of the three twinned orthorhombic cells from 31/2 results in a three-component splitting of each spot in the X-ray photograph, which on cooling to room temperature coalesce into single ones, thus restoring the original b/a ratio. The diffuse streaks disappear in the high-temperature photographs due to a decrease in the probability of error in repetition along the b axes of the three orthorhombic cells with temperature. A successful refinement of the heat-treated ordered Zn(ClO4)2.6H2O crystal verifies the continuous perchlorate–water arrangement and three-component twinning of the orthorhombic cell. Low-temperature X-ray photographs indicate no structural change.