X-ray study of M-II(ClO4)(2).6H2O (M-II=Zn, Ni): Twinning, disorder and phase transitions

X-ray studies of zinc and nickel perchlorate hexahydrate, Zn(ClO4)(2).6H(2)O and Ni(ClO4)(2).6H(2)O, respectively, at different temperatures have been carried out to correlate the structural changes with phase transitions in the compounds. The crystals are pseudohexagonal (P6(3)mc), exhibiting a three-component orthorhombic twinning (Pmn2(1)). At high temperatures a slight deviation of the bla ratio of the three twinned orthorhombic cells from 3(1/2) 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 bla 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).6H(2)O crystal verifies the continuous perchlorate-water arrangement and three-component twinning of the orthorhombic cell. Low-temperature X-ray photographs indicate no structural change.