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)₂.6H₂O 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 (P6₃mc), 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 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 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.

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