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)m c), exhibiting a three-component orthorhombic twinning (Pmn2(1)). At h igh temperatures a slight deviation of the bla ratio of the three twin ned orthorhombic cells from 3(1/2) results in a three-component splitt ing of each spot in the X-ray photograph, which on cooling to room tem perature coalesce into single ones, thus restoring the original bla ra tio. 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 ver ifies the continuous perchlorate-water arrangement and three-component twinning of the orthorhombic cell. Low-temperature X-ray photographs indicate no structural change.