STRUCTURAL TRANSITIONS IN NICKEL PERCHLORATE HEXAHYDRATE. Soma Ghosh, Monika Mukherjee, Siddhartha Ray, Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India

X-ray studies of the phase transitions in Ni(ClO₄)₂.6H₂O (at ~361K,~311K & ~245K) were undertaken to find out the possible mechanisms of phase transitions. The complex crystallizes in space group P6₃ mc, however, the true symmetry is orthorhombic (space group Pmn2₁) due to the distribution of metal atoms in twice their number of available sites. This orthorhombic cell occurs in three orientations giving rise to a three-component twinning. Moreover, due to antiphase formation, diffuse streaks along odd layer lines in oscillation photograph about c-axis and some diffuse spots in corresponding Weissenberg photographs are observed. At ~323K, these streaks disappeared, with no change in cell dimensions, but all spots in first layer Weissenberg photographs were elongated along [[omega]]-direction. On cooling, elongation of spots vanished and the diffuse streaks originally present, did not reappear. Above ~363K, each spot splitted into two/three components along [[omega]]-direction which coalesced into single sharp spots on cooling to room temperature. The overall hexagonal symmetry remained unchanged throughout. This indicated that the perchlorate-water arrangement was continuous throughout the crystal and the peculiar three-component twinning existed even in the transformed phase. Experiments conducted at low temperature (~223K) indicated no structural changes.

The splitting of the spots at high temperature in the crystal is probably due to slight deviation of the ratio of b/a of the three orthorhombic cells from [[radical]]3 so i that interplanar spacings of each of the three orthorhombic cells become different. The decrease in the probability of mistakes in repetition along b-axis of orthorhombic cells with increase in temperature accounts for the disappearance of diffuse streakings.