Structural change induced by dehydration in ikaite (CaCO_3•6H_2O)

摘要

Dehydration-induced structural change in ikaite, CaCO_3•6H_2O, is investigated using a low-temperature single-crystal X-ray diffraction study. At -50 ℃, the crystal structure of ikaite is monoclinic, of space group C2lc with the unit cell parameters a = 8.8134 (1), b = 8.3108 (1), c = 11.0183 (1) A, and β= 110.418 (1)°. The measurements were performed in 10 ℃ steps, revealing a monotonous increase of unit cell volume from 756.3 to 758.0 A~3, up to -20 ℃. The unit cell volume then jumps to 771.0 A~3 at -10 ℃. The unit cell expands anisotropically along the a-axis followed by the c-axis. The ikaite structure is finally lost at 0 ℃, which is a much lower temperature for decomposition than previously reported values. The low temperature decomposition is attributable to the aridity of the sample. The elongation of the O1-O4 intermolecular distance parallel to the (101) plane engenders the substantial increase in the a-axis and c-axis. The two-dimensional molecular sheets composed of the CaCO_3•6H_2O molecules are stacked with hydrogen bondings along the c-axis. The expansion of the c-axis is affected by variations in the hydrogen bondings between the sheets. The intramolecular Ca-O2 and Ca-O5 bond lengths and the intermolecular O1-O5 distance are greatly elongated immediately before the decomposition of ikaite structure. These expansions along the b-axis, however, are offset by the increase in the O2-C-O2 bond angle in the CO_3 geometry, aligned perfectly parallel to the A-axis. The intermolecular angles are maintained as almost constant until the ikaite structure is lost. It can be concluded therefore that the movement of H_2O molecules from the crystal lattice occurs simultaneously because the CaCO_3-6H_2O molecules are stabilized by the hydrogen-bonding network immediately before dehydration.