Single crystals of CaCO3•H2O were synthesized by a silica-gel technique using the following reaction with sugar (C12H22O11) added: Na2CO3+ CaCl2 = CaCO3+ 2NaCl. Precession and Weissenberg X-ray diffraction photographs indicated one of two enantiomorphic polar space groups, P3121 or P3221, which should exhibit piezoelectricity. The subcell lattice parameters are a = 6.092(2) Å and c = 7.529(2) Å with three formula units in the unit cell. The presence of very weak superstructure-type reflections indicated that the real structure has a tripled unit cell with [as,bs,cs] = [(1/3 1/3 0) (–1/3 2/3 0) (0 0 1)][ao,bo,co]. The substructure, assuming half occupancy of carbon and oxygen in general positions, refined to an R factor of 3.4% using anisotropic temperature factors. Ca is surrounded by eight oxygens at distances of 2.42–2.49 Å. One extraordinarily long C-O distance in the carbonate group, 1.367 Å, compared with the other two of 1.251 and 1.273 Å, suggested that hydrogen was bonded to it forming an HCO3 anion, also suggested by the infrared absorption spectra. The structure is further characterized by disordered CO3 groups, pointing up and down the c axis. The superstructure is proposed to result from the ordering of the carbonate groups.
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