Low-temperature x-ray diffraction studies of the crystallographic parameters and thermal expansion of (CH3)(2)NH2Al(SO4)(2)center dot 6H(2)O crystals

The a, b, c, and beta crystallographic parameters of the (CH3)(2)NH2Al(SO4) (2) . 6H(2)O crystal (DMAAS) have been measured by x-ray diffraction in the 90-300-K temperature range. The thermal expansion coefficients along the p rincipal crystallographic axes alpha(a), alpha(b), and alpha(c) have been d etermined. It was shown that, as the temperature is increased, the paramete r alpha decreases and b increases, whereas c decreases for T < T-c (where T -c is the transition temperature) and increases for T > T-c, so that one ob serves a minimum in the c = f(T) curve in the region of the phase transitio n (PT) temperature T-c similar to 152 K. The thermal expansion coefficients alpha(a), alpha(b), and alpha(c) vary in a complicated manner with increas ing temperature, more specifically, alpha(a) and alpha(c) assume negative v alues at low temperatures, and the alpha(a) = f(T), alpha(b) = f(T), and al pha(c) = f(T) curves exhibit anomalies at the PT point. The crystal has bee n found to be substantially anisotropic in thermal expansion. (C) 2000 MAIK "Nauka/ Interperiodica".