Low-temperature X-ray studies of a [CH3)2NH2]2•Cul4 crystal

The a, b, c lattice parameters of a [(CH3)(2)NH2](2).CuCl4 crystal have been measured by the X-ray diffraction method within the temperature range of 100-300 K. The temperature dependences of thermal expansion coefficients alpha(a) =f(T), alpha(b) =f(T), and alpha(c) =f(T) along the principal crystallographic axes and thermal expansion coefficient of the unit-cell volume alpha(V) =f(T) are determined. It is found that all the three parameters, a, b, and c, vary with temperature in a complicated way and show jumplike anomalies in the a =f(T), b = f(T), and c =f(T) curves at phase-transition temperatures T-c1 = 255 K and T-c2 = 279 K. An incommensurate phase with the modulation wave vector q(i) = (1/2 + delta)(a* + c*) is revealed in the temperature range 279-296 K. It is shown that the incommensurability parameter delta increases with an increase in temperature. (C) 2002 MAN "Nauka/Interperiodica".