Low-temperature X-ray studies of a [(CH3)2NH2]2 · CuCl4 crystal

The a, b, c lattice parameters of a [(CH3)2NH2]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 αa = f(T), αb = f(T), and αc = f(T) along the principal crystallographic axes and thermal expansion coefficient of the unit-cell volume αV = 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 Tc1 = 255 K and Tc2 = 279 K. An incommensurate phase with the modulation wave vector qi = (1/2 + δ)(a* + c*) is revealed in the temperature range 279–296 K. It is shown that the incommensurability parameter δ increases with an increase in temperature.