STRUCTURE OF AMMONIUM HYDROGEN SUCCINATE AT LOW TEMPERATURES. S. Kashino, T. Yoshida, Y. Kubozono, T. Urakawa, Y. Yoshida, H. Ishida, H. Maeda, Faculty of Science, Okayama University, Tsushima, Okayama 700, Japan

Crystals of ammonium hydrogen succinate undergo the second order phase transition around 170 K, and the space group is P with Z=2 at 293 K (Haussühl & Schreuer, 1993). We report the structural change accompaning the phase transition based on the structures determined at temperatures from 297 to 20 K. The space group P is retained, and hydrogen atoms involved in two O--H...0 hydrogen bonds between hydrogen succinate ions are disordered at all temperatures. Temperature dependency of the structure is remarkable in the O...O lengths of these hydrogen bonds. These lengths decrease with a decrease in temperature from room temperature to the phase-trasition temperature, but increase after the phase transition. It is worthy of note that peak resolution of the difference Fourier maps in regions of these hydrogen bonds becomes lower at 80 and 20 K than at 150 K, suggesting some fluctuations in hydrogen- atom positions.

Intensity data were measured on a Hüber off-center four-circle diffractometer in 2 [[theta]] range of 3-78deg. by using Mo K[[alpha]] X radiation. Final R values were 0.033 for 2720 reflections at 80 K, and 0.034 for 2819 reflections at 20 K.

Haussühl, S. G. & Schreuer, J. (1993). Z. Kristallogr. 206, 255-265.