STRUCTURE OF AMMONIUM HYDROGEN SUCCINATE ABOVE AND BELOW THE PHASE-TRANSITION AROUND 170 K

For crystals of ammonium hydrogen succinate it is known that the space group is <P(1)over bar> with Z = 2 at 293 K and the second-order phas e transition occurs around 170 K. X-ray crystal structure analyses abo ve and below 170 K have been carried out in order to study the change in mode of short hydrogen bonds between the hydrogen succinate ions. T he space group was determined to be <P(1)over bar> at 150 and 190 K by structure analysis. No ordering of the H-atom positions in the short hydrogen bonds occurs by the phase transition. The hydrogen bonds show a decrease in the O ... O distances with a decrease in temperature fr om 290 to 190 K, but no significant change in the geometries between 1 90 and 150 K. Disorder of the NH4+ ion is not observed at 297, 190 and 150 K. Significant change through the phase transition is found only in the geometry of one of the N-H ... O hydrogen bonds between ammoniu m and hydrogen succinate ions.