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 P1̄ with Z = 2 at 293 K and the second-order phase transition occurs around 170 K. X-ray crystal structure analyses above 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. The space group was determined to be P1̄ 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 from 290 to 190 K, but no significant change in the geometries between 190 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 ammonium and hydrogen succinate ions.