Superlattice structure of ars in the polar phase

X-ray diffraction studies of ammonium Rochelle salt at T=103K in the polar phase are carried out using the Weissenberg camera with an imaging plate. The crystal structure in the polar phase is successfully determined. The structural description in the polar phase is discussed with the previous report in the non-polar phase of ammonium Rochelle salt. The interatomic distances N(1)-O(1), N(2)-O(4), N(2)-O(5), O(8)-O(2) and O(8)-O(3) change accompanying the phase transition, which do not show a practical temperature dependence in the non-polar phase. The orientation of the tartrate molecule also changes with the phase transition. The superlattice structure in the polar phase is made by the drastic structural change.