Structure of sodium ammonium (+)-tartrate tetrahydrate at 200, 151 and 120 K

The crystal structure investigation of NaNH 4 (+)-C 4 H 4 O 6 .4H 2 O, known also as NAT, SAT or ARS (ammonium Rochelle salt), was carried out in order to determine the absolute chirality and to trace possible structural changes in the paraelectric phase from room temperature down to the region close to the ferroelectric phase transition at about 110 K. The observed relative elongation of the thermal ellipsoids in the [100] direction and hydrogen-bonding reorganization near the temperature 150 K support the idea of either a long-range preparation to the ferroelectric phase transition or an additional structure reconstruction, possibly without symmetry changes, at about 150 K