STRUCTURE OF SODIUM AMMONIUM (+)-TARTRATE TETRAHYDRATE AT 200-K, 151-K AND 120-K

The crystal structure investigation of NaNH4(+)-C4H4O6.4H2O, 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. Additionally, the structure at room temperature has been discussed from the point of view of the intermolecular atomic arrangements responsible for the sign of the gyration tensor components. Crystal data: NaNH4[(2R,3R)-C4H4O6].4H2O, M(r) = 261.16, orthorhombic, P2(1)2(1)2, Z = 4, lambda(Cu Kalpha) = 1.54178 angstrom. At 200 K, a = 12.112 (1), b = 14.310 (17), c = 6.204 (1) angstrom, V = 1075 (1) angstrom3, D(x) = 1.613 g cm-3, mu = 17.11 cm-1, R = 0.0344 for 1220 observed unique reflections. At 151 K, a = 12.088 (2), b = 14.281 (31), c = 6.192 (1) angstrom, V = 1069 (2) angstrom3, D(x) = 1.623 g cm-3, mu = 17.22 cm-1, R = 0.0334 for 1221 observed unique reflections. At 120 K, a = 12.065 (6), b = 14.346 (7), c = 6.165 (3) angstrom, V = 1067 (1) angstrom3, D(x) = 1.626 g cm-3, mu = 17.20 cm-1, R = 0.0376 for 1216 observed unique reflections.