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The crystal structures of the ferroelectric NaK0.72 (NH4)0.28(+)-C4H4O6.4H2O (RS0.72ARS0.28) have been studied in the paraelectric phase [Mr = 276.32, orthorhombic, P21212,° Z = 4, F(000) = 575.04, λ(Mo Kα1) = 0.70926 Å]; T = 213 (1), 253 (1) and 293 (1) K. The crystal data are as follows: (I) at T = 213 (1) K: R = 0.0424, wR = 0.0399, number of reflections = 3414, a = 11.899 (2), b = 14.273 (2), c = 6.211 (1) Å, V = 1054.9 (2) Å3, Dx = 1.740 g cm−3; (II) at T = 253 (1) K: R = 0.0498, wR = 0.0442, number of reflections = 3284, a = 11.929 (1), b = 14.297 (1), c = 6.221 (1) Å, V = 1061.0 (2) Å3, Dx = 1.730 g cm−3; (III) at T = 293 (1) K: R = 0.0473, wR = 0.0392, number of reflections = 3179, a = 11.955 (1), b = 14.317 (1), c = 6.231 (1) Å, V = 1066.5 (1) Å3, Dx = 1.721 g cm−3. X-ray investigations reveal details of structural change with temperature in the paraelectric phase. The carboxylate anion O(3)—C(4)—O(4) in the tartrate molecule is displaced along the a axis as the temperature decreases; moreover, the O(8) atom is displaced in the opposite direction against the shift of the carboxylate anion. The temperature dependence of the thermal motions of O(8) shows different behavior from the other O atoms of the water molecules. The amplitude of the thermal motion of O(8) does not tend to zero at T = 0 K, while the other O atoms of the water molecules tend to zero.

Supporting information

cif

Crystallographic Information File (CIF)
Contains datablocks text, oh0056a, oh0056b, oh0056c

CCDC references: 131913; 131914; 131915

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