Temperature dependence of crystal structure of NaK_0.72(NH₄)_0.28(+)-C₄H₄O₆.4H₂O in the paraelectric phase

The crystal structures of the ferroelectric NaK_0.72 (NH₄)_0.28(+)-C₄H₄O₆.4H₂O (RS_0.72ARS_0.28) have been studied in the paraelectric phase [M_r = 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) ų, D_x = 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) ų, D_x = 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) ų, D_x = 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.

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