TEMPERATURE-DEPENDENCE OF CRYSTAL-STRUCTURE OF NAK0.72(NH4)(0.28)(-C4H4O6.4H(2)O IN THE PARAELECTRIC PHASE())

The crystal structures of the ferroelectric NaK0.72(NH4)(0.28)(+)-C4H4 O6.4H(2)O (RS(0.72)ARS(0.28)) have been studied in the paraelectric ph ase [M(r)=276.32, orthorhombic, P2(1)2(1)2, Z=4, F(000)=575.04, lambda (MoK alpha(1))=0.70926 Angstrom]; T=213 (1), 253 (1) and 293 (1) K. Th e 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) Angstrom, V=1054.9 (2) Angstrom(3), 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) Angstrom, V=1061.0 (2) Angstrom(3), D-x=1.73 0 g cm(-3); (III) at T=293 (1) K: R=0.0473, wR=0.0392, number of refle ctions=3179, a=11.944 (1), b=14.317 (1), c=6.231 (1) Angstrom, V=1066. 5 (1) Angstrom(3), D-x=1.721 g cm(-3). X-ray investigations reveal det ails of structural change with temperature in the paraelectric phase. the carboxylate anion O(3)-C(4)-O(4) in the tartrate molecule is displ aced 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 o f O(8) shows different behavior from the other oxygens of the water mo lecules. The amplitude of the thermal motion of O(8) does not tend to zero at T=0 K, while the other oxygens of the water molecules tend to zero.