TEMPERATURE-DEPENDENCE OF THE THERMAL VIBRATIONS IN NAK1-X(NH4)(X)C4H4O6-CENTER-DOT-4H(2)O (X=0.28)

The thermal vibrations of atoms except for hydrogen are studied in the paraelectric phase of NaK1-x(NH4)(x) C4H4O6 . 4H(2)O (x = 0.28). Most atoms, cations K/N, Na, constituent atoms of the tartrate molecule, C , O, and three oxygens of three kinds of water molecules exhibit a nor mal temperature dependence of the thermal vibrations, whose square amp litudes tend linearly to zero towards 0 K. On the contrary, one oxygen of the rest of the water molecules, which connects the tartrate molec ules along the b-axis, has another temperature dependence. Its thermal motion is strongly anisotropic parallel to the ferroelectric a-axis a nd the amplitude of that component does not tend to zero towards 0 K.