Dielectric constants were measured for the pseudo-hexagonal c -plates of K 3 H(SO 4 ) 2 , Rb 3 H(SO 4 ) 2 , and their deuterated analogues below room temperature. The dielectric constants of K 3 H(SO 4 ) 2 and Rb 3 H(SO 4 ) 2 monotonically increase with decreasing temperature down to 4.2 K showing no anomalies correspoinding to phase transitions. On the other hand, the dielectric constants of K 3 D(SO 4 ) 2 and Rb 3 D(SO 4 ) 2 show λ-type anomalies at 84 K and 71 K, respectively. The results indicate that the deuteration in these compounds promotes phase transitions in low temperature region.

Dielectric Properties and Phase Transitions in X3H(SO4)2 and X3D(SO4)2 Crystals(X:K,Rb)

AC electrical properties of the mixed crystal (NH4)3H(SO4)1.42(SeO4)0.58

Structure analysis of K 3 H(SO 4 ) 2 was performed by X-ray diffraction experiments in order to study the hydrogen bond character in connection with the geometrical isotope effect. Crystal data at room temperature were obtained as a=9.777(1)A, b=5.674(1)A, c=14.667(4)A, β=102.97(2)° and the space group A 2/ a , Z =4. The hydrogen bond length R OO determined from the positional parameters was 2.493(1)A which was close to but still longer than the so called critical bond length r c . The electron density of split hydrogen atoms was observed on a differential Fourier map. It is concluded that the hydrogen atoms take the disordered state at room temperature and the distance between hydrogen atoms R HH is approximately 0.65 A. The possibilities for the existence both of the low temperature phase and the high temperature prototype phase are discussed.

Structure Analysis and Hydrogen Bond Character of K3H(SO4)2

Dielectric and dilatometric measurements were made for the deuterated crystal (ND 4 ) 3 D(SeO 4 ) 2 . For comparison the dilatometric measurement was also made for the undeuterated crystal (NH 4 ) 3 (SeO 4 ) 2 . Two phase transitions were found at +55^° C and at about +29^° C for (NH 4 ) 3 H(SeO 4 ) 2 besides the two phase transitions previously reported. Deuteration rises the Curie temperature by about 34°C, showing important role of the O–H–O bonds playing in the ferroelectric transition. The Curie-Weiss constant and spontaneous polarization of the deuterated crystal are 3.7×10 3 K and 2.4 µC·cm -2 at -170^° C , respectively.

Dielectric and Dilatometric Studies of Phase Transitions in (ND 4 ) 3 D(SeO 4 ) 2 and Isotope Effect

The dielectric constants along the c ' axis of Cs 3 H(SeO 4 ) 2 and its deuterated crystal were measured in a temperature range from 5 K to 460 K. It was found that Cs 3 H(SeO 4 ) 2 undergoes three phase transitions at 451 K, 364 K and 50 K. The dielectric constant of Cs 3 H(SeO 4 ) 2 shows a small peak at 50 K, and it obeys the Curie-Weiss law above 50 K. The transition temperature (50 K) is increased to 168 K by deuteration, showing a large isotope effect, while other two transition temperatures do not change appreciably.

Y. Makita

Papers

The crystal structure of triammonium hydrogen disulphate, (NH4)3H(SO4)2