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The temperature dependences of the proton Zeeman and dipolar spin–lattice relaxation rates have been measured in a Rb1–x(NH4)xH2PO4 mixed single crystal both in the paraelectric and the ferroelectric phase. The correlation time for ammonium ion reorientation has been determined and it is found that ultraslow reorientational motion of the H2PO4 groups exists above room temperature. Es werden die Temperaturabhangigkeiten der Protonen-Zeeman- und der Dipol-Spin–Gitterrelaxationsraten in einem einkristallinen Rb1–x(NH4)xH2PO4-Mischkristall sowohl in der paraelektrischen als auch in der ferroelektrischen Phase untersucht. Die Korrelationszeit fur die Reorientierung des Ammoniumions wird bestimmt und gefunden, das ultralangsame Reorientierungsbewegung der H2PO4-Gruppen oberhalb Zimmertemperatur auftritt.

The observation of a thermal hysteresis indicates that the phase transitions in KDA, KD†A, RDA and CDA crystals are of the first order. The static dielectric properties of these ferroelectrics resemble those of KD†P.

Both polarization and dielectric constant measurements at 1 kcs, 2 . 105 kcs and 8.4 . 103 Mcs show that NaH3(SeO3)2 undergoes a second order ferroelectric transition at Tc = −80°C, whereas NaD3(SeO3)2 makes a transition to an apparently anti-ferroelectric state at Tc = −2.5°C. It is believed to be the first time that such drastic changes in the general dielectric behaviour, Curie-Weiss constant and type of the transition have been observed on deuteration. The antiferroelectric state in NaD3(SeO3)2 seems to be very close in energy to a field induced ferroelectric one, the difference in the free energy being of the order of 1 calmol a few degrees below the transition temperature.

The dependence of the spontaneous polarization on temperature and electric field has been measured for ferroelectric triglycine sulphate (TGS) and deuterated triglycine sulphate (D-TGS), and the critical exponents γ3, γ4, γ7, γ9, and γ10 have been determined. The results are consistent with the predictions of the mean field theory for ϵ = (T c − T) T c ⩾ 1.2.10 −4 and E > 30 V/cm. For lower values of either ϵ or E systematic deviations seem to occur and the values of the exponents seem to deviate from the mean field theory and approach those predicted by the 3-dimensional Ising model.

M.A.

NMR data show that the triclinic ferroelectric phase which exists in NaH 3 (SeO 3 ) 2 between -79°C and -173°C does not appear in NaD 3 (SeO 3 ) 2 . The transition from 2/m →1 in NaH 3 (SeO 3 ) 2 is connected with a doubling of the unit cell dimensions both along the a and b axes, whereas in the low temperatures phases of symmetry m in NaH 3 (SeO 3 ) 2 and NaD 3 (SeO 3 ) 2 the unit cell is doubled in the b direction only. The transitions are of the order-disorder type.