COEXISTENCE OF PARAELECTRIC/PROTON-GLASS AND FERROELECTRIC (ANTIFERROELECTRIC) ORDERS IN Rb_{1-x}(NH_{4})_{x}H_{2}AsO_{4} CRYSTALS
This paper reviews the results of dielectric studies of the proton glass state in a mixed crystal Rubidium Ammonium Dihydrogen Arsenate (RADA) The coexistence of paraelectric/proton glass and ferroelectric or antiferroelectric orders, confirmed by other studies, has been discussed in detail. The phase diagram of RADA is asymmetric. The proton glass state exists for ammonium concentration in the range of 0.1 < x < 0.5 . The phase diagram of deuterated DRADA is presented. The proton glass state region in DRADA, 0.2 < x < 0.35 is narrower than that for non-deuterated RADA. The effects of hydrostatic pressure on the dielectric properties in the proton glass state are presented. The glass temperature Tg decreases with pressure and is expected to vanish at 5 kbar. Low temperature behaviour is still an open question, since there is no experimental evidence of Tg(p) dependence below 4K for proton glass systems.
A re-entrant glasslike phase in Rb1−x(NH4)xH2PO4
The mixed crystal Rb 1- x (NH 4 ) x H 2 PO 4 shows ferroelectricity for small x and antiferroelectricity for large x . The frequency dispersion of the dielectric constant was studied in the concentration range 0.5≤ x . The low-frequency dispersion was found for 0.5≤ x ≤0.8 below 40 K. From the results, the phase diagram was obtained. Around x =0.75, a re-entrant glasslike phase with the dispersion of the dielectric constant was found.
Crystal structure and ferroelectricity of cesium dihydrogen phosphate CsH2PO4
Structural and dielectric studies have been performed on CsH2PO4 for scrutinizing the validity of the previously reported structure. It is found that the symmetry in the paraelectric state is not orthorhombic as previously assumed but monoclinic with the space group of C. The net configuration of hydrogen bond and the sequence of Cs and P are found to differ substantially from those of KH2PO4. Structural evidences combined with dielectric measurements indicate that the ferroelectricity of CsH2PO4 takes place through different atomic structure from that of KH2PO4 type ferroelectrics. 
 
 
 
L'etude structurale et dielectrique de CsH2PO4 a ete effectuee en vue de scruter la structure anterieurement rapportee. Nous avons trouve que la symetrie a la phase paraelectrique n'est pas orthorhombique comme prealablement connue mais monoclinique au groupe spatial C. La conformation de la liaison hydrogenee et la sequence de Cs et P different solidement de celles de KH2PO4. Ces faits structural combines avec les mesures dielectrique indiquent que la ferroelectricite de CsH2PO4 a lieu sous la structure atomique differente de celle des ferroelectriques au type de KH2PO4.