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.
Investigation of ferroelectric-antiferroelectric mixed compounds of RADA type
The model describing mixed ferroelectric-antiferroelectric compounds of RADA-type is studied in detail. The equations for both ferro- and antiferroelectric order parameters and pair correlation functions of nearest neighbors are solved and analyzed. The phase diagram “concentration-temperature” has been found and the conditions for realization of different phases are established. The dipole glass state is interpreted as a temperature and concentration peculiarity in the pair correlation function behaviour.
Isotope Effect of Monoclinic Rb(D1-xHx)2PO4 on the Phase Transitions
Dielectric constant along the b axis of crystals in monoclinic Rb(D 1- x H x ) 2 PO 4 system was measured in a temperature range from -190°C to 160°C. Phase diagram of the system obtained in a range between x =0 and 0.8 shows a relatively large isotope effect on the III-II phase transition with a shift of its transition temperature to higher temperature for the decrease of x , and a small isotope effect on the II-I phase transition but with a shift of its transition temperature to lower temperature for the decrease of x .