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.
On the Phase Transitions in Ferroelectric-Antiferroelectric Mixed Systems
A new theoretical approach to the investigation of phase transitions in ferroelectric-antiferroelectric mixed systems is proposed. The model Hamiltonian describes both long and short range interactions between particles the total dipole momentum of which can be oriented only along the z-axis (ferroelectric type of ordering), or along the x-axis (antiferroelectric type of ordering). Using the replica method for calculation of partition function for randomly distributed sites with ferroelectrically and antiferroelectrically ordering particles, the complete analysis of possible phase transitions is performed. The problem of coexistence of para-, ferro-, antiferroelectric, spin-glass and mixed phases are discussed in detail.
Neutron Scattering Study of the Dynamics of (KCN) 0.5 (KBr) 0.5
Our earlier measurements on pure KCN and (KCN)/sub 0.25/(KBr)/sub 0.75/ have been extended to mixed crystals of (KCN)/sub 0.5/(KBr)/sub 0.5/. From a comparison of the observed neutron scattering line shapes to those predicted by a simple extension of the theory used to describe pure KCN, it is concluded that this theory does not describe the mixed systems correctly. This Letter suggests that the current results may be indicative of the formation of a ''glass'' phase at low temperatures in these mixed systems.