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.
Mixed crystals of the KH2PO4 family
Abstract A review of past and current work with mixed crystals of the KH2PO4 family is presented. It concentrates on solid solutions where the cations have been mixed in large proportions to achieve properties different from those of the parent crystals. A large part of the review is devoted to mixed crystals of RbH2PO4 and NH4H2PO4, or RADP, which is a model system for structural glasses that has recently received considerable attention.
Scaling dielectric data on Rb1-x(NH4)xH2PO4 structural glasses and their deuterated isomorphs.
Temperature- (T) and frequency- (\ensuremath{\nu}) dependent dielectric losses on ${\mathrm{Rb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$(${\mathrm{NH}}_{4}$${)}_{\mathrm{x}}$${\mathrm{H}}_{2}$PO $_{4}$ single crystals (RADP), and their deuterated isomorphs (D-RADP), can be collapsed to single curves with a scaling variable E(T,\ensuremath{\nu}). This applies for T sufficiently high that equilibrium is established within the sample-thermalization time. If E(T,\ensuremath{\nu}) follows the Vogel-Fulcher law, the scaling will depend only on the freezing temperature ${T}_{0}$ and on the attempt frequency ${\ensuremath{\nu}}_{0}$. For RADP, values in excellent agreement with a study covering a very broad frequency range are thus obtained, while significantly different values apply to D-RADP.