FINITE-SIZE EFFECTS IN PROTON GLASS

The random distribution of Rb and ND4 in the solid solution Rb1-x(ND4) (x)D2PO4 (D-RADP-X) leads to an extremely rich x-T phase diagram. It c ontains long-range ordered ferroelectric (FE) and antiferroelectric (A FE) phase states, a, short-range ordered glass phase consisting of AFE clusters with disordered inter cluster regions,;ls well as a concentr ation range where stable FE islands are embedded in a sea of glass. Co operative phenomena such as phase transitions are vary sensitive to vi olations of the translational invariance. While Rb-rich regions of the crystal serve as nucleation centers with individual phase transition temperatures, the ND4-rich regions serve as AFE ones. The size and the density of these regions depend on the concentration parameter a: and determine the behavior of the system. With our Rb-87 1D- and 2D-NMR i nvestigations we were able to detect and prove unambiguously most of t he above mentioned features and extract important information concerni ng the dynamics of the system such as the transition from a soft mode behavior to a relaxer behavior, phonon assisted tunneling at low T and bias fluctuations of the hydrogen bonds.