Glassy behavior of mixed hydrogen-bonded systems

Hydrogen-bonded ferroelectrics and glasses are studied both theoretically ( using the Ising pseudospin operator formalism) and experimentally (via NQR and quadrupole-perturbed NMR). Random electric and elastic fields coexistin g with competing random interbond interactions are shown to smear the trans ition from the paraelectric into the glassy phase with frozen pseudospins. Random-bond effects dominate over random-field effects even in the limit of a weak substitutional disorder where a long-range ferroelectric or antifer roelectric and glassy order coexist below T-C. Measurements of the pressure dependence of the glass-transition temperatures and the Rb-87 and H-2 spin -lattice relaxation data prove that, at low temperatures, we deal with quan tum glasses. In proton and deuteron glasses, the degree of the static glass disorder described by the Edwards-Anderson order parameter q(EA) and the l ocal polarization distribution function W(p) were determined from the secon d moments of the deuteron NMR line shapes. At low temperatures, the classic al 1D NMR methods fail in determining q(EA) because of the extremely slow i ntrabond deuteron dynamics; therefore, the 2D exchange H-2 NMR method was s uccessfully applied to measure both q(EA) and the intrabond exchange correl ation time. The unusually low activation energy and the long correlation ti mes of this process show that, at low temperatures, the phonon-assisted tun neling of the D3AsO4 and DAsO4 defects plays an important role in the dynam ics of these systems.