NMR relaxation study of the H-bonded glassRb1−x(NH4)xH2PO4

Abstract

The existence of a minimum in the temperature dependence of the $^{87}\mathrm{Rb}$ spin-lattice relaxation time ${T}_{1}$ in ${\mathrm{Rb}}_{1\ensuremath{-}x}{(\mathrm{N}{\mathrm{H}}_{4})}_{x}{\mathrm{H}}_{2}\mathrm{P}{\mathrm{O}}_{4}$ with $x=0.35$ indicates a tremendous progressive slowing down of the proton intrabond (O-H\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}O) jump motion from ~ ${10}^{\ensuremath{-}11}$ to ~ ${10}^{\ensuremath{-}7}$ sec not found in ${\mathrm{RbH}}_{2}$${\mathrm{PO}}_{4}$ or other members of the ${\mathrm{KH}}_{2}$${\mathrm{PO}}_{4}$ family exhibiting ferro- or antiferroelectric transitions. The occurrence of a linewidth broadening together with the ${T}_{1}$ minimum exhibits a wide distribution of correlation times that develops near the onset of the low-frequency dielectric dispersion and loss characteristic of the glass state. The results can be described in terms of a cluster freeze-out model originally developed for spin-glasses. The ${\mathrm{NH}}_{4}$ reorientations freeze out much before the low-temperature glass state has been reached.