NMR relaxation study of the H-bonded glass ${\mathrm{Rb}}_{1-x}{\left(\mathrm{N}{\mathrm{H}}_4\right)}_x{\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4$

The existence of a minimum in the temperature dependence of the ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ spin-lattice relaxation time $T_1$ in ${\mathrm{Rb}}_{1-x}{\left(\mathrm{N}{\mathrm{H}}_4\right)}_x{\mathrm{H}}_2\mathrm{P}{\mathrm{O}}_4$ with $x=0.35\mathrm{}$ indicates a tremendous progressive slowing down of the proton intrabond (O-H···O) jump motion from ~ ${10}^{-11\mathrm{}}$ to ~ ${10}^{-7\mathrm{}}$ 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.