Dielectric measurements of the proton-glass state in ${\mathrm{Rb}}_{0.65}$(${\mathrm{NH}}_4$${)}_{0.35}$${\mathrm{H}}_2$${\mathrm{AsO}}_4$

Dielectric measurements of the ${\mathrm{Rb}}_{1\mathrm{-}\mathrm{x}/\mathrm{e}\mathrm{m}\mathrm{p}\mathrm{h}>(\mathrm{N}{\mathrm{H}}_4}$ ${)}_{\mathrm{x}}$ ${\mathrm{H}}_2$ ${\mathrm{AsO}}_4$ (x=0.35) mixed crystals in the a and c tetragonal directions in the temperature range from 3 to 300 K are reported. Below $T_g$ a dielectric dispersion of ε’(T) and ε’ ’(T) in the applied frequency range from 1 Hz to 30 kHz was observed. A small anisotropy of the proton-glass transition temperature $T_g$ was detected, but we did not find the large anisotropy reported by J. Kim, N. Kim, and K. Lee [J. Phys. C 21, L-663 (1988)].