The mixed crystal ${\mathrm{Rb}}_{1\mathrm{-}\mathit{x}}$(${\mathrm{NH}}_4$ ${)}_{\mathit{x}}$ ${\mathrm{H}}_2$ ${\mathrm{AsO}}_4$ (RADA) has been investigated for several values of (x=0,0.12,0.15, and 0.20) by measuring the complex dielectric permittivity along the a tetragonal axis in the temperature range from 3 to 300 K and frequency range from 1 Hz to 30 kHz. We find coexistence of ferroelectric and proton-glass order in RADA for x=0.12 and probably at x=0.15. The long-range ferroelectric order is not destroyed by the onset of proton-glass ordering. This behavior is similar to that of other proton-glass systems and of the magnetic Ising glass systems reported by Wong et al. in ${\mathrm{Fe}}_{0.55}$ ${\mathrm{Mg}}_{0.45}$ ${\mathrm{Cl}}_2$ and by Yoshizawa et al. in ${\mathrm{Fe}}_{\mathit{x}}$ ${\mathrm{Mn}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{TiO}}_3$. The partial phase diagram for ${\mathrm{Rb}}_{1\mathrm{-}\mathit{x}}$(${\mathrm{NH}}_4$ ${)}_{\mathit{x}}$ ${\mathrm{H}}_2$ ${\mathrm{AsO}}_4$ including the coexisting proton-glass and ferroelectric phases is presented.

We have presented the first experimental data of the temperature dependence of ε′_c and ε″_c for Rb_1−x (NH₄)_x H₂AsO₄. The results are demonstrated by the presence of a glassy state in Rb_1−x (NH₄)_x H₂AsO₄ for x = 0.31.