The complex dielectric constant of ${\mathrm{Rb}}_{1\mathrm{-}\mathit{x}}$(${\mathrm{NH}}_4$${)}_{\mathit{x}}$${\mathrm{H}}_2$${\mathrm{AsO}}_4$ mixed crystals is measured for the concentrations x=0.17 and 0.19. It reveals a proton-glass state preceded by the ferroelectric phase transition as the temperature decreases. A complementary laser optical study suggests that the ferroelectric orthorhombic symmetry remains down to the low temperature where the glass state is established. Hence, the proton-glass state coexists with the long-range ferroelectric order of orthorhombic symmetry, and the glass freezing may occur quite independently of the ferroelectric order.

• Received 8 March 1991

DOI:https://doi.org/10.1103/PhysRevB.44.2826