The complex dielectric constants and polarization of the mixed crystals of ${\mathrm{Rb}}_{1-x}({\mathrm{NH}}_4{)}_x{\mathrm{H}}_2{\mathrm{PO}}_4$ (RADP) with $x=0.15\mathrm{}$ and 0.17 have been measured along the tetragonal a and c axes as a function of temperature. Phase coexistence of ferroelectric order and proton glass in RADP can be confirmed from a dielectric dispersion distinguished from the ferroelectric domain wall freezing at low temperature. The dielectric dispersion, which was ascribed to the proton glass state, occurred at much lower temperatures compared to pure proton glass composition. The detection of depolarization after field-cooling or zero-field cooling the sample, showed that the ferroelectric domain and polarization were retained in the glass freezing process. The result is qualitatively consistent with the expectation of the mean-field model in the presence of macroscopic polarization. The phase coexistence may be a common feature of ${\mathrm{KH}}_2{\mathrm{PO}}_4$ family mixed crystals.

• Received 13 May 1999

DOI:https://doi.org/10.1103/PhysRevB.62.223