A high-resolution Brillouin scattering study of the soft acoustic mode of K${\mathrm{H}}_2$P${\mathrm{O}}_4$ in the hydrostatic-pressure range 0-4 kbar is presented. All spectra could be fitted closely by the usual mean-field coupled-mode model. From those fits, the pressure dependence of the ferroelectric-mode relaxation rate is derived. At pressures near and above the tricritical point, the transition can be approached so closely that the Brillouin spectrum becomes strongly overdamped and very narrow (∼ 50 MHz). Under those conditions, the elastic central peak of even the best available crystal becomes a dominant spectral feature, while the maximum dielectric constant is close to ${10}^6$.

• Received 8 November 1982

DOI:https://doi.org/10.1103/PhysRevB.27.4359