Brillouin scattering investigation of paraelectric KH2PO4near the tricritical point

Abstract

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 (\ensuremath{\sim} 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 ${10}^{6}$.