Internal-mode line-broadening by proton jumps in KH2PO4

The Raman linewidth of the 914 cm^?1 line of KH₂PO₄ in the ferroelectric phase has been measured as a function of temperature. The drastic decrease in linewidth with cooling near T_c is in fair agreement with a recent pseudo-spin-phonon coupling theory. A quantitative correlation has been found between the Raman linewidth, the spotaneous polarization and the proton spin-lattice relaxation time. A search for a similar effect on the corresponding line in KH₂AsO₄ gave negative results.