The pressure dependence of the coupled-mode spectra in potassium dihydrogen phosphase (KDP) was measured at room temperature for hydrostatic pressures from 0 to 9.3 kbar. The soft mode, which is overdamped for all temperatures at atmospheric pressure, becomes underdamped for pressures ≥ 6 kbar. This is the first observation of an underdamped soft mode for a crystal of the KDP class and resolves the question of whether the excitation should be considered as diffusive or oscillatory in nature.

• Received 6 June 1973

DOI:https://doi.org/10.1103/PhysRevLett.31.379