Vanishing of the Ferroelectric and Antiferroelectric States in KH2PO2-Type Crystals at High Pressure

The ferroelectric phase of K${\mathrm{H}}_2$P${\mathrm{O}}_4$ and the antiferroelectric phase of N${\mathrm{H}}_4$ ${\mathrm{H}}_2$P${\mathrm{O}}_4$ vanish at hydrostatic pressures >17 and 33 kbar, respectively. The results are qualitatively discussed in terms of the coupled proton-lattice model developed by Kobayashi and others, and they reemphasize the important role of the hydrogen bond in these crystals. We discuss possible important changes with increasing pressure in the character of the ferroelectric mode in K${\mathrm{H}}_2$P${\mathrm{O}}_4$.