Pressure and Temperature Dependence of the Dielectric Properties of Hydrogen-Bonded Ferroelectrics: Li${\mathrm{H}}_3$${\left(\mathrm{S}\mathrm{e}{\mathrm{O}}_3\right)}_2$ and Li${\mathrm{D}}_3$${\left(\mathrm{S}\mathrm{e}{\mathrm{O}}_3\right)}_2$

The dielectric properties of Li${\mathrm{H}}_3$ ${\left(\mathrm{S}\mathrm{e}{\mathrm{O}}_3\right)}_2$ (LHS) and Li${\mathrm{D}}_3$ ${\left(\mathrm{S}\mathrm{e}{\mathrm{O}}_3\right)}_2$ (LDS) were investigated as functions of hydrostatic pressure to 26 kbar and temperature 20-140°C. A ferroelectric-to-paraelectric transition can be induced in both crystals at high pressure, and their Curie points decrease linearly with pressure with slopes of -6.0±0.2°K/kbar for LHS and -5.9±0.4°K/kbar for LDS. The high-pressure results indicate that LHS and LDS would have Curie points of 147°C and 172°C, respectively, at 1 bar were it not for the fact that both melt at 110°C. The room-temperature phase of LDS may be antiferroelectric, but the experimental evidence is not conclusive. The experimental results on LHS are correlated by means of a phenomenological theory. The transition entropy calculated on the basis of this theory is 0.61 cal/mole °K as compared with a value of 0.69 cal/mole °K predicted from statistical mechanics for an order-disorder transition of the K${\mathrm{H}}_2$P${\mathrm{O}}_4$ type. The decrease of the Curie points of LHS and LDS with pressure are compared with the results on K${\mathrm{H}}_2$P${\mathrm{O}}_4$ recently reported elsewhere by the author and others. The similarities between LHS and K${\mathrm{H}}_2$P${\mathrm{O}}_4$ are discussed, and the point of view is taken that the ordering of the protons (which in turn leads to a distortion of the lattice) is the triggering mechanism for the ferroelectric transition in these crystals. The effects of pressure on their Curie points are discussed in terms of a model in which the proton moves in a double-minimum potential well along the length of the H bond. Increasing pressure decreases both the separation of the potential minima and the height of the potential barrier between them. Both effects lead to a lower Curie point.