Pressure and temperature dependence of the dielectric properties of Cs${\mathrm{H}}_2$P${\mathrm{O}}_4$ and Cs${\mathrm{D}}_2$P${\mathrm{O}}_4$

The pressure (0-7 kbar) and temperature (-200-+10°C) dependence of the dielectric properties (relative permittivity ${\epsilon }_r$ and spontaneous polarization $P_s$) of Cs${\mathrm{H}}_2$P${\mathrm{O}}_4$ and Cs${\mathrm{D}}_2$P${\mathrm{O}}_4$ were investigated. By the observation of the $P-E$ double hysteresis loops it is confirmed that the pressure-induced phase (at pressures above the critical pressure $p_c$) is antiferroelectric for both compounds. The pressure-temperature phase diagrams of both compounds were dielectrically determined. The pressure effects on the dielectric properties were found to be striking in comparison with those of the other K${\mathrm{H}}_2$P${\mathrm{O}}_4$-type crystals: (i) the $P_s$ and the Curie constant decrease strongly with pressure at pressures below $p_c$; and (ii) the critical electric field which induces the forced transition has a very small value and its pressure dependence is very large at pressures above $p_c$. Such facts reflect the one-dimensional nature characterized by the chainlike ordering of the hydrogen bonds along the $b$ axis. The dielectric properties associated with the paraelectric-ferroelectric and -antiferroelectric transitions are explained by introducing into an Ising spin model (containing the proton tunneling) a spin-spin interaction term within the chain and an interaction term between spins among the chains along the $b$ axis.