Transverse susceptibility in KH2PO4-type crystals

A well-known model of ferroelectricity in potassium dihydrogen phosphate type crystals is extended to provide a description of observed transverse susceptibility in K${\mathrm{H}}_2$P${\mathrm{O}}_4$ and K${\mathrm{H}}_2$AsP${\mathrm{O}}_4$, the temperature dependence of which exhibits the expected antiferroelectric characteristics. Experimental data can be used in principle to determine the bond-bond interaction parameter $J$ and the tunneling integral $\Gamma$. Available data provide satisfactory order-of-magnitude results, and are in qualitative agreement with determinations based on other experiments with regard to the relative values of $\Gamma$ in two isomorphs. It is shown that more refined experimental data would contribute to a further understanding of this example of simultaneous ferroelectricity and antiferroelectricity in a single medium.