New Ferroelectrics of the Tetramethylammonium-Trihalo-Mercurate Family

Three new ferroelectrics belonging to a family of the general formula $\left[X{\left(\mathrm{C}{\mathrm{H}}_3\right)}_4\right]\mathrm{Hg}{Y}_3$ (where $X=\mathrm{N},\mathrm{P}$; $Y=\mathrm{C}\mathrm{l},\mathrm{B}\mathrm{r},\mathrm{I}$) were discovered. They are: [N${\left(\mathrm{C}{\mathrm{H}}_3\right)}_4$]Hg${\mathrm{Br}}_3$, [N${\left(\mathrm{C}{\mathrm{H}}_3\right)}_4$]Hg${\mathrm{I}}_3$, and [P${\left(\mathrm{C}{\mathrm{H}}_3\right)}_4$]Hg${\mathrm{Br}}_3$. The preparation of single crystals of these compounds (monoclinic, space group $P{2}_1$) is described. The dielectric constants are of the order of 10, the spontaneous polarizations between 1 and 3 μcoul/${\mathrm{cm}}^2$. Curie points could not be observed because of thermal decomposition of the compounds around 170°C. The switching time varies with the seventh power of the applied field $E$.