Ferroelectricity in H·NH4(ClCH2COO)2

H·N${\mathrm{H}}_4$ ${\left(\mathrm{C}\mathrm{l}\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\right)}_2$ is ferroelectric below -145°C. At room temperature, the crystal is monoclinic with $a=10.56\mathrm{}$, $b=11.51\mathrm{}$, $c=8.36\mathrm{}$ Å, $\beta =119\mathrm{}°{50}^{\prime }$, and space group is most probably $\frac{C2\mathrm{}}{c}$. The density is 1.528 g/${\mathrm{cm}}^3$. The unit cell contains four formula units. The ferroelectric axis is along the [102] direction. The spontaneous polarization is 0.18μC/${\mathrm{cm}}^2$ and the coercive field is 1.2 kV/cm at -195°C. Along the ferroelectric axis the dielectric constants obey the Curie-Weiss law with a Curie-Weiss constant of 44°C. The domain-wall motion seems to contribute to the initial dielectric constants if the frequency of applied ac field is as low as 10 kc/sec. The phase transition seems to be of the first order.