Ferroelectric Transition in Rubidium Bisulfate

RbHS${\mathrm{O}}_4$ is ferroelectric below -15°C. The room-temperature phase is monoclinic, with space group $\frac{P{2}_1}{c}$, $a={14.35}_6$ A, $b={4.62}_2$ A, $c={14.80}_7$ A, $\beta =121.0\mathrm{}°$ and $Z=8\mathrm{}$. The symmetry of the ferroelectric phase is $\mathrm{Pc}$, as established by systematic x-ray absences and the fact that spontaneous polarization appears along the $c$ axis below -15°C. Both the high- and low-temperature phases are pseudo-orthorhombic. The dielectric constant ${\epsilon }_c$ at 10 kc/sec and for a field of 5 v/cm is 10 at room temperature; as the temperature is lowered, ${\epsilon }_c$ rises to a sharp peak of ∼240 at -15°C and falls to ∼5 at -196°C. The transition appears to be of second order. No second transition, as in the case of isomorphous N${\mathrm{H}}_4$HS${\mathrm{O}}_4$, could be detected in the temperature range -15°C to -196°C.