Ferroelectric Transition in Rubidium Bisulfate

RbHSO₄ is ferroelectric below -15^∘{}C. The room-temperature phase is monoclinic, with space group P2₁c, a=14.35₆ A, b=4.62₂ A, c=14.80₇ A, β=121.0^∘ and $Z=8$. The symmetry of the ferroelectric phase is 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 εc at 10 kc/sec and for a field of 5 v/cm is 10 at room temperature; as the temperature is lowered, εc 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 NH₄HSO₄, could be detected in the temperature range -15^∘{}C to -196^∘{}C.