Ammonium Hydrogen Sulfate: A New Ferroelectric with Low Coercive Field

(NH₄)HSO₄ is ferroelectric in the temperature range between -3^∘{}C and -119^∘{}C. The symmetry of the room-temperature phase is P2₁c; the ferroelectric phase is also monoclinic, with space group Pc; and the lowest phase, which is piezoelectric, has triclinic symmetry $P1$. All three phases are pseudo-orthorhombic. The coercive field is very low: about 150 v/cm at -13^∘{}C. The spontaneous polarization Pₛ is about 0.4 {μ}coul/cm² at that temperature, and rises to a maximum of about 0.8 {μ}coul/cm² just above -119^∘{}C. Pₛ falls to zero abruptly at the lower transition point. The higher transition is of second order, and the lower of first order.