Pressure-Induced Ferroelectricity in (NH4)3H(SO4)2

Dielectric properties of single crystal (NH 4 ) 3 H(SO 4 ) 2 were measured in a pressure and temperature region of \(p{}8.5\) kbar and -100°C\({}T{}RT\). A broad peak of the dielectric constant along the c * -direction which is found around -25°C at 1 atm becomes sharp and intense as pressure increases, and then above 5.0 kbar a ferroelectric phase (Phase VI) appears. Further increase in pressure stabilizes another ferroelectric phase (Phase VII) in a lower temperature region. The VI-VII transition is of the first order. Temperature dependence of the spontaneous polarization was measured at various pressures. A two-sublattice model is presented to interpret the pressure and temperature dependence of the dielectric properties of (NH 4 ) 3 H(SO 4 ) 2 .