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

Dielectric properties of single crystals of (NH 4 ) 3 H(SO 4 ) 2 were measured at various hydrostatic pressures up to about 8 kbar below room temperature. The broad peak of dielectric constant along the c * -direction which appears around -2.5°C at 1 atm becomes more and more prominent as pressure increases, and then a ferroelectric phase (Phase VI) develops above 4.9 kbar. Another ferroelectric phase (Phase VII) is also found above about 5 kbar in a lower temperature region. The pressure-temperature phase diagram is given.