Pressure-Temperature Phase Diagram of Ferroelectric Ammonium Bisulfate NH₄HSO₄

The pressure-temperature phase diagram of ferroelectric NH 4 HSO 4 was studied by means of dielectric and differential thermal analysis (DTA) measurements in a pressure and temperature region of \(p{}8×10³\) kg cm -2 , –120°C \({}T{}\)200°C. The high pressure phase found by Bridgman above about 1×10 3 kg cm -2 at room temperature is proven not to be identical with the low temperature non-ferroelectric phase (Phase III) at 1 atm. The liquidus line was newly determined by DTA. A new high pressure phase (Phase VI) was found above 4×10 3 kg cm -2 just below the melting point. A change in the order of the transition at the upper Curie point was found at about 0.6×10 3 kg cm -2 .