Pressure-Temperature Phase Diagram of Triammonium Hydrogen Disulfate (NH4)3H(SO4)2

The pressure-temperature phase diagram of (NH 4 ) 3 H(SO 4 ) 2 was studied by means of dielectric and differential thermal analysis measurements in a range of \(p{}8.5\) kbar, –100°C \({}T{}\)150°C. The I-II transition point (140°C at 1 atm) linearly decreases with increasing hydrostatic pressure with a slope of -5.76 deg kbar -1 . The pressure dependence of the II-III transition point is expressed by a quadratic equatuon T c = T 0 c + K p +γ p 2 with parameters of T 0 c =-8.1^° C , K =-3.7 deg kbar -1 , and γ=-0.13 deg kbar -2 . Two pressure-induced ferroelectric phases (Phases VI, VII) are found above about 5 kbar. The II-III-VI triple point is found at p =7.0 kbar, T =-40.5^° C .