Tricritical point in the p-T phase diagram of Ag2HgI4

It is proven by differential scanning calorimetry that silver mercury iodide has a tricritical point at 0.474 GPa and 382 K. The phase transition is of second order on the low-pressure side, while both the transition enthalpy Δ H and the transition volume Δ V reduce gradually to nil when the tricritical point is approached from the high-pressure side. Furthermore, both Δ H and the inverse of the heat capacity anomaly, 1/Δ C p , are proportional to the pressure difference p-p cr and to the temperature difference T-T cr which is in accordance with a theory by Landau from 1935. It is essential for the existence of the tricritical point that the phase transition occurs between two disordered phases. The probability of tricritical points in other salts with a high ionic conductivity is discussed.