High-pressure phase diagram of silver mercury iodide (Ag2HgI4).

Abstract

Differential scanning calorimetry has been used to study the phase diagram of ${\mathrm{Ag}}_{2}$${\mathrm{HgI}}_{4}$ in the range up to 1 GPa and from 278 to 440 K. Stoichiometric samples were prepared by a solid-state reaction which gives pure ${\mathrm{Ag}}_{2}$${\mathrm{HgI}}_{4}$, while previous high-pressure studies have been on samples prepared by precipitation, a technique where the presence of minor amounts of AgI as impurity hardly can be avoided. The phase diagram is characterized by two triple points: 0.477 GPa, 328.5 K and 0.74 GPa, 400.5 K. At ambient temperature a transition between the ordered phases occurs at 0.62 GPa when the pressure is increased but at 0.47 GPa when the pressure is decreased. In both cases the transition enthalpy is strongly temperature dependent, indicating that there is either a triple point or a critical point near 270 K. The temperature of the order-disorder transition between the \ensuremath{\beta} and \ensuremath{\alpha} phases increases from 325 K at normal pressure to a maximum of 334 K at 0.42 GPa. The enthalpy of the \ensuremath{\beta}\ensuremath{\rightleftarrows}\ensuremath{\alpha} transition is 8.5 kJ/mol at normal pressure and 6.2 kJ/mol in the range 0.0001\char21{}0.477 GPa. The enthalpy of the \ensuremath{\gamma}\ensuremath{\rightleftarrows}\ensuremath{\alpha} order-disorder transition increases from 1.3 kJ/mol at 0.477 GPa to 6.8 kJ/mol at 0.74 GPa. Perhaps the most remarkable features of this phase diagram concern the transition of the disordered \ensuremath{\alpha} phase to other disordered high-temperature phases. At normal pressure the transition enthalpy is 11.5 KJ/mol at 425 K. The transition temperature has a maximum of 428 K at about 0.045 GPa. At higher pressures the transition changes its character, and it is clearly second order from, say, 0.16 GPa to a singularity point (minimum) at 0.474 GPa and 382 K. Beyond this point the transition enthalpy increases linearly from zero up to 11.9 kJ/mol at 0.74 GPa.