Phase relations of HgI2 at pressures < 0.7 GPa studied by differential thermal analysis especially the red ⇌ yellow transition

Abstract

Differential thermal analyses of high-purity HgI₂, under Ar pressure of ≲ 0.7 GPa, narrowly constrain the location of the phase boundary between the high-temperature yellow and low-temperature red polymorphs. This transition is unique among solid–solid transitions because of its maximum in transition temperature, with the initially less dense yellow phase becoming, with pressure, as dense and then denser than the red phase. The red → yellow and yellow → red transition temperatures, obtained mostly at heating and cooling rates of 30–60 K min^–1, were from four runs on one sample plus a single run on another. The data suggest an initial slope dT_t/dp= 0.237 ± 0.021 µK Pa^–1 and initial curvature –d²T_t/dp²= 5.2–5.8 mK Pa^–2. From the areas of the differential temperature peaks transition enthalpies and then transition entropies were estimated; ΔS_t/R decreases rapidly with pressure from 0.81 ± 0.06 at 0.1 MPa to ca. 0.2₅ near the maximum at ca. 0.4₃ GPa. That the hysteresis in the transition varies little, despite the considerable variation in transition volume, is ascribed to the negligible role of strain energy, because of plastic deformation. A single run corroborated the Bardoll–Tödheide results for the high-temperature, high-pressure transitions between yellow solid, liquid and the S₃ polymorph; the triple point is located near 0.32 ± 0.01 GPa and 335 ± 3 °C and changes in entropy and volume are estimated there.