Phase Transition and EOS of Cinnabar (α-HgS) at High Pressure and High Temperature

Phase relations and equation of state (EOS) of natural cinnabar (α-HgS) are investigated by high-pressure and high-temperature synchrotron x-ray powder diffraction. The unambiguous cinnabar-rocksalt structure phase boundaries are determined to be P_lower(GPa)=15.54-0.014T°C and P_upper(GPa)=23.84-0.014T(°C) at 300-623K. With K' fixed at 4, we obtain K₀= 37(4)GPa, (∂K/∂ T)_P=-0.025(2)GPaK^-1, and α_0=3.79(20)×10^-5K^-1 for the cinnabar phase of α-HgS. The (K/∂T)_P and α₀ of cinnabar phase are obtained for the first time. A nearly isotropic compression of cinnabar phase is observed by linear regressions.