Calculation of Thermodynamic and Crystal Structure Changes of HgS Phase Transition at Conventional Temperature

Cinnabar is a commonly used red natural mineral pigment in various kinds of cultural relics discovered around the world due to its bright color and stable properties. Cinnabar pigments fades to dark red or even black after long-term aging, affecting further research on the historical, artistic and scientific values of cultural relics. The study on the mechanism of cinnabar discoloration can provide a scientific basis for the restoration and preventive protection of cultural relics. The phase transformation from cinnabar to black β-HgS is one of the possible mechanisms of cinnabar discoloration. The theoretical calculation based on density functional theory was carried out to reveal the crystal structure and properties of α-HgS (main component of cinnabar) and its paramorphs. Crystallographic alteration and thermodynamic in the phase transformation process among HgS paramorphs were investigated. The results indicated that it is impossible for α-HgS to transform into β-HgS at room temperature and pressure, and the discoloration of cinnabar was caused by other external factors.