Abstract
The experimental study of the phase diagram is hampered by strong kinetic effects leading to wide regions of metastability and to large uncertainties in the location of some phase boundaries. Here, we determine ’s thermodynamic phase boundaries by means of ab initio calculations of the Gibbs free energy of several solid phases of up to 50 Gigapascals. Temperature effects are included in the quasiharmonic approximation. Contrary to previous suggestions, we find that the boundary between molecular forms and the nonmolecular phase V has, indeed, a positive slope and starts at 21.5 GPa at . A triple point between phase IV, V, and the liquid phase is found at 35 GPa and 1600 K, indicating a broader region of stability for the nonmolecular form than previously thought. The experimentally determined boundary line between and phases is reproduced by our calculations, indicating that kinetic effects do not play a major role in that particular transition. Our results also show that is stabilized at high temperature and its stability region coincides with the conditions where phase VII has been reported experimentally; instead, phase II is the most stable molecular phase at low temperatures, extending its region of stability to every condition where phase III is reported experimentally.
- Received 25 October 2019
- Accepted 10 February 2020
DOI:https://doi.org/10.1103/PhysRevLett.124.095701
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