Theoretical phase diagram of boron carbide B_xC from ambient to high pressure (<i>P</i>) and high temperature (<i>T</i>): the “single phase” regime in the light of the density functional theory (DFT) results

The phase diagram of boron carbide is calculated within DFT as a function of T and P up to 80 GPa, accounting for icosahedral, graphite- and diamond-like atomic structures [1]. Only some icosahedral phases turn out to be thermodynamically stable with atomic carbon concentrations (c) of resp. 8.7% (B_10.5C), 13.0% (B_6.7C), 20% (B₄C) and 28.6% (B2.5C).
Their respective ranges of stability under pressure and temperature are calculated, and the theoretical T-P-c phase diagram boundaries are discussed. At ambient conditions, the introduction in the phase diagram of the new phase B_10.5C, with an ordered crystalline motif of 414 atoms, is shown to bring the theoretical solubility range of carbon in boron close to the experimental one. The effect of configurational entropy is studied at finite temperature, and the convex hull modified. We discuss the occurrence of the “single phase regime” in the light of these new results.
[1] A. Jay, O. Hardouin Duparc, J. Sjakste and N. Vast, J. Appl. Phys. 125, 185902 (2019). https://doi.org/10.1063/1.5091000