Prediction of orientational phase transition in boron carbide

The assessed binary phase diagram of boron–carbon exhibits a single intrinsically disordered alloy phase designated “B₄C” with rhombohedral symmetry occupying a broad composition range that falls just short of the nominal carbon content of 20%. As this composition range is nearly temperature independent, the phase diagram suggests a violation of the third law of thermodynamics, which typically requires compounds to achieve a definite stoichiometry at low temperatures. By means of first principles total energy calculations we predict the existence of two stoichiometric phases at T = 0 K: one of composition B₄C with monoclinic symmetry; the other of composition B₁₃C₂ with rhombohedral symmetry. Using statistical mechanics to extend to finite temperatures, we demonstrate that the monoclinic phase reverts to the observed disordered nonstoichiometric rhombohedral phase above T = 600 K, along with a slight reduction on carbon content.