A Free Energy Model of Boron Carbide

Pittsburgh, PA UNITED STATES Corresponding Author Secondary Information: Corresponding Author's Institution: Corresponding Author's Secondary Institution: First Author: William P. Huhn First Author Secondary Information: Order of Authors: William P. Huhn Michael Widom Order of Authors Secondary Information: Abstract: The assessed binary phase diagram of boron-carbon exhibits a single intrinsically disordered alloy phase designated ``B\sub{4}C'' 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=0K: one of composition B\sub{4}C with monoclinic symmetry; the other of composition B\sub{13}C\sub{2} 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=600K, along with a slight reduction on carbon content.