Prediction of Orientational Phase Transition in Boron Carbide

Abstract

The assessed binary phase diagram of boron-carbon exhibits a single 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=0K: 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 rhombohedral phase above T=600K, along with a slight reduction on carbon content.

9 pages; 3 figures; 1 table; 17th International Symposium on Boron, Borides, and Related Materials

References in corpus (1)

• Symmetry-broken crystal structure of elemental boron at low temperature

Cited by in corpus (8)

• First-principles study of configurational disorder in B${}_4$C using a superatom-special quasirandom structure method
• Predicted phase diagram of B-C-N
• Phase Transitions of Boron Carbide: Pair Interaction Model of High Carbon Limit
• A Free Energy Model of Boron Carbide
• Phase Diagram of Boron Carbide With Variable Carbon Composition
• Understanding first order Raman spectra of boron carbides across the homogeneity range
• Properties of B4C in the shocked state for pressures up to 1.5 TPa
• Hydrogenation, dehydrogenation of $\alpha$-tetragonal boron and its transition to $\delta$-orthorhombic boron