Prediction of Orientational Phase Transition in Boron Carbide

TL;DR

This paper predicts a temperature-driven phase transition in boron carbide, revealing two distinct low-temperature phases and their transformation at higher temperatures through first principles calculations and statistical mechanics.

Contribution

It introduces the prediction of two stoichiometric phases in boron carbide and details their phase transition at finite temperatures, which was not previously understood.

Findings

Two low-temperature phases: B4C with monoclinic symmetry and B13C2 with rhombohedral symmetry.

Monoclinic phase reverts to rhombohedral above 600K.

Slight reduction in carbon content at transition temperature.

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.