Predicted phase diagram of B-C-N

TL;DR

This paper predicts the phase diagram of B-C-N materials using first principles and statistical mechanics, revealing negligible solubility between certain ultra hard materials and a stable B13CN compound with an order-disorder transition.

Contribution

It introduces a comprehensive predicted phase diagram for B-C-N compounds, highlighting the stability of B13CN and solubility behaviors at finite temperatures.

Findings

Negligible mutual solubility between diamond and cubic BN.

Continuous solubility range connecting boron carbide and boron subnitride at high temperatures.

B13CN is stable up to melting and has an order-disorder transition around 500K.

Abstract

Noting the structural relationships between phases of carbon and boron carbide with phases of boron nitride and boron subnitride, we investigate their mutual solubilities using a combination of first principles total energies supplemented with statistical mechanics to address finite temperatures. Owing to large energy costs of substitution, we find the mutual solubilities of the ultra hard materials diamond and cubic boron nitride are negligible, and the same for the quasi-two dimensional materials graphite and hexagonal boron nitride. In contrast, we find a continuous range of solubility connecting boron carbide to boron subnitride at elevated temperatures. The electron precise compound B$_{13}$CN consisting of B$_{12}$ icosahedra with NBC chains is found to be stable at all temperatures up to melting. It exhibits an order-disorder transition in the orientation of NBC chains at approximately T=500K.