Structural, mechanical and electronic properties and phase transitions in superhard BC1-xNx: A first-principles study

First-principles investigations of the stability, mechanical properties, electronic and phonon structures, as well as molecular dynamics simulations of temperature- and pressure-induced phase transitions in boron carbonitrides, BC1-xNx, for x = 1.0 (C0), 0.75 (C1), 0.5 (C2), 0.25 (C3) and 0.0 (C4), were carried out. The plausible mechanisms of the phase transitions in BNs were proposed and the transformation paths were established for the h → w, r → c, c → r and w → h’ transitions, where h’ was the new hP4-187 phase (space group P-6m2, #187). The non-layered C0–C3 compounds transformed into the layered ones at 2000–3000 K. The new BC1-xNx structures are brittle materials and exhibit the best combination of hardness (23.1–60.8 GPa) and fracture toughness (4.03–5.23 MPa m1/2). The C1–C4 structures are metallic, except for three compounds: C1-oP8-25 and C4-tP8-136 are semi-metals, and C4-mP8-10 is a semiconductor with a very narrow bandgap of 0.2 eV.