Phase Diagram of the B–BN System at Pressures up to 24 GPa: Experimental Study and Thermodynamic Analysis

Phase relations in the B–BN system have been studied ex situ and in situ at pressures 2–20 GPa and temperatures up to 2800 K. The evolution of topology of the B–BN phase diagram has been investigated up to 24 GPa using models of phenomenological thermodynamics with interaction parameters derived from our experimental data on phase equilibria at high pressures and high temperatures. There are two thermodynamically stable boron subnitrides in the system i.e. B$_{13}$N$_2$ and B$_{50}$N$_2$. Above 16.5 GPa, the $B_{50}N_2 ⇄ L + B_{13}N_2$ peritectic reaction transforms to the solid-phase reaction of $B_{50}N_2$ decomposition into tetragonal boron $(t′-B_{52})$ and B$_{13}$N$_2$, while the incongruent type of B$_{13}$N$_2$ melting changes to the congruent type only above 23.5 GPa. The constructed phase diagram provides fundamentals for directed high-pressure synthesis of superhard phases in the B–BN system