On the problem of the phase relations in the B-BN system at high pressures and temperatures

Phase relations and chemical interaction in the B-BN system have been studied in situ using X-ray diffraction with synchrotron radiation at pressures to 5.3 GPa and temperatures to 2700 K. The results obtained have been used for the thermodynamic analysis of this system and for the construction of its phase diagram at 5 GPa. The values of thermodynamic functions of the phases competing in the B-BN system at high pressures and temperatures have been calculated in the framework of phenomenological models, unknown parameters of which have been found from the experimental data. It has been shown that only one thermodynamically stable boron subnitride (rhombohedral B₁₃N₂) exists in the system. It melts incongruently and forms an eutectic equilibrium with boron.