X-ray diffraction with synchrotron radiation has been used to study in situ the chemical interaction of beta-rhombohedral boron with boron (III) oxide and phase relations in the B-B2O3 system at pressures up to 6 GPa in the temperature range from 300 to 2800 K. The B-B2O3 system has been thermodynamically analyzed, and its equilibrium phase diagram at 5 GPa has been constructed. Only one thermodynamically stable boron suboxide, B6O, exists in the system. It forms eutectic equilibria with boron and B2O3.

The chemical interaction and phase relations in the B-BN system have been in situ studied at 5 GPa and temperatures up to 2800 K using X-ray diffraction with synchrotron radiation. The thermodynamic analysis of the B-BN system based on experimental data allowed us to construct equilibrium and metastable phase diagrams of the system at 5 GPa. The only thermodynamically stable boron subnitride, B(13)N(2), melts incongruently at 2600 K and forms eutectic equilibrium with boron at 2300 K and 4 at. % of nitrogen.

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