B-N-O System at High Pressures and High Temperatures: Chemical Interaction and Phase Relations Vladimir L. Solozhenko LSPM–CNRS, Université Paris Nord, 93430 Villetaneuse, France The B–B2O3 and B–BN binary subsystems of the ternary B–N–O system have been studied in situ up to 6 GPa and 2800 K using MAX80 multianvil system and synchrotron X-ray powder diffraction at HASYLAB-DESY, and corresponding phase diagrams have been constructed. At 5 GPa, the equilibrium phase diagram of the B–B2O3 system is characterized by the congruent melting of B6O boron suboxide, and two eutectic equilibria, i.e. L ⇆ B6O + B at 2400 K and L ⇆ B2O3 + B at 1290 K. B6O was found to be the only thermodynamically stable boron suboxide which is a congruently melting compound that enters into eutectic reactions with boron and B2O3. Metastable B–B2O3 diagram is represented by the liquidus lines of -rhombohedral boron and -B2O3, and contains metastable eutectic equilibrium at 1130 K. New rhombohedral boron subnitride B13N2 has been synthesized by crystallization from the B–BN melt at 5 GPa. The structure of this phase belongs to the R-3m space group (a = 5.4585 Å, c = 12.253 Å) and represents a new structural type produced by the distorted B12 icosahedra linked together by N–B–N chains and inter-icosahedral B–B bonds. B13N2 is thermodynamically stable boron subnitride, while previously reported B50N2 is metastable. At 5 GPa, the phase diagram of the B–BN system is characterized by the following nonvariant equilibria: L + BN ⇆ B13N2 of peritectic type at ~2600 K; L ⇆ -B + B13N2 of eutectic type at ~2300 K; and L ⇆ -B + BN metastable eutectic at 2120 K that assures the appearance of the liquid phase, from which B13N2 crystallizes. The melting diagram of the B–B2O3–BN ternary system at 5 GPa has been constructed by combining the previously reported data for binary sub-systems and conventional phenomenological model describing ternary interactions. The interaction parameter of the model of ternary liquid phase has been determined by adjustment of experimental melting points to the calculated monovariant eutectic curve. The phase diagram is characterized by two ternary eutectics, one ternary transition-type equilibrium, and the maximum in monovariant eutectic curve.