Vladimir L Solozhenko
CNRS, France
Title: Chemical interaction in the Bâ€P system at high pressure: A route to new advanced materials
Biography
Biography: Vladimir L Solozhenko
Abstract
Chemical interaction and phase relations in the boron–phosphorus system have been systematically studied up to 8 GPa and 2800 K using synchrotron X-ray diffraction. In the whole studied pressure range solid phosphorus does not react with boron. At 5.5 GPa, phosphorus melts at 1300 K that is accompanied by appearance of cubic BP. However, unreacted solid boron and liquid phosphorus are still present, even at 2000 K. For the mixture with BP stoichiometry, the reaction is completed only above 2100 K, and quenched samples are single-phase boron phosphide. For reaction mixture of the B6P stoichiometry, formation of BP is observed immediately after phosphorus melting, while formation of rhombohedral boron subphosphide B12P2 starts only above 1750 K, and both boron phosphides coexist in the 1750-2000 K range. Above 2000 K, only B12P2 is present in the system, and quenched samples are single-phase boron subphosphide.
Similar behavior in the B–P system is observed at lower pressures. All this is indicative of the substantial kinetic barrier of reaction between elemental boron and liquid phosphorus at temperatures below 2000 K, perhaps due to a high viscosity of phosphorus melt under pressure.
At pressures to 9 GPa BP melts congruently, and the melting curve exhibits negative slope (-60 K/GPa), which is indicative of a higher density of the melt as compared to the solid phase. B12P2 also melts congruently, but the melting curve has a positive slope of +23 K/GPa. At pressures to 26 GPa, the icosahedral crystal structure of B12P2 remains stable up to the melting.
Bulk polycrystalline BP and B12P2 synthesized at 7.7 GPa by crystallization from B–P melt exhibit Vickers hardness of 28 GPa and 35GPa, respectively, which is in agreement with theoretical values predicted by the thermodynamic model of hardness.