In situ studies of boron nitride crystallization from BN solutions in supercritical N–H fluid at high pressures and temperatures

Abstract

The reaction of boron with products of the thermal decomposition of hydrazine and the crystallization of boron nitride from BN solutions in supercritical N–H fluid have been studies in situ at pressures up to 5.2 GPa and temperatures up to 1600 K using angle- and energy-dispersive X-ray diffraction with synchrotron radiation. Graphite-like boron nitride produced by the reaction between boron and N–H supercritical fluid dissolves in the latter to form associated solutions of different concentrations. On cooling, the disappearance of short-range order in this solution is observed, accompanied by the precipitation of solid phases (cBN or hBN and BN–NH₃ intercalation compound depending on the pressure, temperature and concentration). Spontaneous crystallization of cubic boron nitride has been observed down to 1.9 ± 0.2 GPa, which is the lowest pressure of the cBN crystallization from a solution reported so far. The BN–NH₃ hypothetical quasibinary section of the phase diagram of the B–N–H system at 4 GPa has been constructed.