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

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.