New Experimental Results on the Phase Diagram of Boron Nitride

In order to clarify the discrepancy of the phase diagram on boron nitride as it is found in the literature we have made numerous in situ diffraction experiments using synchrotron radiation. The conditions were temperatures in the range of 1600?C and pressures up to 6.5 GPa. For the experiments diamond anvil squeezers and the MAX80 high-pressure/high-temperature device installed at the DESY synchrotron facility in Hamburg/Germany were used. We studied the transformation from hBN to cBN at 6.5 GPa/1200?C and the backtransformation from cBN to hBN around 0.9 to 2 GPa. The experiments included kinetics measurements. The experiments verified the theoretical results by V. L. Solozhenko (1991, High Pressure Res. 7, 201) and J. Maki et al. (1991, "Proceedings II: International Conference on New Diamonds Research and Technology"). Further on the calculations for the equilibrium boundary between cBN and hBN were repeated including uncertainties of the thermodynamic data. The cubic phase, cBN, is definitely the stable phase, in contrast to metastable diamond