In-situ investigations of the reversible hBN-cBN-hBN-transformation in the Li3N−BN\mathrm{Li_3N-BN} catalyst system using synchrotron radiation

The kinetics of the transformation of boron nitride's hexagonal form (hBN) into the polymorphic cubic high pressure phase (cBN) were studied in the Li3N-BN catalyst system under in-situ pressure and temperature conditions. Energy-dispersive X-ray experiments were performed in a MAX 80 high pressure cell at HASYLAB, DESY, Hamburg using synchroton radiation. The transformation of hBN into cBN and the reverse transformation of cBN into hBN within the same experimental run were examined. Simultaneously the kinetics of the transformation were determined in the pressure range 0.65-6.5 GPa and at temperatures between 600–1400 °C. It could be shown that, in most cases, the transformation was rather fast and it was completed in less than 5 min. The observed data confirm the experimental results and the phase diagram as given by Bundy and Wentorf, but are in contrast to the experimental results and thermodynamic calculations by Solozhenko and by Maki