Phase formation in the B-C-N system at high-pressures and temperatures: in situ studies

Thermal phase stability of graphite-like BC4N and BC3.88N has been studied up to 7 GPa and 2100 K using in situ powder diffraction of synchrotron radiation. Measurement of 298-K equation of state of g-BC4N resulted in zero-pressure bulk modulus of 18.1+/-0.2 GPa and its pressure derivative of 6.6+/-0.1. It has been shown that the process of g-BC4N decomposition at ambient pressure starts at 2050 K and occurs to form highly ordered hexagonal graphite-like boron nitride (hBN) and disordered graphite. With the pressure increase up to 6.6 GPa, the temperature of the decomposition onset drops linearly to 1070 K. At this pressure in the 1070-1400 K range, the decomposition products remain the same, while at higher temperatures, decomposition of g-BC4N is accompanied by the formation of cubic boron nitride (cBN) and disordered graphite. From the kinetic analysis the activation energy of direct transformation g-BC4N-to-cBN was found to be 185+/-5 kJ/mole at 6.6 GPa. Thermodynamic analysis has shown that the g-BC4N decomposition is a nonequilibrium process and the phase itself is metastable. Thermal decomposition of g-BC3.88N at 6.6 GPa is observed at considerably higher temperatures (above 1360 K) and is also accompanied by formation of hBN, disordered graphite and cBN (starting from 1550 K only). In the 1650-1900 K range, a simultaneous formation of both cBN and hBN is observed.