Phase Stability of Graphitelike BC 4 N up to 2100 K and 7 GPa

Thermal phase stability of graphitelike BC 4 N (g‐BC 4 N) has been studied up to 7 GPa and 2100 K using in‐situ powder diffraction of synchrotron radiation. It has been shown that the process of g‐BC 4 N decomposition at ambient pressure starts at 2050 K and occurs to form highly ordered hexagonal graphitelike boron nitride (hBN) and disordered graphite. As the pressure increases up to 6.6 GPa, the temperature of the decomposition onset decreases to 1070 K. At this pressure in the 1070‐1400 K temperature range (decomposition degree of less than or equal to 0.1), the decomposition products remain the same, whereas at higher temperatures, the decomposition of g‐BC 4 N is accompanied by the formation of cubic boron nitride (cBN) and disordered graphite. Measurement of 298 K equation of state of g‐BC 4 N results in a zero‐pressure bulk modulus of 18.1 ± 0.2 GPa and its pressure derivative of 6.6 ± 0.1. Thermodynamic analysis has shown that the g‐BC 4 N decomposition is a nonequilibrium process and the phase itself is metastable.