Refined Phase Diagram of Boron Nitride

The equilibrium phase diagram of boron nitride thermodynamically calculated by Solozhenko in 1988 has been now refined on the basis of new experimental data on BN melting and extrapolation of heat capacities of BN polymorphs into high-temperature region using the adapted pseudo-Debye model. As compared with the above diagram, the hBN ⇆ cBN equilibrium line is displaced by 60 K toward higher temperatures. The hBN−cBN−L triple point has been calculated to be at 3480 ± 10 K and 5.9 ± 0.1 GPa, while the hBN−L−V triple point is at T = 3400 ± 20 K and p = 400 ± 20 Pa, which indicates that the region of thermodynamic stability of vapor in the BN phase diagram is extremely small. It has been found that the slope of the cBN melting curve is positive whereas the slope of hBN melting curve varies from positive between ambient pressure and 3.4 GPa to negative at higher pressures