Thermoelastic behaviour of hexagonal graphite-like boron nitride

A synchrotron X-ray diffraction study on hexagonal graphite-like boron nitride (h-BN) was performed under high pressures and temperatures. From the measured P-V-T relation for h-BN (with a three-dimensional ordering parameter P-3 = 0.9) in the temperature range from 298 to 1273 K and up to 6.7 GPa, the thermoelastic parameters are derived by fitting a modified high temperature Birch-Murnaghan equation of stale. The results are: bulk modulus B-0[GPa] = 27.6 - 0.0081(T[K] - 298) and its pressure derivative B' = 10.5 + 0.0016(T[K] - 298). These values are for samples with P-3 = 0.9 and are quite different for samples with different values of the order parameter. This parameter is shown to have a leading role in the determination of the thermoelastic properties of h-BN, which explains and reconciles the differences between previous results.