A synchrotron X-ray diffraction study on hexagonal graphite-like boron nitr
ide (h-BN) was performed under high pressures and temperatures. From the me
asured 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 tempera
ture Birch-Murnaghan equation of stale. The results are: bulk modulus B-0[G
Pa] = 27.6 - 0.0081(T[K] - 298) and its pressure derivative B' = 10.5 + 0.0
016(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 pa
rameter is shown to have a leading role in the determination of the thermoe
lastic properties of h-BN, which explains and reconciles the differences be
tween previous results.