  Compressibility of high-purity nanostructured cBN has been studied under
quasi-hydrostatic conditions at 300 K up to 35 GPa using diamond anvil cell and
angle-dispersive synchrotron X-ray powder diffraction. A data fit to the Vinet
equation of state yields the values of the bulk modulus B0 of 375(4) GPa with
its first pressure derivative B0' of 2.3(3). The nanometer grain size (\sim20
nm) results in decrease of the bulk modulus by ~9%.
