Neutron- and x-ray-diffraction measurements of the bulk modulus of boron

The bulk modulus of \ensuremath{\beta}-boron has been measured up to 10 GPa by neutron powder diffraction using an opposed-anvil pressure cell. The bulk modulus of \ensuremath{\alpha}-boron has also been measured to 5 GPa by single-crystal x-ray techniques. The values have been determined to be 185(7) GPa for \ensuremath{\beta}-boron and 224(15) GPa for \ensuremath{\alpha}-boron. These values are markedly smaller than those measured for other hard materials, but total-energy calculations for \ensuremath{\alpha}-boron are in reasonable agreement.