High-pressure neutron diffraction studies of boron carbide, ${\mathrm{B}}_4$C, to 11 GPa show that the icosahedral structural units are 23(4)% more compressible than the structure between them. This inverted-molecular compression is in accordance with qualitative predictions based on models of the bonding, but had not previously been observed directly. The results show the effect of the structural compression on the electrical resistivity to be more complex than present, semiquantitative models suggest.

• Received 18 August 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.2268