Lattice properties ofMgB2versus temperature and pressure

We have determined the structural properties of the superconducting compound ${\mathrm{MgB}}_2$ as a function of temperature from 11 to 297 K and as a function of hydrostatic pressure up to 0.62 GPa using neutron powder diffraction. This compound, when compared to other diborides with the same structure, is characterized by unusually large anisotropies of both the thermal expansion and compressibility, with the c-axis responses being substantially larger. We speculate that the comparatively weaker metal-boron bonding in ${\mathrm{MgB}}_2$ manifested by these lattice responses, is important for establishing the structural features that give rise to high-$T_c$ superconductivity in this structure type.