  We have determined the structural properties of the superconducting compound
MgB2 as a function of temperature from 11 K 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 in both
cases. We speculate that the comparatively weaker metal-boron bonding in MgB2,
manifest by these lattice responses, is important for establishing the
structural features that give rise to high Tc superconductivity in this
structure type.
