Lattice Properties of MgB2 versus Temperature and Pressure

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.