  Considerable excitement has been caused recently by the discovery that the
binary boride system with stoichiometry MgB2 is superconducting at the
remarkably high temperature of 39 K (1). This potentially opens the way to even
higher Tc values in a new family of superconductors with unexpectedly simple
composition and structure. The simplicity in the electronic and crystal
structures could allow the understanding of the physics of high-Tc
superconductivity without the presence of the multitude of complicated
features, associated with the cuprates. Synchrotron X-ray diffraction was used
to measure the isothermal compressibility of MgB2, revealing a stiff
tightly-packed incompressible solid with only moderate bonding anisotropy
between intra- and inter-layer directions. These results, combined with the
pressure evolution of the superconducting transition temperature, Tc establish
its relation to the B and Mg bonding distances over a broad range of values.
