Considerable excitement has been caused recently by the discovery that the binary-boride system with stoichiometry ${\mathrm{MgB}}_2$ is superconducting at the remarkably high temperature of 39 K [J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, and J. Akimitsu, Nature 410, 63 (2001)]. This potentially opens the way to even higher-$T_c$ 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-$T_c$ 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 ${\mathrm{MgB}}_2,$ revealing a stiff tightly packed incompressible solid with only moderate bonding anisotropy between intralayer and interlayer directions. These results, combined with the pressure evolution of the superconducting transition temperature, $T_c,$ establish its relation to the B and Mg bonding distances over a broad range of values.

• Received 27 February 2001

DOI:https://doi.org/10.1103/PhysRevB.64.012509