Compressibility and electronic structure ofMgB2up to 8 GPa

The lattice parameters of ${\mathrm{MgB}}_2$ up to a pressure of 8 GPa were determined using high-resolution x-ray powder diffraction in a diamond anvil cell. The bulk modulus, $B_0,$ was determined to be $151\pm 5\hspace{0.5em}\mathrm{GPa}.$ Both experimental and first-principles calculations indicate nearly isotropic mechanical behavior under pressure. This small anisotropy is in contrast to the two-dimensional nature of the boron p states. The pressure dependence of the density of states at the Fermi level and a reasonable value for the average phonon frequency $〈\omega 〉$ account within the context of BCS theory for the reduction of $T_c$ under pressure.