Absence of a structural transition up to 40 GPa in MgB2 and the relevance of magnesium nonstoichiometry -: art. no. 172502

We report measurements on MgB2 up to similar to 40 GPa. Increasing pressure yields a monotonous decrease of the lattice parameters and of the c/a ratio, but no structural transition down to parameters smaller than those of AlB2 The transition superconducting temperature also decreases with temperature in a sample-dependent way. The results are explained by an increase of the filling of the two-dimensional p(xy) bands with pressure, the Mg stoichiometry determining the starting position of the Fermi level. Our measurements indicate that these hole bands are the relevant ones for superconductivity.