Absence of a structural transition up to 40 GPa in (formula presented) and the relevance of magnesium nonstoichiometry

We report measurements on (formula presented) up to (formula presented) Increasing pressure yields a monotonous decrease of the lattice parameters and of the (formula presented) ratio, but no structural transition down to parameters smaller than those of (formula presented) 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 (formula presented) 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.