Loss of superconductivity with the addition of Al to MgB₂ and a structural transition in Mg_1-xAl_xB₂

The basic magnetic and electronic properties of most binary compounds have been well known for decades. The recent discovery¹ of superconductivity at 39 K in the simple binary ceramic compound magnesium diboride, MgB₂, was therefore surprising. Indeed, this material has been known and structurally characterized since the mid 1950s (ref. 2), and is readily available from chemical suppliers (it is commonly used as a starting material for chemical metathesis reactions³). Here we show that the addition of electrons to MgB₂, through partial substitution of Al for Mg, results in the loss of superconductivity. Associated with the Al substitution is a subtle but distinct structural transition, reflected in the partial collapse of the spacing between boron layers near an Al content of 10 per cent. This indicates that superconducting MgB₂ is poised very near a structural instability at slightly higher electron concentrations.