Loss of superconductivity with the addition of Al to MgB2 and a structuraltransition in Mg1-xAlx

The basic magnetic and electronic properties of most binary compounds have been well known for decades. The recent discovery(1) of superconductivity a t 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 reactions(3)). Here we show that the addition of electrons to M gB2, through partial substitution of Al for Mg, results in the loss of supe rconductivity. Associated with the Al substitution is a subtle but distinct structural transition, reflected in the partial collapse of the spacing be tween boron layers near an Al content of 10 per cent. This indicates that s uperconducting MgB2 is poised very near a structural instability at slightl y higher electron concentrations.