  The basic magnetic and electronic properties of most binary compounds have
been well known for decades. Therefore the recent announcement of
superconductivity at 39 K in the simple binary ceramic compound MgB2 is
surprising. This compound, available from common chemical suppliers, and used
as a starting material for chemical metathesis reactions, has been known and
structurally characterized since the mid 1950's. 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 10% Al content. This indicates that
superconducting MgB2 is poised very near a structural instability at slightly
higher electron concentrations.
