  Superconductivity and structural properties of Mg1-xAlxB2 materials have been
systematically investigated. Evident modifications in both superconductivity
and microstructure are identified to originate from the Al ordering along the
c-axis direction. The resultant superstructure phase has an optimal composition
of MgAlB4 with the superconducting transition at around 12K. Brief diagrams
illustrating the superconductivity and structural features of Mg1-xAlxB2
materials along with the increase of Al concentration are presented.
