  The two-gap model for superconductivity in MgB2 predicts that interband
impurity scattering should be pair breaking, reducing the critical temperature.
This is perhaps the only prediction of the model that has not been confirmed
experimentally. It was previously shown theoretically that common
substitutional impurities lead to negligible interband scattering - if the
lattice is assumed not to distort. Here we report theoretical results showing
that certain impurities can indeed produce lattice distortions sufficiently
large to create measurable interband scattering. On this basis, we predict that
isoelectronic codoping with Al and Na will provide a decisive test of the
two-gap model.
