  The variation of TC in MgB2 has been systematically investigated for
monovalent cation substitution in Mg1-xLixB2 and in Mg1-xCuxB2 and simultaneous
cation and Carbon substitution in Mg0.80Li0.20B2-xCx and Mg0.95Cu0.05B2-xCx
using dc resistivity and ac susceptibility techniques. TC seems to be uniquely
determined by the electron count in the sample, being constant at the MgB2
value for electron counts lower than MgB2 but rapidly decreasing for larger
electron counts. The Debye temperature extracted from fitting normal state
resistivity to the Bloch-Gruneisen formula shows no systematics with TC while
the residual resistivity versus TC indicates that interband/intraband
scattering is affected by the substitutions.
