  We have studied the composition-induced metal-to-insulator transitions of
cation substituted Lithium Titanate, in the forms Li1+xTi2-xO4 and
LiAlyTi2-yO4, utilising a quantum site percolation model, and we argue that
such a model provides a very reliable representation of the noninteracting
electrons in this material if strong correlations are ignored. We then
determine whether or not such a model of 3d electrons moving on the Ti
(corner-sharing tetrahedral) sublattice describes the observed
metal-to-insulator transitions, with the critical concentration defined by the
matching of the mobility edge and the chemical potential. Our analysis leads to
quantitative predictions that are in disagreement with those measured
experimentally. For example, experimentally for the LiAlyTi2-yO4 compound an Al
concentration of y_c approximately 0.33 produces a metal-to-insulator
transition, whereas our analysis of a quantum site percolation model predicts
y_c approximately 0.83. One hypothesis that is consistent with these results is
that since strong correlations are ignored in our quantum site percolation
model, which includes the effects of configurational disorder only, such strong
electronic correlations are both present and important.
