  Metal to insulator transitions (MITs) driven by strong electronic
correlations are common in condensed matter systems, and are associated with
some of the most remarkable collective phenomena in solids, including
superconductivity and magnetism. Tuning and control of the transition holds the
promise of novel, low power, ultrafast electronics, but the relative roles of
doping, chemistry, elastic strain and other applied fields has made systematic
understanding difficult to obtain. Here we point out that existing data on
tuning of the MIT in perovskite transition metal oxides through ionic size
effects provides evidence of systematic and large effects on the phase
transition due to dynamical fluctuations of the elastic strain, which have been
usually neglected. This is illustrated by a simple yet quantitative statistical
mechanical calculation in a model that incorporates cooperative lattice
distortions coupled to the electronic degrees of freedom. We reproduce the
observed dependence of the transition temperature on cation radius in the
well-studied manganite and nickelate materials. Since the elastic couplings are
generically quite strong, these conclusions will broadly generalize to all MITs
that couple to a change in lattice symmetry.
