  We consider semiclassically scaled Schrodinger equations with an external
potential and a highly oscillatory periodic potential. We construct asymptotic
solutions in the form of semiclassical wave packets. These solutions are
concentrated (both, in space and in frequency) around the effective
semiclassical phase-space flow obtained by Peierl's substitution, and involve a
slowly varying envelope whose dynamics is governed by a homogenized Schrodinger
equation with time-dependent effective mass. The corresponding adiabatic
decoupling of the slow and fast degrees of freedom is shown to be valid up to
Ehrenfest time scales.
