  The interplay of the electron-phonon interaction and of the double and super
exchange magnetic effects is analyzed in the $La_{1-x}Ca_xMnO_3$ perovskites
with $0<x<0.5$. By using an analytical variational scheme that allows to treat
the electron-phonon interaction in a fully quantum manner it is shown that in
the intermediate electron-phonon coupling regime a charge density instability
occurs near the metal-insulator transition, induced by either temperature or
hole doping. The system segregates in antiferromagnetic or paramagnetic small
polarons and ferromagnetic large polarons domains characterized by different
values of the lattice distortions that are satisfactory compared with those
experimentally observed.
