  Employing a variational approach that takes into account electron-phonon and
magnetic interactions in $La_{1-x}A_xMnO_3$ perovskites with $0<x<0.5$, the
effects of the magnetic field and the oxygen isotope substitution on the phase
diagram, the electron-phonon correlation function and the infrared absorption
at $x=0.3$ are studied. The lattice displacements show a strong correlation
with the conductivity and the magnetic properties of the system. Then the
conductivity spectra are characterized by a marked sensitivity to the external
parameters near the phase boundary.
