  The crystal and magnetic structures of the specified CMR manganite system
have been studied as a function of $y=(0.2-1)$ across the metal-insulator (MI)
transition, and of the oxygen mass ($^{16}$O, $^{18}$O). We quantitatively show
how the polaronic narrowing of the carrier bandwidth and the crystal lattice
micro-strains control the volume fractions of the mesoscopic ferro- and
antiferromagnetic clusters. A well-defined dip in the transition temperatures
and the suppression of all the types of long range ordering seen near the
MI-transition at $y\simeq0.9$ indicate a key role of the quenched disorder for
the formation of the long-scale phase separated state.
