  High resolution neutron diffraction shows that the mesoscopic separation into
ferromagnetic (FM) and antiferromagnetic (AFM) phases and the FM transition
temperature T_C in the perovskite manganite (La_{1-y}Pr_{y})_{0.7}Ca_{0.3}MnO_3
strongly depend on the quenched correlated disorder. The different disorder
strengths are achieved by different procedures of the sample synthesis and
quantitatively characterized by the micro-strain-type diffraction peak
broadening. The system shifts to predominantly one phase state with smaller T_C
as the correlated disorder strength is decreased supporting the viewpoint that
the origin of phase separation in the indicated manganite system is the
correlated quenched disorder. The ground state of an ultimately chemically
homogeneous sample is FM-like containing about 20% of the AFM minority phase.
This FM-like state can be readily transformed to the AFM-like one having <20%
of FM-phase by the decreasing of the effective charge carrier bandwidth via
oxygen isotope substitution.
