We address the problem of the percolative phase separation in polycrystalline
samples of Pr0.5−δCa0.2+δSr0.3MnO3 for −0.04≤δ≤0.04 (hole doping n between 0.46 and 0.54). We perform
measurements of X-ray diffraction, dc magnetization, ESR, and electrical
resistivity. These samples show at TC a paramagnetic (PM) to ferromagnetic
(FM) transition, however, we found that for n>0.50 there is a coexistence of
both of these phases below TC. On lowering T below the charge-ordering
(CO) temperature TCO all the samples exhibit a coexistence between the FM
metallic and CO (antiferromagnetic) phases. In the whole T range the FM phase
fraction (X) decreases with increasing n. Furthermore, we show that only
for n≤0.50 the metallic fraction is above the critical percolation
threshold XC≃15.5. As a consequence, these samples show very
different magnetoresistance properties. In addition, for n≤0.50 we
observe a percolative metal-insulator transition at TMI, and for
TMI<T<TCO the insulating-like behavior generated by the enlargement of
X with increasing T is well described by the percolation law ρ−1=σ∼(X−XC)t, where t is a critical exponent. On the basis of
the values obtained for this exponent we discuss different possible percolation
mechanisms, and suggest that a more deep understanding of geometric and
dimensionality effects is needed in phase separated manganites. We present a
complete T vs n phase diagram showing the magnetic and electric properties
of the studied compound around half doping.Comment: 9 text pages + 12 figures, submitted to Phys. Rev.