  A systematic study of phase separation effects in polycrystalline La_0.5
Ca_0.5 Mn O_3 obtained under different thermal treatments is reported. Samples
with average grain size ranging from 200 to 1300 nm were studied. Magnetic and
electrical measurements show quantitative differences among samples in their
low temperature behavior. indicating that the fraction of the ferromagnetic
(FM) phase gradually decreases as the grain size increase. Percolation of the
FM phase in samples with a small fraction of this phase suggests that grain
boundaries play a distinctive role in the spatial distribution of coexisting
phases. The defective structure at the grain surface could explain the local
inhibition of the antiferromagnetic charge ordered phase, an effect that is
gradually removed with increasing grain size. Qualitative agreement of the data
with this description is found. This effect is also found to be highly
dependent on the oxygen content of the samples and its spatial distribution.
