  We present results of electrical resistivity, magnetoresistance and ac and dc
magnetic susceptibility on polycrystalline samples of the type
La(0.5)Ca(0.5-x)Ba(x)MnO(3) synthesized under identical heat treatment
conditions. The substitution of larger Ba ions for Ca results in a non-
monotonic variation of the curie temperature as the system evolves from a
charge ordered insulating state for x=0 to a ferromagnetic metallic state for
x=0.5. An intermediate compositino, x=0.1, interestingly exhibits
ferromagnetic. insulating behaviour with thermal hysteresis in ac chi around
the curie tem- perature (120K). The x=0.2 and 0.3 compounds exhibit
semiconducting like behavior as the temperature is lowered below 300K, with a
broad peak in rho around 80-100K: These compositions exhibit a weak increase in
rho as the temperature lowered below 30K, indicative of electron localization
effects. These compositions also undergo ferromagnetic transitions below about
200 and 235K respectively, though these are non-hysteretic; above all, for
these compositions, MR is large and conveniently measurable over the entire
tempera- ture range of measurement below Tc. This experimental finding may be
of interest from the application point of view. We infer that the A-site
ionic-size mismatch plays a crucial role in the deciding these properties.
