We show that for La 0:5 Ca 0:5Àx Ba x MnO 3 , the sample is single phase with an orthorhombic structure for x < x d % 0:28 while a phase separation occurs for x ! x d . The observed phase separation is a chemically driven inhomogeneity caused by the large size-mismatch (s 2 ) which occurs as s 2 ! 1:42 Â 10 À4 nm 2 . In the single-phase region, increasing x to !0:14 causes a transition to metallic state, and the magnetoresistance near the transition shows behavior similar to that in doubleexchange manganite oxides. In the two-phase region, although the compound also shows an insulator-metal transition, both transport and magnetoresistance show behavior completely different from that in the single-phase region. Some anomalous transport and magnetoresistance behaviors are also revealed at low temperatures. A possible phase diagram is presented as a function of T and x and the experimental findings are discussed according to different ground states.
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