Phase separation and transport behavior in La0.5Ca0.5-xBaxMnO3

We show that for La0.5Ca0.5-xBaxMnO3, the sample is single phase with an or thorhombic structure for x < x(d) approximate to 0.28 while a phase separat ion occurs for x greater than or equal to x(d). The observed phase separati on is a chemically driven inhomogeneity caused by the large size-mismatch ( sigma (2)) which occurs as sigma (2) greater than or equal to 1.42 x 10(-4) nm(2). In the single-phase region, increasing x to greater than or equal t o0.14 causes a transition to metallic state, and the magnetoresistance near the transition shows behavior similar to that in double-exchange manganite oxides. In the two-phase region, although the compound also shows an insul ator-metal transition, both transport and magnetoresistance show behavior c ompletely different from that in the single-phase region. Some anomalous tr ansport and magnetoresistance behaviors are also revealed at low temperatur es. A possible phase diagram is presented as a function of T and x and the experimental findings are discussed according to different ground states.