Phase separation in structural and magnetic transitions in Pr0.5Ca0.5-xSrxMnO3 (x=0.15 and 0.3)

An earlier study of the structural and magnetic transitions in Pr0.5Ca0.5-xSrx MnO3 perovskites has been completed by a combination of the investigation of the NMR and transport properties together with a consistent application of a two-phase approach in the evaluation of the neutron diffraction data. The compositions with x = 0.15 and x = 0.3 were chosen as typical representatives for two different kinds of behaviour in the course of these transitions. For both compositions the existence of the first-order structural transition between the high-temperature pseudocubic and low-temperature charge-ordered tetragonally contracted low-temperature phases was found with critical temperatures T-co = 210 K and 200 K for x = 0.15 and x = 0.3, respectively. The coexistence of both phases below T-co was observed and their relative volumes were determined. Two different Mn-55 NMR FM signals were detected and one possessed a complex dynamics of nuclear spins. This signal was observed in both materials for T < T-co and was ascribed to FM inclusions within the 1:1 Mn3+/Mn4+ ordered phase with the CE-type antiferromagnetic ground state. The second signal, found for x = 0.3 only, was identified with ferromagnetism within the pseudocubic phase with T-C = 215 K and almost temperature-independent magnetization similar to2 mu (B)/Mn. Its appearance was found to be accompanied by a distinct phase separation of electronic origin, leading to stabilization of FM clusters below T-C. Due to the phase separation this second FM phase for x = 0.3 coexists with the PM phase, gradually increases its volume and, finally, vanishes with the onset of CE-type antiferromagnetism at T-N(C) = 170 K (T-N(W) = 180 K). A detailed study of the thermal and electronic transport in magnetic fields up to 5 T (9 T) has revealed the metallic character of small ferromagnetic clusters imbedded into an insulating matrix. The influence of the magnetic field upon the overall electrical conductivity in various temperature ranges was investigated and the role of percolation discussed.