Structural phase diagram of La1-xSrxMnO3+delta: Relationship to magnetic and transport properties

The structural properties of La1-xSrxMnO3+delta have been studied using neutron powder diffraction as a function of both Sr doping (0 less than or equal to x less than or equal to 0.225) and oxygen partial pressure during synthesis [2.1x10(-4) atm less than or equal to P(O-2)less than or equal to 1 atm]. A structural phase diagram constructed as a function of these parameters has a rhombohedral phase (R3c), an orthorhombic phase (Pbnm), and a monoclinic phase (P2(1)/c). For a given x, decreasing P(O-2) yields smaller cation vacancy concentrations. At low temperature, the R phase is ferromagnetic, while the M phase is antiferromagnetic. The O phase is ferromagnetic for x greater than or equal to 0.125, and the ferromagnetism is independent of the O-R phase transition that coincides with the transition from nonmetal to metal. Transport measurements made between 20 and 350 K show that O and M samples are nonmetallic (d rho/dT<0), while the R samples exhibit a temperature-dependent nonmetal-to-metal transition at temperatures close to the Curie temperature. Magnetoresistance (MR) is observed in all three phases. The largest value at 9 T, found in the orthorhombic and monoclinic samples, is of similar order (Delta rho=rho(0)-rho(9T)similar to 10(4) Omega cm) to that reported for 10(6)% colossal MR powder samples. However, our lower sintering temperatures result in large rho(9T) values that yield Delta rho/rho(9T)similar to 230%.