THE STRUCTURAL PHASE DIAGRAM OF THE MAGNETORESISTIVE OXIDE LA_1-XSR_XMNO_3+d; RELATIONSHIP TO MAGNETIC AND TRANSPORT PROPERTIES. D. N. Argyriou, Material Science Division, Argonne National Laboratory, Argonne, IL 60439

The structural phase diagram of La_1-xSr_xMnO_3+d has been constructed as a function of both Sr doping (0 <= x <= 0.225) and oxygen partial pressure during synthesis (2.1 x 10^-4 atm <= P(O₂) <= 1 atm). The phase diagram contains a rhombohedral phase ( ), an orthorhombic phase (Pbnm), and a monoclinic phase (P2₁/c). For a given doping level x, decreasing P(O₂) yields smaller cation defect concentrations and consequently a reduction of the average Mn oxidation state. The orthorhombic and monoclinic phases synthesized under low P(O₂) exhibit a cooperative Jahn-Teller distortion of the MnO₆ octahedra, consistent with a high Mn³⁺/Mn⁴⁺ ratio. For the La_1-xSr_xMnO_3+d system a strong correlation is observed between structural and transport properties; orthorhombic and monoclinic samples are nonmetallic, while rhombohedral samples exhibit a temperature-dependent nonmetal-metal transition at a temperature close to the Curie temperature. Magnetoresistance is observed for all three phases. We also find that for samples of constant Sr doping the effect of P(O₂) on magnetic transition temperature is significant. On cooling both rhombohedral and orthorhombic samples exhibit structural anomalies close to the magnetic ordering temperature. This anomaly exhibited by the orthorhombic phase will be briefly discussed. This work was supported by the NSF Office of Science and Technology Centers under contract DMR 91-2000.