Orbital and oxygen vacancy ordering in La$_{1-x}$Sr$_{x}$MnO$_{3-\delta }$ (x$\ge $0.8, $\delta \ge $0.15)

We have developed synthesis method, obtained, and studied the highly Sr substituted LSM manganites. New oxygen and orbital ordered perovskite type phase SrMnO$_{2.6}$ (nominally Sr$_{5}$Mn$_{5}$O$_{13})$ was found tetragonal P4/m, a=8.61328(18) and c=3.80997(11) {\AA}. Mn ion shows charge separation to Mn$^{3+}$ in a pyramidal and Mn$^{4+}$ in a regular octahedral coordination. Nonstoichiometry has been observed with oxygen compositions between 0.25$<\delta <$0.4; upon oxidation a monoclinic distortion is observed. Analogous tetragonal and monoclinic phases have been found for La$_{1-x}$Sr$_{x}$MnO$_{3-\delta }$ (0.3$<\delta <$0.5) with x=0.05 and 0.2, respectively. In-situ neutron powder diffraction as well as susceptibility measurements show different magnetic arrangements below RT depending on x and $\delta $. This work was supported by the U.S. Department of Transportation and NSF-DMR-0302617.