Structure and properties of high-oxygen-pressure annealed Sr$_{1-x}$La$_{x}$Co$_{0.5}$Fe$_{0.5}$O$_{3-d}$ (0$\le $x$\le $0.5)

Synthesis, oxygen content, structural, magnetic, and resistive properties will be discussed for the Sr$_{1-x}$La$_{x}$Co$_{0.5}$Fe$_{0.5}$O$_{3-d}$ perovskites. The x=0 sample shows oxygen-vacancy ordered Sr$_{8}$Co$_{4}$Fe$_{4}$O$_{23}$ tetragonal I4/mmm structure. With an increase of the La content the materials became oxygen stoichiometric and a lowering of the crystal symmetry is observed from cubic Pm3m (x=0.1 and 0.2) to tetragonal I4/mcm (x=0.3 and 0.4), and finally to monoclinic I12/c1 (x=0.5). All samples show ferromagnetic ordering with the maximum Curie temperature near 290 K at x=0.2. Conductivity is enhanced and small negative magneto-resistance is observed below T$_{C}$. Transport measurements up to 1100$^{o}$C show high conductivity that is affected by the varying oxygen content. Work at NIU was supported by the NSF (DMR-0706610) and at ANL by the U.S. DOE under contract No. DE-AC02-06CH11357.