THE SYNTHESIS AND PROPERTIES OF THE CHEMICALLY OXIDIZED PEROVSKITE, LA1-XSRXCOO3-DELTA LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.9)

The series of La1-xSrxCoO3-delta (0.5 less than or equal to x less tha n or equal to 0.9) perovskites have been prepared using the Pechini ge l technique. The products mere chemically oxidized by stirring in a so dium hypobromite solution, The samples have been characterized by powd er X-ray diffraction, thermal analysis, iodometric titration, Co K edg e X-ray absorption spectroscopy, temperature-dependent SQUID magnetic susceptibility, and temperature-dependent electrical resistivity. The Sr-rich samples(x > 0.7) have the brownmillerite-type structure prior to oxidation and the cubic perovskite structure after treatment with s odium hypobromite. Iodometric titration shows as much as a similar to 14% increase in Co(IV) concentration in the Sr-rich samples after chem ical oxidation, The Co K edge spectra show that there is very little c hange in the formal cobalt valence with increasing Sr content. The eff ective magnetic moments of the oxidized compounds level off with incre asing Sr concentration. All of the chemically oxidized samples exhibit small-bandgap semiconducting behavior. The data lead to the postulati on of an equilibrium between CoO(IV) and O- in the series, (C) 1998 Ac ademic Press.