Synthesis, crystal chemistry, and electrical and magnetic properties of Sr3Fe2-xCoxO7-delta (0 <= x <= 0.8)

Sr3Fe2-xCoxO7-delta oxides crystallizing in a tetragonal perovskite-related intergrowth structure have been synthesized for 0 less than or equal to x less than or equal to 0.8 and characterized by X-ray powder diffraction, we t-chemical analysis, and electrical and magnetic measurements. The lattice parameters, oxygen content, and average oxidation state of iron and cobalt decrease with increasing Co content x. However, the samples show instabilit y on exposure to ambient air for prolonged periods of time due to a possibl e reaction with H2O and CO2. Resistivity measurements indicate that the sam ples exhibit a semiconducting behavior at low temperatures (80 less than or equal to T less than or equal to 680 K) and the resistivity value decrease s with increasing Co content. For T > 680 K, the resistivity versus tempera ture plots exhibit a change in the sign of the slope from negative to posit ive due to oxygen loss at higher temperatures. Low-temperature resistivity data show an exponential temperature dependence of rho proportional to exp( T-0/T)(nu) with nu = 1/2 for samples with 0 less than or equal to x less th an or equal to 0.25 and nu = 1/4 for samples with 0.6 less than or equal to x less than or equal to 0.8 suggesting a transition from activated carrier s involving a coulomb gap to thermally activated hopping as the Co content increases. SQUID magnetometer measurements indicate a transition from antif erromagnetic behavior to weak ferromagnetic behavior as the Co content incr eases. (C) 2001 Academic Press.