(Bi0.4Sr0.45Co0.15)Sr2CoO5−δ: a new 1201-type oxygen dependent series of cobaltites

A new series of cobaltites (Bi0.4Sr0.45Co0.15)Sr2CoO5−δ(0.15≤δ≤0.55) related to the 1201-type structure, has been synthesized. The X-ray and neutron powder diffraction studies, coupled with transmission electron microscopy (TEM) observations, evidence an orthorhombic cell (Pnnn, a≈b≈2ap, c≈2c1201) for the lowest oxygenated phases, whereas a tetragonal cell (I4/mmm, a=b≈ap2, c≈2c1201) is observed for the highest ones. The main structural feature of this series deals with the ordering between Bi3+(Co3+) and Sr2+within the rocksalt layer [(Bi/Co,Sr)O]∞leading to the doubling of the c1201parameter. The structural transition, observed versus oxygen content, is correlated to a specific oxygen arrangement around cobalt species. The XANES investigations carried out for two limit oxygen contents, 4.53 and 4.85, show two distinct formal cobalt valences. These materials have a semi-conducting behavior with negative magnetoresistance properties which are sensitive to their oxygen content. In particular, for the tetragonal phase, a spin glass like behavior (Tcusp=38.5 K) has been observed.