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

Abstract A new series of cobaltites (Bi 0.4 Sr 0.45 Co 0.15 )Sr 2 CoO 5− 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 ≈2 a p , c ≈2 c 1201 ) for the lowest oxygenated phases, whereas a tetragonal cell ( I4/mmm , a = b ≈ a p 2 , c ≈2 c 1201 ) is observed for the highest ones. The main structural feature of this series deals with the ordering between Bi 3+ (Co 3+ ) and Sr 2+ within the rocksalt layer [(Bi/Co,Sr)O] ∞ leading to the doubling of the c 1201 parameter. 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 ( T cusp =38.5 K) has been observed.