Synthesis and structural characterization of LaxSr1−xMnO2.6+δ (0.1

LaxSr1−xMnO2.6+δ (x=0.1–0.4) compounds have been obtained by low-temperature annealing of stoichiometric materials in hydrogen. La0.1Sr0.9MnO2.6+δ (δ=0.15) and La0.3Sr0.7MnO2.6, tetragonal (P4/m), and La0.2Sr0.8MnO2.6, pseudo-tetragonal monoclinic (P2/m), structures are isostructural with oxygen-vacancy-ordered Sr5Mn5O13 (image, c≈aP). La0.4Sr0.6MnO2.6 shows cubic perovskite structure with disordered oxygen vacancies. In the vacancy-ordered (LaxSr1−x)5Mn5O13 phases four out of five Mn cations are Mn3+ and show a typical Jahn-Teller elongated pyramidal coordination while the fifth one Mn(4−5x)+, in octahedral environment, shows decreasing formal charge from Mn4+ (x=0) to Mn2.5+ x=0.3. This unusual selective doping of the octahedral site produces structural strain due to increasing size of the Mn(4−5x)+ and, in the case of (La0.2Sr0.8)5Mn5O13, the unusual compressed octahedral arrangement of oxygen atoms around it. The coordination geometry implies that either the image orbital is occupied, which would be a rare example of inverted occupancy of eg orbitals in manganites, or that disordered Mn3+ apically elongated MnO6 octahedra are present with normal electronic configuration image, and the observed bond distances are the average of the long and intermediate in-plane Mn–O bonds. Several structural features favor the second case.