Synthesis and structural characterization of LaxSr1−xMnO2.6+δ (0.1<x<0.4) compounds displaying compressed octahedral coordination of Mn(4−5x)+

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 (a=b≈5aP, 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 dx2-y2dx2-y2 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 dt2g3dx2-y20dz21, 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.

LaxSr1−xMnO2.6 compounds (0.1⩽x⩽0.4) show a structural phase transition from Sr5Mn5O13-type oxygen-vacancy-ordering (x⩽0.3) to vacancy-disordered x=0.4, showing clear indications of short-range ordering. Compressed MnO6 octahedra are observed for Mn3+-containing x=0.2 compound that is probably consequence of the presence of elongated MnO6 octahedra with the long-axis disordered in the x–y plane .Figure optionsDownload as PowerPoint slide