Structural phase diagram of La1-xSrxMnO3 for low Sr doping

The Mn-based perovskite oxides are of strong current interest because they show a colossal negative magnetoresistance near the ferromagnetic ordering temperature. The samples become metallic below T-c for a content of approx. 30% Mn4+. We present in this article a structural study of La1-xSrxMnO3 for compositions lying below the optimum magnetoresistance. At high temperature, the system is rhombohedral (R (3) over bar c). On cooling, two structural transitions to orthorhombic Pbnm phases (O and O') take place successively. The transition from the O to the O'-phase is due to the setting up of co-operative Jahn-Teller distortions that become static in the O'-phase. The O'-phase is characterised by the presence of an anti-ferrodistorsive Mn3+ orbital ordering. This ordering is evidenced by a peculiar arrangement of distorted MnO6 octahedra: the connection between octahedra in the basal plane (a, b) shows long Mn-O bonds adjacent to short Mn-O bonds. The peculiarity of samples around the composition x = 0.125 is that they undergo another transition to an orthorhombic O"-phase at lower temperatures. This phase has the same average structural features as the O-phase but the Jahn-Teller distortion seems to be completely suppressed. (C) Elsevier Science S.A.