Preparation and electrical properties of La_2-xSr_xMnO_4+δ with K₂NIF₄-structure

Polycrystalline bodies of La_2-xSr_xMnO_4+δ (1.0≤x ≤2.0) with a K₂NiF₄-structure were prepared by a standard ceramic method. Lattice parameters, electrical conductivity and Seebeck voltage were measured. Furthermore, optical absorption spectra were recorded to obtain information on changes in the electronic structure in the vicinity of the energy band gap. Changes in lattice parameters were attributed to the disappearance of Jahn-Teller distortion of MnO₆-octahedra upon Sr-doping. The results suggested that holes doped in the present system were mainly localized on manganese ions. The temperature dependence of electrical conductivities of all the samples was almost Arrhenius-type. Seebeck voltages and optical absorption spectra changed drastically in the composition ranges of 1.0<x<1.2 and 1.6<x<2.0. It was suggested that the electronic structure near the energy band gap changed drastically in those composition ranges.