In this study, structural and electrical properties of La0.5Sr1.5Mn0.5T0.5O4 (T = Mn, Fe, and Co) manganites, ith K2NiF4 structure, have been investigated and compared with those of La0.5Sr0.5Mn0.5T0.5O3 perovskite manganites. The room temperature X-ray diffraction patterns of the synthesized La0.5Sr1.5Mn0.5T0.5O4 layered-structures and La0.5Sr0.5Mn0.5T0.5O3 perovskites were indexed with tetragonal and orthorhombic structures, respectively. The FE-SEM images revealed that the average size of La0.5Sr1.5Mn0.5T0.5O4 particles prepared by solid state reaction method using La0.5Sr0.5Mn0.5T0.5O3, as the starting materials, is larger compared with that of La0.5Sr0.5Mn0.5T0.5O3 synthesized by the modified sol-gel route. The SEM micrographs of the sintered ceramics indicated that the average grain size of La0.5Sr1.5Mn0.5T0.5O4 layered- structures is larger than that of the corresponding La0.5Sr0.5Mn0.5T0.5O3 perovskites, which is supported by FE-SEM images. The electrical conductivity measurements, in the range of room temperature to 800 °C, indicated that these oxides have semiconducting behavior, and their transport properties can be described by small polaron conduction mechanism. Results showed that La0.5Sr1.5Mn0.5T0.5O4 layered-structures have lower electrical conductivity in comparing to the corresponding La0.5Sr0.5Mn0.5T0.5O3 perovskites, since the perovskite structure provides three-dimensional (Mn/T)3+–O–(Mn/T)4+ interaction, whereas it is two-dimensional interaction in the layered-structure. The reduction in conductivity with Fe doping could be due to the presence of Fe 3+ ions, which causes a decrease in Mn3+/Mn4+ ratio and the number of electrons involved in hopping mechanism. The electrical conductivity enhancement of La0.5Sr1.5MnO4 by doping Co can be associated with reducing Mn/Co‒O bond length and the increase in the overlap between O 2p and Mn/Co 3d orbitals.

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