| Crystalline perovskite is a promising thermoelectric material among the metal oxides. Especially high concentration-doped SrTiO3 exhibits a rather large TE figure of merit, which is the largest among n-type metal oxides reported to date, but unfortunately, it has a high thermal conductivity. Comparing with SrTiO3, the Ruddlesden-Popper phase layered oxides SrO(SrTiO3)n ( n=1, 2 ) have the lower thermal conductivity due to their character of nanoblock integration. But, S2·σvalue drastically decreases. On the other hand, the synthesis of Ruddlesden-Popper compound is rather difficult. The problems that how to reduce the thermal conductivity, make use of the layered perovskite structure and not affect the transport properties, have been received much attention. To solve the problem, it is necessary to understand the effect of layered structure in the materials on their transport properties. In this paper, two series of Ruddlesden-Popper type compounds, SrO(SrTiO3)n ( n=1, 2,∞) and LaO(LaNiO3)n ( n=1, 2, 3,∞), were studied, their electronic structure, synthesis method and transport properties were investigated systematically.In the present work, SrO(SrTiO3)n ( n=1, 2,∞) nanopowders were prepared by a microwave-assisted sol–gel self-propagating method which is a combination of the combustion and the chemical gelation processes. The reaction course can be determined by the XRD and DTA analysis of as-synthesized samples under different sintered temperatures. The synthesis temperature of the materials was chosen to be 850℃. The SEM analysis indicates the layered perovskites have plate-like morphology. The plate-like particles have a smooth and well-developed plane with a side length of about 1μm and an aspect ratio (side length/thickness) of about 10. The form of the nanosized flake-like morphology is mainly ascribed to the crystal layered structure.We provide a comparative study on the optical absorption properties of SrO(SrTiO3)n ( n=1, 2,∞) using linear IR and UV-visible diffuse reflectance spectroscopy, respectively. The infrared spectra indicate that the width of TiO6 octahedron in SrO(SrTiO3)n ( n=1, 2) stretching vibration absorption peak is broaden, the width decreases with the increasing of n value. This means that the bond strength of Ti–O in TiO6 octahedron becomes different. The theoretical study of three compounds was undertaken by density functional theory and their properties were compared. The UV–vis spectra indicate that the absorption edges of SrO(SrTiO3)n ( n=1, 2) shift to the short wave with the decrease of n value. Based on proper electronic structures, the interband transitions are assigned. The evolution of the material absorption property is mainly ascribed to the stretching of the octahedral and the layered structure.To clarify the effect of substitutional La3+ doping, measurements were conducted for the transport properties of SrO(Sr1-xLaxTiO3)n ( n=1, 2,∞) materials. The band structure, total density of states (DOS), and partial density of states (PDOS) of SrO(Sr1-xLaxTiO3)n ( n=1, 2,∞) were also calculated in order to study their electronic structures. The La doping leads to the lift of the Fermi plane, which makes the materials, have electric property. However, the semiconductor-metal transition only exists in the perovskite Ti-O-Ti plane. The vertical direction over perovskite layer is always with high resistance.The first-principles calculations were carried out to study the electronic structures of SrO(SrTiO3)n ( n=1, 2,∞) by using CASTEP software based on the Density Functional Theory(DFT). The band structure, total density of states (DOS), partial density of states (PDOS) and population analysis of SrO(SrTiO3)n were calculated in order to explore the electronic structures of SrO(SrTiO3)n. The effect of two-dimensional structure on the materials electronic structure was investigated. The calculated results indicated: The oxygen atoms in different crystallographic positions in SrO(SrTiO3)n (n = 1, 2 ) have different PDOS. The covalent nature and binding energy of Ti-O bands located on the edge of perovskite layer are weaker than that of other Ti–O bonds, which agrees with the results of population analysis. Compared with the simple perovskite structure, layered structure possesses much lower carrier mobility and higher effective carrier mass. The transport behavior of SrO(SrTiO3)n ( n=1, 2) have obvious two-dimensional character.LaO(LaNiO3)n ( n=1, 2, 3,∞) compounds were prepared by a modified sol-gel auto-combustion method, which is a low-temperature combustion synthesis procedure using microwave-assisted sol-gel as precursors. The high-temperature transport properties of the samples were investigated. The experiment results show that the electrical conductivity of LaO(LaNiO3)n (n=1, 2, 3,∞) is increased with the increase of n, which mainly originates from the increase of the concentration of Ni3+, namely the concentration of carrier, with increase of n. Hereinto, the compound LaO(LaNiO3)n (n=1) has a change from semiconductor to metal at 700K. The first-principles calculations were carried out to study the electronic structures of LaO(LaNiO3)n (n=1) with different phases (I4/mmm, Cmca and Fmmm) by using CASTEP software based on the Density Functional Theory (DFT). A metal-semiconductor transition exists in the calculated band structure of La2NiO4 as it changes from high-temperature orthorhombic to low-temperature tetragonal phase transition. The semiconductor-metal transition of La2NiO4 only exists in the Ni-O-Ni plane, the vertical direction over perovskite layer is always has high resistance.
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