The research on ordered perovskite-type oxide Cd_3TeO_6 with the aim of making new thermoelectric materials

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Electron doping into Cd_3TeO_6 with a 1:1 ordered perovskite-type structure was carried out by two methods : the introduction of oxygen vacancies and the atomic substitution of In, Bi and La for Cd. Both methods turned polycrystalline samples into metallic conductors while single crystals with metallic conductivity were obtained by the second method. The resistivity and thermoelectric power of the single-crystal indium-substituted sample were 0.6 mΩ cm and -50 μV/K, respectively, giving a power factor of 4×10^<-4> W K^<-2> m^<-1>, which is the highest among non-transition metal oxides. The good thermoelectric performance of the electron-doped Cd_3TeO_6 derives from its excellent electrical conductivity. Though the reason for the good conductivity is not clear at present, the structural feature of the electron-doped Cd_3TeO_6 may be considered to provide the special electric characteristics of Cd_3TeO_6. The structure of the ordered perovskite oxide Cd_3TeO_6 is strongly deformed (monoc … More linic, P2_1/n), and the Cd-O distances are similar for the cadmium ions in both the A and B sites. Therefore, in the electron-doped Cd_3TeO_6, electrons introduced into the empty 5s orbitals of Cd^<2+> and Te^<6+> can move in the structure through the overlap of the 2p orbitals of the oxide ions not only with the 5s orbitals of the the B-site cations but also with the 5s orbitals of the A-site cadmium ions. This conducting network unique to Cd_3TeO_6 may be related to the high conductivity of this material.In conclusion, we successfully modified the polycrystalline and single-crystal Cd_3TeO_6 samples also by substituting In^<3+> cations for Cd^<2+> ions. The electron doping of the polycrystalline samples could be attained also by introducting oxygen vacancies. This investigation of thermoelectric properties has revealed that the thermoelectric figure-of-merit Z (Z=S^2ρ^<-1>κ^<-1>) of a single crystal, Cd_<3-x>In_xTeO_6, is approximately 1.0×10^<-4> K^<-1> at room temperature. The electron-doped Cd_3TeO_6 is a promising n-type thermoelectric material. Less