Effects of Pd substitution on the thermoelectric and electronic properties of delafossite Cu1-xPdxFeO2 (x=0.01, 0.03 and 0.05)

Abstract

Cu(1-x)Pd(x)FeO2 (x=0.01, 0.03 and 005) delafossite was prepared by solid state reactions and was calcined/sintered at 1050 °C. The effect of Pd2+ substitution for the Cu1+ sites on the thermoelectric and electronic properties of Cu (1-x)Pd(x)FeO2 were investigated. The crystal structure, oxygen decomposition, thermoelectric and electronic properties were characterized by X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy (XPS), Seebeck coefficient, electrical conductivity and thermal conductivity measurements. The characterization showed that Cu (1-x)Pd(x)FeO2 formed a hexagonal delafossite structure with R3-m symmetry. The existence of Pd2+, Cu1+, Cu2+, Fe3+, Fe4+ and O was revealed from the XPS results. Confirmation of Pd2+ substitution for the Cu 1+ sites occurred by increasing the c-axis in the lattice parameter with a Pd content. The O content intercalated at the center of the triangular Cu acted as a support to produce Cu2+ ions and was reduced with an increasing Pd content. The mixed valencies of Cu1+/Cu2+ and Cu1+/Pd2+ in the Cu layer changed the electrical conductivity and the Fe3+/Fe4+ mixed valencies in the FeO6 layer caused the Seebeck coefficient to increase. Both the electrical conductivity and Seebeck coefficient for Pd contents of x=0.01 and 0.03 were higher than that of non-doped CuFeO2. The low thermal conductivity of Cu(1-x)Pd(x)FeO2 resulted from the substitution of Pd, which has a large atomic mass, into structure. The Jonker plot indicated that the electronic properties displayed a degenerate density of states and that Cu(1-x)Pd(x)FeO2 was a semiconductor. A high ZT value of 0.055 was obtained for a Pd content of 0.03 at 950 K. The Pd2+ substitution for the Cu1+ sites influenced the thermoelectric and electronic properties of the delafossite Cu(1-x)Pd(x)FeO2 samples. © 2014 Elsevier Inc.