Effect of excess oxygen for CuFeO2.06 delafossite on thermoelectric and optical properties

This work presents the role of excess oxygen in CuFeO2.06 compounds on thermoelectric and optical properties. The CuFeO2.06 specimens were synthesized by solid state reaction method. X-ray diffraction technique has confirmed the CuFeO2 structure for the specimens. In particularly, CuFeO2.06 specimen revealed the structural extension of lattice parameter: a and c. Also, the specimen found increasing excess oxygen of approximately 3% as a resulted enhancement of mixed valence state of Cu+ and Cu2+ ions. XPS showed mixed valence state of the Cu+/Cu2+ ions, and Fe3+ and Fe2+ ions was also found in the CuFeO2.06 specimen. Mixed valence states contributed the co-existence of hole and electron carriers for conduction. Consequently, electrical conductivity of the CuFeO2.06 specimen increased up to 23 S/cm at 873 K. Also, increasing Seebeck coefficient was shown to be approximately 302 μV/K at 873 K. The CuFeO2.06 specimen was found power factor to be approximately 2.1 × 10−4 W/m∙K2 at 873 K. The indirect optical gap of CuFeO2.06 (2.40 eV) was lower than that of the CuFeO2 (2.60 eV). Thus, thermoelectric and optical properties were governed by an existence of excess oxygen.