Electrical conductivity, Seebeck coefficient, and defect structure of oxygen nonstoichiometric Nd2−xSrxNiO4+δ

To elucidate the electronic state and the conduction mechanism of Nd2NiO4+δ series oxides at high temperatures, the electrical conductivity, Seebeck coefficient, and nonstoichiometric oxygen content of Nd2−xSrxNiO4+δ (x = 0, 0.2, 0.4) were measured as a function of the Sr content, temperature, and oxygen partial pressure. The hole mobility is estimated from the electrical conductivity and the hole concentration which is defect chemically determined. The mobility slightly decreases as temperature increases as in metals at high temperatures. The relationships between the Seebeck coefficient, electrical conductivity, and hole concentration can be explained by Mott's equation, which expresses the Seebeck coefficient for metals. Semi-quantitative analyses strongly indicate that the electron or hole is itinerant in Nd2−xSrxNiO4+δ, and the conduction mechanism is metal-like band conduction at high temperatures. Based on the experimental results, schematics for energy level and band structure are proposed. At high temperatures, free holes in the σx2−y2 band composed of dx2−y2 orbitals contribute to metallic conduction.