Electronic state of oxygen nonstoichiometric La_2-xSr _xNiO_4+δ at high temperatures

In order to elucidate the electronic state and the conduction mechanism of La_2-xSr_xNiO_4+δ at high temperatures, electrical conductivity, Seebeck coefficient, and nonstoichiometric oxygen content of La_2-xSr_xNiO_4+δ (x = 0, 0.2, 0.4) were measured as a function of Sr content, temperature, and oxygen partial pressure. The hole mobility is estimated from the electrical conductivity and oxygen content. The mobility decreases slightly as temperature increases, like metals at high temperatures. The relationships between Seebeck coefficient and electrical conductivity, and Seebeck coefficient and hole concentration can be explained by the metallic conduction model. Semi-quantitative analyses strongly indicate that electrons (holes) are itinerant in La_2-xSr _xNiO_4+δ and the conduction mechanism of La _2-xSr_xNiO_4+δ is metal-like band conduction at high temperatures. Based on the experimental results and discussions, a schematic of the energy levels and band structure is proposed. At high temperatures, metallic conduction is induced by a free hole in the σ_x2-y2 band composed of a d_x2-y2 orbital.