Defect-chemical analysis of the nonstoichiometry, conductivity and thermopower of La2NiO4+δ

Abstract

La₂NiO_4+δ is an oxygen excess compound (δ > 0) with oxygen interstitials and holes (h˙) in majority, which deviates positively from the ideal-dilute-solution behavior of defects. It was earlier attempted to interpret this positive deviation by taking into account the activity coefficients of both and h˙. In this work, we examined the nonstoichiometry, electrical conductivity, and thermopower against oxygen activity in the entire stability range of the oxide at 800°, 900° and 1000 °C, respectively. It has been found that the positive deviation is ascribed essentially to the hole degeneracy and quantitatively described by using the Joyce–Dixon approximation of the Fermi–Dirac integral. Consequently evaluated are the effective density of states of the valence band (20.3 ≤ log(N_v/cm⁻³) ≤ 20.5), the Fermi energy relative to the valence band edge (−5 ≤ (E_F − E_V)/kT ≤ 1.5), and the effective mass (), mobility (0.14–0.17 cm² V⁻¹ s⁻¹), and reduced heat-of-transport (ca. 0.02 eV) of holes. The stability limit of the oxide is also evaluated and compiled together with the reported values.