Oxygen excess nonstoichiometry and thermodynamic quantities of La2NiO4 + δ

The oxygen excess nonstoichiometry of La2NiO4 + δ is measured as a function of temperature and oxygen partial pressure (pO2) by coulometric titration method. A positive deviation from the ideal dilution solution behavior is exhibited, and the partial molar thermodynamic quantities of La2NiO4 + δ are calculated from the Gibbs–Helmholtz equation for regular solution by introducing the activity coefficient of the charge carriers. The activity coefficient of holes is successfully calculated by using the Joyce–Dixon approximation of the Fermi–Dirac integral. The effective mass of holes ( $ m_\texth^* $ ) is 1.27–1.29 times the rest mass (m h), which indicate the action of band-like conduction and allow the effect of the small degree of polaron hopping to be ignored. The activity coefficient of holes calculated against the oxygen nonstoichiometry clearly illustrates the early positive deviation of the activity coefficient of holes from unit, leading to $ γ_\texth^ • $  ≈ 14 at δ ≈ 0.08, which is quite close to the literature value of $ γ_\texth^ • $  ≈ 10 at δ ≈ 0.08. All the evaluated thermodynamic quantities are in good agreement with the experimental literature values.