Oxygen nonstoichiometry and electron-hole transport in La2Ni0.9Co0.1O4+δ

The p(O2)-T-δ diagram of La2Ni0.9Co0.1O4+δ with K2NiF4-type structure was determined by coulometric titration technique at 923–1223 K in the oxygen partial pressure range from 10−4 to 0.6 atm, where the content of extra oxygen is 0.08–0.15. The Seebeck coefficient and total electrical conductivity, predominantly p-type electronic, were analyzed as functions of the oxygen nonstoichiometry. The p(O2)-T-δ diagram can be adequately described by equilibrium process of oxygen incorporation, with electron hole localization on Ni2+ and Co3+ cations and the Co4+ state blocking neighboring nickel sites. As expected, the thermodynamic functions of these processes and the blocking factor are independent of defect concentrations. The concentration of oxygen vacancies, formed in the perovskite-like layers of K2NiF4-type lattice due to intrinsic Frenkel disorder, was found negligible. The temperature-activated character of hole mobility confirms a hopping conduction mechanism, whilst the corresponding activation energy, 10.6–16.0 kJ×mol−1, decreases with increasing oxygen content. The partial molar enthalpy and entropy for overall oxygen intercalation reaction vary in the ranges −286 to −177 kJ mol−1 and 153 to 164 J mol−1 K−1, respectively.