Abstract
The total conductivity, Seebeck coefficient and oxygen non-stoichiometry for La1.2Sr0.8Ni0.9Fe0.1O4+δ have been measured vs temperature and oxygen partial pressure P(O2). The measurements were carried out at 800, 850, 900 and 950 °C within the P(O2) range of 10−5–0.21 atm. La1.2Sr0.8Ni0.9Fe0.1O4+δ was shown to be oxygen deficient in all temperature and P(O2) ranges studied. The calculated values of the partial molar enthalpy of oxygen depend very slightly on oxygen content (δ), indicating that La1.2Sr0.8Ni0.9Fe0.1O4+δ with the oxygen deficiency can be considered an ideal solution. The model of point defect equilibria in La1.2Sr0.8Ni0.9Fe0.1O4+δ has been proposed and fitted to experimental dependencies. Subsequent joint analysis of the defect structure and transport properties revealed that electron holes can coexist in both localized and quasi-delocalized states in the oxide: the former corresponded to high-spin state Ni3+ and the latter – to low-spin state Ni3+. The mobilities of localized electron holes were shown to be significantly lower in comparison to quasi-delocalized ones. The behavior of localized electron holes was explained in terms of a small polaron conduction mechanism; in contrast, quasi-delocalized electron holes were described in terms of a band conduction approach. The small polaron conduction mechanism was shown to be predominant in the Sr- and Fe-co-doped lanthanum nickelate.
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