Oxygen non-stoichiometry of Ln4Ni2.7Fe0.3O10−δ (Ln=La, Pr)

The oxygen deficiency of iron-substituted nickelates Ln4Ni2.7Fe0.3O10−δ (Ln=La, Pr) with the orthorhombic Ruddlesden-Popper structure was studied by thermogravimetric analysis and coulometric titration in the oxygen partial pressure range 6×10−5 to 0.7 atm at 973–1223 K. In air, the non-stoichiometry values vary in the relatively narrow ranges (2.4−4.2)×10−2 for La- and (0.01−2.0)×10−2 for Pr-containing compositions, increasing with temperature. Due to the smaller size of praseodymium cations, Pr4Ni2.7Fe0.3O10−δ exhibits a substantially lower thermodynamic stability in comparison with La4Ni2.7Fe0.3O10−δ and La4Ni3O10−δ, although the oxygen content in Pr4Ni2.7Fe0.3O10−δ lattice is higher. The partial substitution of iron for nickel has no essential effect on the low-p(O2) stability limit corresponding to the transition of Pr4Ni3O10−δ into K2NiF4-type Pr2NiO4+δ. On the contrary, doping of La4Ni3O10−δ with iron decreases the oxygen vacancy concentration and shifts the phase stability boundary towards lower oxygen chemical potentials, suggesting a stabilization of the transition metal-oxygen octahedra in lanthanum nickelate lattice. The Mössbauer spectroscopy showed that the predominant state of iron cations, statistically distributed between the nickel sites, is trivalent.