Oxygen non-stoichiometry of Ln 4Ni2.7Fe0.3O10-δ (Ln=La, Pr)

The oxygen deficiency of iron-substituted nickelates Ln 4Ni2.7Fe0.3O10-δ (Ln=La, Pr) with the orthorhombic Ruddlesden-Popper structure was studied by thermogravimetric analysis and coulometric titration in the oxygen partial pressure range 6x10-5 to 0.7 atm at 973-1223 K. In air, the non-stoichiometry values vary in the relatively narrow ranges (2.4-4.2)x10-2 for La- and (0.01-2.0)x10-2 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 Moessbauer spectroscopy showed that the predominant state of iron cations, statistically distributed between the nickel sites, is trivalent. - Graphical abstract: Oxygen deficiency of Ruddlesden-Popper nickelates substituted with iron