Intolerance of the Ruddlesden-Popper La₂NiO₄₊ Structure to A-Site Cation Deficiency

Tuning the cation nonstoichiometry is an effective approach to modify the stability and functional properties and to assist the surface redox engineering of perovskite oxides. This work addresses the possibility of the introduction of cation vacancies into the Ln sublattice of perovskite-related Ruddlesden-Popper Ln(2)NiO(4+delta) nickelates. La2-xNiO4 +/-delta (x = 0-0.10) and Nd1.95NiO4 +/-delta were selected as model compositions. Ceramic materials were sintered in air at 1350-1450 degrees C for 10-40 h and characterized by the combination of experimental (X-ray diffraction, neutron diffraction, scanning electron microscopy, energy-dispersive spectroscopy, thermogravimetric analysis, and measurement of electrical transport properties) and computational (static lattice and molecular dynamics simulations) methods. All nominally A-site-deficient materials comprised nickel oxide as a secondary phase. The fraction of NiO impurities in the La2-xNiO4 +/-delta series increased with x, while the parameters of the orthorhombic crystal lattice remained composition-independent. Refinement of neutron diffraction patterns of La2NiO4+delta and La1.95NiO4 +/-delta yielded the cation ratio La/Ni = 2:1 in the Ruddlesden-Popper phase for both materials. The results indicate that the concentration of cation vacancies that can be tolerated in the A sublattice of the Ruddlesden-Popper La2NiO4+delta structure is << 1 at. %, if any. The experimental findings are supported by computer simulations, showing that the formation of lanthanum-deficient La1.95NiO4 is energetically less favorable compared to cation-stoichiometric La2NiO4+delta coexisting with NiO or La4Ni3O10 secondary phases and that introduction of lanthanum vacancy results in enhanced diffusivity of A-site cations at elevated temperatures and destabilization of the Ruddlesden-Popper structure. Within experimental error, the nominal cation deficiency had no effect on the electrical conductivity and oxygen permeability of La2-xNiO4 +/-delta ceramics.