Intolerance of the Ruddlesden–Popper La2NiO4+δ structure to A-site cation deficiency

Abstract

Tuning cation nonstoichiometry is an effective approach to modify stability and functional properties and to assist 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 Ln2NiO4+δ nickelates. La2-xNiO4±δ (x = 0-0.10) and Nd1.95NiO4±δ were selected as model compositions. Ceramic materials were sintered in air at 1350-1450°C for 10-40 h and characterized by the combination of experimental (XRD, ND, SEM, EDS, TGA, measurements 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 impurity in the La2-xNiO4±δ series increased with x, while the parameters of the orthorhombic crystal lattice remained composition-independent. Refinement of neutron diffraction patterns of La2NiO4+δ and La1.95NiO4±δ 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 Ruddlesden-Popper La2NiO4+δ structure is ≪1 at.%, if any. The experimental findings are supported by the computer simulations showing that the formation of lanthanum-deficient La1.95NiO4 is energetically less favorable compared to cation-stoichiometric La2NiO4+δ co-existing 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 Ruddlesden-Popper structure. Within experimental error, nominal cation deficiency had no effect on the electrical conductivity and oxygen permeability of La2-xNiO4±δ ceramics.