The effect of interstitial oxygen formation on the crystal lattice deformation in layered perovskite oxides for electrochemical devices

In order to understand the behaviour of the crystal lattice deformation induced by the interstitial oxygen formation in La₂(Ni_0.9M_0.1)O_4+δ (M = Fe, Co, and Cu), thermogravimetry, coulometric titration, and high temperature X-ray diffraction measurements were carried out in the temperature range between 873 and 1173 K and the P(O₂) range between 10^-24 and 1 bar. Compared with non-doped La₂NiO_4+δ, La₂(Ni_0.9Fe_0.1)O_4+δ and La₂(Ni_0.9Co_0.1)O_4+δ have more interstitial oxygen while La₂(Ni_0.9Cu_0.1)O_4+δ has less. The crystal structure at high temperatures was analyzed assuming the tetragonal symmetry, I4/mmm, for all compositions. With increasing interstitial oxygen concentration, the lattice parameter perpendicular to the perovskite layer increased and that parallel to the perovskite layer decreased. Consequently, the change of the cell volume by the interstitial oxygen formation was small, meaning that macroscopic chemical expansion was small. Chemical and thermal deformation behaviour could be explained by assuming a linear relation of the lattice constants to T and δ. Apparent and true thermal expansion coefficients and chemical expansion coefficient were calculated and compared with oxygen deficient perovskite- and fluorite-type oxides. It was found that the chemical expansion coefficients of La₂NiO₄-based oxides which are induced by the formation/annihilation of interstitial oxygen are smaller than those of perovskite- and fluorite-type oxides which are induced by the formation/annihilation of oxygen vacancies.