Chemically Induced Expansion of La₂NiO₄₊_δ-Based Materials

Abstract

The equilibrium chemical strains induced by the oxygen hyperstoichiometry variations in mixed-conducting La 2 Ni 1 - x M x O 4+ δ (M = Fe, Co, Cu; x = 0−0.2) with K 2 NiF 4 -type structure, were studied by controlled-atmosphere dilatometry at 923−1223 K in the oxygen partial pressure range 5 × 10 - 4 to 0.7 atm. In combination with the oxygen content measured by coulometric titration and thermogravimetry, the results reveal a very low chemical expansivity, favorable for high-temperature electrochemical applications. Under oxidizing conditions, the isothermal expansion relative to atmospheric oxygen pressure (ε C ) is less than 0.02%. The ratio between these values and the corresponding nonstoichiometry increment varies from −3 × 10 - 3 to 6 × 10 - 3, which is much lower compared to most permeable mixed conductors derived from perovskite-like cobaltites and ferrites. Consequently, the chemical contribution to apparent thermal expansion coefficients at a fixed oxygen pressure, (13.7−15.1) × 10 - 6 K - 1, does not exceed 5%. The high-temperature X-ray diffraction studies showed that this behavior results from strongly anisotropic expansion of the K 2 NiF 4 -type lattice, namely the opposing variations of the unit-cell parameters on changing oxygen stoichiometry.