Chemically induced expansion of La2NiO4+δ-based materials

The equilibrium chemical strains induced by the oxygen hyperstoichiometry variations in mixed-conducting La2Ni1-xMxO4+delta (M = Fe, Co, Cu; x = 0-0.2) with K2NiF4-type structure, were studied by controlled-atmosphere dilatometry at 923-1223 K in the oxygen partial pressure range 5 x 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 (epsilon(C)) is less than 0.02%. The ratio between these values and the corresponding nonstoichiometry increment varies from -3 x 10(-3) to 6 x 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) x 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 K2NiF4-type lattice, namely the opposing variations of the unit-cell parameters on changing oxygen stoichiometry.