The equilibrium chemical strains induced by the oxygen hyperstoichiometry variations in mixed-conducting La2Ni1
-
x
M
x
O4+
δ (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 × 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
(εC) 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 K2NiF4-type lattice, namely the opposing variations of the unit-cell parameters
on changing oxygen stoichiometry.
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