Strong coupling between the chemical composition and crystal lattice dimensions resulting in the contraction or expansion of a material upon change of its chemical composition is known as chemical expansion or chemical strain. This phenomenon significantly influences the performance of oxide materials in different energy conversion and storage devices. In many such applications, e.g., in oxygen-permeating membranes or solid oxide fuel cells (SOFCs), the materials are subject to significant in situ variation of their chemical composition, which is accompanied by large volume changes. This may often be detrimental to the operation of the electrochemical device. Not only is chemo-mechanical coupling crucial for various practical applications, but also, when measured accurately and discussed appropriately, the chemical strain of an oxide material allows better understanding of its local electronic and defect structure. Chemical strain is also strongly correlated with diffusion phenomena in high-temperature electrochemical devices – the subject that for many years has been of particular scientific interest for John Kilner and where he holds a lot of pioneering achievements. This article reviews the state of the art in the field of chemical strain of various oxide materials, primarily those intended to operate at elevated temperatures, and aims at summarizing the available experimental, theoretical and computational insights into its origins, factors impacting its magnitude, and the available means for its a priori quantitative estimation.

Perovskites based upon substituted LaCoO_3−δ are the state-of-the-art materials for a variety of different devices for moderate high temperature applications such as solid oxide fuel cells (SOFCs) and mixed ionic and electronic conducting (MIEC) membranes. The unique feature of the oxides is their ability to undergo both thermal expansion and that induced by the defects of oxygen nonstoichiometry in the oxide lattice. The latter is chemical or defect-induced expansion. This property is extremely sensitive to the defect structure of the oxide material. Therefore reliable data on the defect structure of oxide materials is of key importance to understanding the origin of their chemically induced lattice strain. On the other hand, chemical expansion of oxide crystal cell precisely measured with ppm-resolution allows better understanding its local electronic and defect structure.

This paper presents original results on the oxygen nonstoichiometry and defect structure modeling of LaNi_0.6Fe_0.4O_3−δ in relation to its physico-chemical properties.

Essential thermodynamic functions, obtained for Sr₂NiMoO₆ and Sr₂CoMoO₆ in broad temperature range, allowed to evaluate their stability in reducing conditions.