The YBaCo4O7+δ‐Based Functional Oxide Material Family: A Review

The complex cobalt oxide YBaCo4O7+δ is an exciting material from both a fundamental scientific and an applied technological point of view. It has a unique ability to reversibly absorb and desorb large amounts of oxygen in an exceptionally low‐temperature range, which makes it a promising candidate for applications in which efficient oxide ion conductivity or large oxygen storage capacity is required. It has also been considered as a model system for geometrically frustrated magnetism, which originates from the nature of the layered crystal structure of YBaCo4O7+δ, with alternating triangular and Kagome‐type cobalt oxide layers. The structure is also extremely flexible in a chemical sense, with all three cation constituents accepting a number of different substitutions, thereby allowing its properties to be tuned. In this review, our aim is to provide a compact but comprehensive overview of the work published so far on this interesting family of functional oxides.