RETRACTED: The valence states of Europium in Eu-based intermetallic compounds: A review

The valence state of Eu in intermetallic compounds (IMCs) is a key factor that governs their electronic and magnetic behavior. Despite extensive studies across different material families and external conditions, a unified understanding of how Eu valence state is controlled and how it translates into measurable functionalities remains incomplete. In most reported Eu-based IMCs, Eu is predominantly divalent (Eu 2+ ), yet the average valence can be driven toward trivalency (Eu 3+ ) under perturbations such as temperature, pressure, chemical substitution, and structural engineering. This review summarizes and organizes the literature on Eu valence state in IMCs, with emphasis on the spatial distribution of Eu 2+ and Eu 3+ , the microscopic mechanisms that enable valence tuning, and the resulting evolution of physical properties. We highlight that Eu valence state is closely linked to the local crystallographic environment and bonding, which provides a consistent framework for interpreting changes in magnetism and electronic transport. Across bulk and thin-film platforms, tuning approaches including epitaxial strain and interfacial chemical interactions can be rationalized by their impact on electronic structure and lattice degrees of freedom. By integrating these relationships, this review aims to guide the design of Eu-based IMCs and to inform studies of other variable-valence rare-earth systems.