Eu 2 CuSe 3 Revisited by Means of Experimental and Quantum‐Chemical Techniques

The bonding nature between chalcogenides and rare‐earth‐elements is typically described as ionic in the spirit of the Zintl‐Klemm formalism; yet, recent efforts showed that lanthanides also act as d ‐metals in transition‐metal‐post‐transition‐metal‐element bonding. Hence, how can we describe the bonding nature between chalcogen and europium atoms, which have frequently acted as electron‐donors like group‐I/II‐elements? To answer this question, we prototypically explored the electronic structure of Eu 2 CuSe 3 , which was obtained in considerable yields from solid‐state reactions of the pure elements at 600 °C. The crystal structure of Eu 2 CuSe 3 was determined based on X‐ray diffraction experiments and it is composed of diverse types of linear chains of selenium polyhedra enclosing the copper and europium atoms. These chains are condensed into [EuCuSe 3 ] layers, which are separated by additional europium atoms. From analyses of the crystal structure and electronic structure of Eu 2 CuSe 3 , it is clear that there are two different europium valence states, whose nature controls if europium acts as an electron‐donor like a group‐I/II‐element or as a d ‐metal.