An Unusual Valence State: Trivalent Europium in Intermetallic Eu₂ Ir₃ Al₉

Eu₂ Ir₃ Al₉ , synthesized from the elements in tantalum tubes, is one of the rare examples for trivalent europium in the field of intermetallic compounds. The compound crystallizes in the Y₂ Co₃ Ga₉ -type structure (space group Cmcm), with lattice parameters fitting in between the isostructural samarium and gadolinium compounds. In the crystal structure, the Eu atoms form Al₃ -triangle-centered honeycomb layers and exhibit a coordination number of 17 in the shape of a fivefold-capped hexagonal prism (Eu@Ir₆ Al₆ +Al₅ ). Magnetic measurements indicate an overall low susceptibility, in line with van Vleck paramagnetism caused by the Eu³⁺ cations. Fits of the susceptibility yield a coupling constant of λ=290(10) K and an effective magnetic moment of μ_eff =4.56(1) μ_B , in line with a slight hybridization of the ⁷ F₀ and ⁷ F₁ state. ¹⁵¹ Eu Mössbauer spectroscopic investigations unambiguously prove the presence of solely Eu³⁺ in the bulk material.