Abrupt Europium Valence Change in Eu₂Pt₆Al₁₅ around 45 K

Eu₂Pt₆Al₁₅ has been prepared from the elements via arc-melting and subsequent temperature treatment; the structure was refined from single crystal X-ray diffraction data. The compound crystallizes in an orthorhombic (3 + 1)D commensurately modulated structure (Sc₂Pt₆Al₁₅ type) with space group Cmcm(α,0,0)0 s0 (α = 2/3). Full ordering of the Pt and Al atoms within the [Pt₆Al₁₅]^δ- polyanion was observed. Magnetic measurements revealed an anomaly in the susceptibility data at T = 41.6(1) K, which was also observed as λ-type anomaly in heat capacity measurements ( T = 40.7(1) K). Temperature dependent powder X-ray diffraction experiments indicated a drastic shortening of the c axis (-18 pm, -1.1%) around 45 K, while the a axis nearly remains the same (-1 pm, -0.2%). Measurements of the electrical resistivity verified the anomaly, indicating a clear change in the electronic structure of the material. The observed anomalies in the physical measurements can be explained by a temperature driven first order valence change from Eu²⁺ at higher temperatures (>55 K) to Eu³⁺ at low temperatures. This valence change was proven by temperature dependent ¹⁵¹Eu Mössbauer spectroscopic investigations. Isostructural Eu₂Pt₆Ga₁₅ was prepared in comparison, and it shows divalent Eu atoms down to 2.5 K along with antiferromagnetic ordering at T_N = 13.1(1) K.