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Radzieowski, Mathis; Stegemann, Frank; Block, Theresa; Stahl, Juliane; Johrendt, Dirk; Janka, Oliver (2018): Abrupt Europium Valence Change in Eu2Pt6Al15 around 45 K. In: Journal of the American Chemical Society, Vol. 140, No. 28: pp. 8950-8957
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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(alpha,0,0)0s0 (alpha = 2/3). Full ordering of the Pt and Al atoms within the [Pt6Al15](delta-) polyanion was observed. Magnetic measurements revealed an anomaly in the susceptibility data at T = 41.6(1) K, which was also observed as lambda-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 Eu-151 Mossbauer 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.