Pure EuC(2), free of EuO impurities, was obtained by the reaction of elemental europium with graphite at 1673 K. By means of synchrotron powder diffraction experiments, the structural behavior was investigated in the temperature range from 10 to 1073 K. In contrast to former results, EuC(2) crystallizes in the ThC(2) type structure (C2/c, Z = 4) at room temperature. A tetragonal modification (I4/mmm, Z = 2) is only observed in a very small temperature range just below the transition to a cubic high-temperature modification (Fm3m, Z = 4) at 648 K. DTA/TG investigations confirm these results. According to Raman spectroscopy, EuC(2) contains C(2)(2-) ions (nu(C[triple bond]C) = 1837 cm(-1)). The divalent character of Eu is confirmed by the results of magnetic susceptibility measurements and (151)Eu Mossbauer spectroscopy. In these measurements a transition to a ferromagnetic state with T(C) = 15 K is observed, which is in reasonable agreement with literature data. Above T(C) EuC(2) is a semiconductor according to measurements of the electric resistivity vs temperature, again in contrast to former results. Around T(C) a sharp maximum of the electric resistivity vs temperature curve was observed, which collapses on applying external magnetic fields. The observed CMR effect (colossal magnetoresistance) is much stronger than that reported for other EuC(2) samples in the literature. These investigations explicitly show the influence of sample purity on the physical and even structural properties of EuC(2).

Table 1 contains several typos and inconsistencies. A revised table is given below. Revised CIFs for each structure are deposited at FIZ Karlsruhe and are given as Supporting Information.We are thankful to Michael H€ ulsen for indicating the inconsistencies in Table 1.

A solid solution EuxSr1–xC2 (0 ≤ x ≤ 1) was synthesized by direct reaction of the elements at 1123 K. The crystal structures of these compounds, investigated by synchrotron powder diffraction, depend upon x. For x > 0.5 the monoclinic ThC2 type structure (C2/c, Z = 4) is observed and for x ≤ 0.5 the ThC2 type structure coexists with the tetragonal CaC2 type structure (I4/mmm, Z = 2). The unit cell volumes per formula unit of all EuxSr1–xC2 compounds show perfect Vegard behavior, which is due to the almost identical ionic radii of Eu2+ and Sr2+. Mössbauer spectroscopic investigations indeed reveal that europium is in the divalent state over the whole composition range. EuxSr1–xC2 exhibits several temperature dependent phase transitions that were studied by synchrotron powder diffraction and differential thermal analysis. The transition to a cubic high‐temperature modification (Fmm, Z = 4) is of special interest, as it contains information about strain effects appearing inside the modifications with ordered C2 dumbbells (ThC2 and CaC2 type structures). The linear temperature dependence of the obtained transition temperatures TPh shows that no observable strain exists in EuxSr1–xC2, which is again due to the almost identical radii of Eu2+ and Sr2+. EuxSr1–xC2 may therefore be described as a strain free dicarbide solid solution with perfect Vegard behavior.

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