Structural Phase Transitions in EuC2

Pure EuC2, 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, EuC2 crystallizes in the ThC2 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 (Fm3hm, Z =4) at 648 K. DTA/TG investigations confirm these results. According to Raman spectroscopy, EuC2 contains C2 2- ions (ν ~(CtC) = 1837 cm-1 ). The divalent character of Eu is confirmed by the results of magnetic susceptibility measurements and 151Eu M€ossbauer spectroscopy. In these measurements a transition to a ferromagnetic state with TC = 15 K is observed, which is in reasonable agreement with literature data. Above TC EuC2 is a semiconductor according to measurements of the electric resistivity vs temperature, again in contrast to former results. Around TC 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 EuC2 samples in the literature. These investigations explicitly show the influence of sample purity on the physical and even structural properties of EuC2