Structural Phase Transitions in EuC₂

Abstract

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).

EuxSr1–xC2 (0 ≤ x ≤ 1): A Dicarbide Solid Solution with Perfect Vegard Behavior

“…The resulting patterns were first indexed and subsequently refined by the Rietveld method to get reliable cell parameters and phase ratios. The known crystal structures of EuC 2 2 and SrC 2 7 were used as starting models for the refinements. Figure 1 shows the Rietveld refinement of Eu 0.5 Sr 0.5 C 2 as an example for the quality of the data and the refinements.…”

“…These observations reveal that EuC 2 resembles more the AE C 2 than the RE C 2 compounds. Accordingly, measurements of the Mössbauer effect show that europium is clearly in a divalent state in EuC 2 2.…”

“…This additional electron is delocalized in a conduction band. Only very recently we showed that the dicarbide of europium (EuC 2 ) shows semiconducting behavior 2. At ambient conditions it crystallizes – differing to all other RE C 2 , which are known to exist in the CaC 2 type structure ( I 4/ mmm , Z = 2) – in the ThC 2 type structure ( C 2/ c , Z = 4).…”

Valence Changes in Dicarbide Solid Solutions Induced by Structural Changes

“…metallic (REC 2 ) versus insulating (AEC 2 ) behaviour. In this context, semiconducting properties with divalent Eu(II) under ambient conditions are found for EuC 2 [1] , but under pressure metallic EuC 2 with trivalent Eu(III) is observed. [2] However, for YbC 2 an intermediate valence of approx.…”

“…The investigation of the photochromic behaviour of azobenzene (AZB) molecules embedded in different metal-organic frameworks [1] revealed that AZB can be switched from its trans-form to the cis isomer by illumination with UV light in AZB@MOF-5, AZB@MIL-68(Ga) and AZB@MIL-68(In), whereas no isomerization occurs in the system AZB@MIL-53(Al). To explain these different results, the crystal structures of AZB@MIL-53(Al), AZB@MIL-68(Ga) and f-AZB@MIL-53(Al) (f-AZB: perfluorinated AZB) were solved and refined from synchrotron powder diffraction data (ESRF: BM01B) at 120 K. Taking into account the van der Waals radii of host material and guest molecules it becomes clear that the isomerization process from trans-AZB to the cis isomer is sterically hindered in AZB@MIL-53(Al) (Figure 1), preventing the photochromic behaviour.…”