ISBN 9783843924504

978-3-8439-2450-4, Reihe Anorganische Chemie

Stefanie Busch
Gemischtkationische Dicarbide der Lanthanoide, Alkali- und Erdalkalimetalle Kristallstrukturen und Phasenumwandlungen

309 Seiten, Dissertation Universität Köln (2014), Softcover, A5

Zusammenfassung / Abstract

The work within presents the synthesis and characterization of two new solid solutions: EuxBa1-xC2 and YbxBa1-xC2. The synthesis is based upon previously known solid solutions LnxEA1-xC2 such as EuxCa1-xC2 prepared by Link. This solid solution was reproduced and fully characterized in this study. To investigate the structural characteristics of the solid solutions EuxCa1-xC2 as well as the already known series YbxSr1-xC2 and YbxEu1-xC2 in more detail, selected compounds were investigated by temperature-dependent synchrotron powder diffraction, with structural characterization carried out using Rietveld refinements. All compounds crystallize in the known structure types of CaC2: CaC2-I (I4/mmm), CaC2-II (ThC2 type, C2/c), CaC2-III (C2/m) and CaC2-IV (Fm3 ̅m). The results suggest that the formation of a complete solid solution or the formation of a miscibility gap is preferred to the formation of new ternary compounds. The results were summarized and compared with literature, whilst similarities and differences were worked out and then placed in relation to each other. The occurrence of various structure types, which form a complete solid solution series as well as the occurrence of a miscibility gap are dependent on the mole fraction x, the temperature, the difference of the ionic radii, the type of cations and the difference of the lattice parameters. The investigated solid solutions were tested if Vegards law was obeyed, and the factors that result in deviations were determined. It was found that miscibility gaps as well as valence changes are the main reasons for deviations from Vegards law. Three different ways to decrease the lattice strain in the solid solutions were observed: decrease of the phase transition temperatures, valence changes of lanthanide cations and formation of a miscibility gap. The valence of ytterbium and europium were elucidated by XANES spectroscopy. While ytterbium reveals significant valence changes, europium is in the divalent valence state in all investigated compounds. Additionally temperature-dependent neutron diffraction was performed on the compound CsNaC2 prepared by Liebig. His model to describe the thermal motion of the C2 dumbbells was confirmed by the data.