1. On the formation of the Gd3Ru4Al12versus the Y2Co3Ga9 type structure – M3Rh4Al12 (M = Ca, Eu) versus M2T3Al9 (M = Ca, Sr, Eu, Yb; T = Ir, Pt)
- Author
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Stegemann, Frank, Zhang, Yuemei, Fokwa, Boniface P. T., and Janka, Oliver
- Subjects
ALKALINE earth metals ,TANTALUM ,X-ray powder diffraction ,TRANSITION metals ,COVALENT bonds ,X-ray diffraction ,CRYSTAL structure - Abstract
The new Y
2 Co3 Ga9 and Gd3 Ru4 Al12 type representatives M2 T3 Al9 (M = Ca, Sr, Eu; T = Ir, Pt) and M3 Rh4 Al12 (M = Ca, Eu) have been synthesized from the elements by heating the respective elemental compositions in sealed tantalum tubes. The samples were analysed by powder X-ray diffraction to check their purity. By applying different temperature treatments, their phase purity and crystallinity were enhanced. The crystal structures of Ca3 Rh4 Al12 and Eu3 Rh4 Al12 (hexagonal Gd3 Ru4 Al12 type, P63 /mmc) as well as Ca2 Ir3 Al9 and Ca2 Pt3 Al9 (orthorhombic Y2 Co3 Ga3 type, Cmcm) were refined from single-crystal X-ray diffraction data. All structures can be described based on distorted cube-like T@Al8 units that are connected to form strands. Additionally, an Al11 supertetrahedral building block can be identified within the structures. While the trigonal bipyramidal core of the cluster contains substantial bonding interactions in the case of the M3 Rh4 Al12 members, the connection via common edges in the case of the M2 Ir3 Al9 compounds seems to weaken these interactions. The differences in the bonding situation and the question why these different structure types are formed for the different transition metals has been targeted by quantum-chemical calculations. The calculated formation energy using three different reaction paths suggests that the stability of these phases is highly dependent on the side phases involved, even though Ca3 T4 Al12 phases are in general thermodynamically more favourable. According to the Bader analysis of the two polyanions, an improved covalent bonding can be observed in the [T4 Al12 ]δ− over the [T3 Al9 ]δ− framework. [ABSTRACT FROM AUTHOR]- Published
- 2020
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