On the formation of the Gd3Ru4Al12versus the Y2Co3Ga9 type structure – M3Rh4Al12 (M = Ca, Eu) versus M2T3Al9 (M = Ca, Sr, Eu, Yb; T = Ir, Pt)

Abstract

The new Y₂Co₃Ga₉ and Gd₃Ru₄Al₁₂ type representatives M₂T₃Al₉ (M = Ca, Sr, Eu; T = Ir, Pt) and M₃Rh₄Al₁₂ (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 Ca₃Rh₄Al₁₂ and Eu₃Rh₄Al₁₂ (hexagonal Gd₃Ru₄Al₁₂ type, P6₃/mmc) as well as Ca₂Ir₃Al₉ and Ca₂Pt₃Al₉ (orthorhombic Y₂Co₃Ga₃ 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 M₃Rh₄Al₁₂ members, the connection via common edges in the case of the M₂Ir₃Al₉ 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 Ca₃T₄Al₁₂ 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 [T₄Al₁₂]^δ− over the [T₃Al₉]^δ− framework.