New alkaline earth and rare earth representatives adopting the Ce₂Al₁₆Pt₉-type structure

Abstract The M 2 Al 16 Pt 9 series with M = Ca, La–Nd and Sm–Gd was synthesized from the elements in analogy to the literature known compounds Ce 2 Al 16 Pt 9 and Sr 2 Al 16 Pt 9 . All compounds crystallize in the orthorhombic crystal system with space group Immm and have been characterized by powder X-ray diffraction. For La 2 Al 16 Pt 9 and Eu 2 Al 16 Pt 9 also single-crystal investigations have been conducted. In all cases, Al 3 Pt 2 as well as the members of the RE Al 5 Pt 3 series (YNi 5 Si 3 type, space group Pnma ) have been observed as side phases. Annealing led in all cases to an increase of the side phases, suggesting that the title compounds decompose peritectically. In the crystal structures, polyanionic [Al 16 Pt 9 ] δ – networks can be found with large fourfold capped hexagonal prismatic cavities ( M @Al 6 Pt 6 Al 4 ), which host the respective M atoms. As seen from the unit cell volumes, the La–Nd, Sm and Gd compounds exhibit a formally trivalent oxidation state while the Ca and Eu atoms in Ca 2 Al 16 Pt 9 and Eu 2 Al 16 Pt 9 are divalent.