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

The M2Al16Pt9 series with M = Ca, La–Nd and Sm–Gd was synthesized from the elements in analogy to the literature known compounds Ce2Al16Pt9 and Sr2Al16Pt9. All compounds crystallize in the orthorhombic crystal system with space group Immm and have been characterized by powder X-ray diffraction. For La2Al16Pt9 and Eu2Al16Pt9 also single-crystal investigations have been conducted. In all cases, Al3Pt2 as well as the members of the REAl5Pt3 series (YNi5Si3 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 [Al16Pt9]δ– networks can be found with large fourfold capped hexagonal prismatic cavities (M@Al6Pt6Al4), 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 Ca2Al16Pt9 and Eu2Al16Pt9 are divalent. [ABSTRACT FROM AUTHOR]