Caught in Action. The Late Rare Earths Thulium and Lutetium Substituting Aluminum Atoms in the Structure of Ca14AlBi11

Described are two unprecedented cases, where the rare earth metals Tm and Lu partially substitute Al atoms in the structure of the Zintl phase Ca14AlBi11. These are the first examples within this large family where lanthanides replace the atoms of a main group element. Such crystal chemistry has never been observed before in other materials with the same structure. The uniqueness of this finding is also amplified by the fact that the rare earth metal atoms in the crystal structure are tetrahedrally coordinated, which is another remarkable trait of the new Tm- and Lu-substituted compounds.