The “Submetalate” Ba₁₇Al₃O₇: The First Member of a New Class of Compounds

[O7(Al2Ba17)] clusters with dialuminate cores [Al2O7]8−, which are topologically closely related to the [O3Cs11] clusters in the Cs suboxides, lie completely separated in the crystal structure of Ba17Al3O7 (together with “metallic” aluminum). Thus, in analogy to the suboxides, the metal-rich oxide can be regarded as the first member of the “submetalates”.