Ba₇C₂Al₂O₉: A Barium Oxide Carbide Oxoaluminate with the Structured Formula Ba₇O[C₂][AlO₄]₂

The mixed‐anionic barium compound Ba 7 O[C 2 ][AlO 4 ] 2 is obtained by serendipity via a high‐temperature synthesis route designed to yield “Ba 5 [AlC 2 ] 2 ”. Its new crystal structure is determined from X‐ray diffraction data of selected single crystals exhibiting the orthorhombic space group Pnma with the lattice parameters a = 2264.21(16), b = 597.08(4), and c = 1114.67(8) pm for Z = 4. Isolated [AlO 4 ] 5− tetrahedra alongside O 2− and [C 2 ] 2− anions provide appropriate coordination spheres for the seven crystallographically independent Ba 2+ cations with C.N. = 7–9. These structural elements and their incremental volumes are compared to those of related compounds. Irregular [OBa 5+1 ] 10+ octahedra and ([C 2 ]Ba 5+2 ) 12+ polyhedra complete the crystal structure of Ba 7 O[C 2 ][AlO 4 ] 2 , whose most covalent features ( ortho ‐oxoaluminate tetrahedra [AlO 4 ] 5− and acetylide dumbbells − [C≡C] − ) can also be identified unambiguously by single‐crystal Raman spectroscopy.