Ba7C2Al2O9 : a barium oxide carbide oxoaluminate with the structured formula Ba7O[C2][AlO4]2

Abstract

The mixed‐anionic barium compound Ba7O[C2][AlO4]2 is obtained by serendipity via a high‐temperature synthesis route designed to yield “Ba5[AlC2]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 [AlO4]5- tetrahedra alongside O2- and [C2]2- anions provide appropriate coordination spheres for the seven crystallographically independent Ba2+ cations with C.N. = 7-9. These structural elements and their incremental volumes are compared to those of related compounds. Irregular [OBa5+1]10+ octahedra and ([C2]Ba5+2)12+ polyhedra complete the crystal structure of Ba7O[C2][AlO4]2, whose most covalent features (ortho‐oxoaluminate tetrahedra [AlO4]5- and acetylide dumbbells -[C≡C]-) can also be identified unambiguously by single‐crystal Raman spectroscopy.