Crystal Structure Refinement and Bonding Patterns of CrB4: A Boron-Rich Boride with a Framework of Tetrahedrally Coordinated B Atoms

Crystals of chromium tetraboride, a recently proposed candidate superhard material, have been grown for the first time to allow for a first structure refinement of the compound [orthorhombic, space group Immm (No. 71), a = 474.82(8) pm, b = 548.56(8) pm, and c = 287.17(4) pm, R value (all data) = 0.018]. The previously proposed structure model is confirmed, and accurate interatomic distances are presented for the first time. First-principles electronic structure calculations emphasize the unique framework of three-dimensionally linked B atoms that are tetrahedrally coordinated and carry a slightly negative charge. All B B bonding is of the 2-center 2-electron type. CrB4 is metallic with a pseudogap at the Fermi level.