Crystal Structures of the Metal Diborides ReB2, RuB2, and OsB2 from Neutron Powder Diffraction

Because of the very small scattering power of the light element, the crystal structures of metal borides that contain heavy metal atoms are difficult to determine unambiguously from X-ray diffraction data only. Using neutron diffraction methods and applying them to isotopically enriched B-11 boride powders, the crystal structures of ReB2, RuB2, and OsB2 were re-determined and analysed with respect to the boron atom arrangement. In accordance with the findings from X-ray diffraction experiments, the structures exhibit corrugated boron atom layers of conjugated six-rings, either seat-like (ReB2) or boat-like (RuB2, OsB2). ReB2 crystallises in the hexagonal crystal system, space group P6(3)/mmc (no. 194, a = 290.05(1) pm, c = 747.72(1) pm); OsB2 and RuB2 are isostructural and crystallise orthorhombically, space group Pmmn (no. 59, a = 464.479(5) pm, b = 286.515(3) pm, c = 404.560(6) pm (RuB2); a = 468.408(5) pm, b = 287.255(3) pm, c = 407.693(6) pm (OsB2)). Boron boron distances vary between 181.7 and 189.9 pm. For RuB2 and OsB2, shortest metal-boron distances range from 217.2 to 217.3 pm, indicating a covalent interaction between metal and boron, compared to 222.7 pm for ReB2. Metal-metal distances are between 286.5 pm and 302.2 pm. All three compounds have been described as very hard or incompressible materials.