ScRu2B3 and Sc2RuB6: Borides Featuring a 2D Infinite Boron Clustering

Two borides, ScRu ₂B ₃ and Sc ₂RuB ₆, were obtained by argon-arc melting of the elements followed by annealing at 800 °C. ScRu ₂B ₃ exhibits a new structure type with the space group Cmcm (a = 3.0195(2) Å, b = 15.4056(8) Å, c = 5.4492(3) Å single crystal X-ray data; R _F ² = 0.0105). Sc ₂RuB ₆ adopts the Y ₂ReB ₆-type structure (space group Pbam; a = 8.8545(2) Å, b = 11.1620(3) Å, c = 3.4760(1) Å single crystal X-ray data; R _F ² = 0.0185). ScRu ₂B ₃ displays an unusual intergrowth of CeCo ₃B ₂- and AlB ₂-related slabs; a striking feature is a boat configuration of puckered boron hexagons within infinite graphite like boron layers (6 ³ nets). Sc ₂RuB ₆ presents two-dimensional planar nets of condensed boron pentagons, hexagons, and heptagons sandwiched between metal layers. In Sc/Y substituted Y ₂ReB ₆-type, Y atoms are distributed exclusively inside the boron heptagons. Exploration of the Sc-Ru-B system at 800 °C including binary boundaries employing EPMA and powder X-ray diffraction technique furthermore rules out the existence of previously reported "ScRuB ₄" but confirms the formation and crystal structure of Sc ₂Ru ₅B ₄. ScRu ₄B ₄ forms in cast alloys (LuRu ₄B ₄-type structure; space group I4 ₁/acd (No. 142), a = 7.3543(2) Å, c = 14.92137(8) Å). Cell parameters and atomic coordinates have been refined for ScRu ₂B ₃, Sc ₂RuB ₆, and ScRu ₄B ₄ in the scope of the generalized gradient approximation. Ab initio electronic structure calculations indicate a moderate electronic density of states at the Fermi level situated near the upper edge of essentially filled d-bands. Electrical resistivity measurements characterize ScRu ₂B ₃ and Sc ₂RuB ₆ as metals in concord with electronic band structure calculations.