ScRu₂B₃ and Sc₂RuB₆, new borides featuring a 2D infinite boron clustering

Abstract

Two borides, ScRu₂B₃ and Sc₂RuB₆ were obtained by argon-arc melting of the elements followed by annealing at 800 ºC. ScRu₂B₃ 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; RF²=0.0105). Sc₂RuB₆ adopts the Y₂ReB₆-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; RF²=0.0185). ScRu₂B₃ displays an unusual intergrowth of CeCo3B₂- and AlB₂- related slabs; a striking feature is a boat configuration of puckered boron hexagons within infinite B6³ nets. Sc₂RuB₆ presents two-dimensional planar nets of condensed boron pentagons, hexagons and heptagons sandwiched between metal layers. In Sc/Y substituted Y₂ReB₆-type, Y atoms are distributed exclusively inside the boron heptagons. Exploration of the Sc-Ru-B system at 800 ºC including binary boundaries employing EPMA and powder X-ray diffraction technique furthermore rules out the existence of previously reported "ScRuB₄" but confirms the formation and crystal structure of Sc₂Ru₅B₄. ScRu₄B₄ forms in cast alloys (LuRu₄B₄-type structure; space group I4₁/acd (no. 142), a=7.3543(2) Å, c=14.92137(8) Å). Cell parameters and atomic coordinates have been refined for ScRu₂B₃, Sc₂RuB₆ and ScRu₄B₄ in the scope of 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 ScRu₂B₃ and Sc₂RuB₆ as metals in concord with electronic band structure calculations.