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Three Haeckelite structures. (A) Oblique O5,6,7, (B) a rectangular R5,7, and (C) hexagonal H5,6,7 (nomenclature from ref. 2), and (D) octagon-square structure. In MC2B2 borocarbides, lighter carbon atoms marked with red circles are replaced with boron in (B) M = Sc and (D) M = Ce, Y, Ca, Ln (metal ions located above the center of heptagons and octagons, respectively). Bonds marked “x,” when rotated, convert a 5–7–7–5 patch into four hexagons.
