Crystal structure of Th2B2C3 with unique mixed B-C structural units

X-ray single crystal data for “ThBC2” defined the crystal structure as orthorhombic (space group Pnnm; a = 1.30655(3) nm, b = 0.39757(1) nm, c = 0.36507(1) nm; RF2 = 0.0130) with the correct formula Th2B2C3. According to the new findings phase relations for the Th-B-C system at 1400 °C have been revised confirming the hitherto known thorium-boron-carbon compounds: ThBC, ThB2C and Th3B2C3. The crystal structure of Th2B2C3 as a unique structure type is characterized by a C2-branched infinite chain … B-C1-B-B-C1-B … whereby each boron atom is additionally linked to a C2-atom. Boron atoms are in a typical triangular prismatic metal coordination, C1-atoms center a bi-pyramid, Th4B2, and C2-atoms are surrounded by 5 pyramidal Th-atoms and one B-atom. Density functional theory calculations were made within the pseudopotential approach of VASP utilizing the general gradient approximation for the exchange correlation functional. Structural parameters were optimized resulting in the orthorhombic lattice parameters of a = 1.3144 nm, b = 0.3989 nm, and c = 0.3656 nm. Relativistic calculations by including spin-orbit coupling were performed for the electronic structure. Atomic volumes and charges were computed by the concept of Bader yielding the ionic charges Δq(Th) = +0.99; Δq(B) = +0.61; Δq(C1) = −1.20; Δq(C2) = −1.10. The analysis of the electronic structure shows features of chains and corresponding structural subunits with σ-like bonding. The compound appears weakly metallic with a Sommerfeld constant of γ = 3.12 mJ/(molK2) per formula unit. © 2017 Acta Materialia Inc.