Thermodynamic Stability and Electronic Structure of Thorium Boron Carbides

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Abstract
This work presents a study of the thermodynamic stabilities of the thorium boron carbides including a revised version of the Th-B binary system: (i) a maximum of 1 at.% Th dissolves in beta boron, (ii) the “hectoboride ThB66(O)” is found at ThB60, (iii) the B-rich boundary of the hexaboride Th1-x□xB6 (□ denotes a vacancy) extends according to its homogeneity region 0≤x≤0.22, (iv) novel thorium diboride with AlB2-type (a = 0.310(1) nm, c = 0.402(1) nm) forms in a peritectic reaction at ~2200°C. Based on DFT derived energies of formation, thermodynamic phase equilibria in the binary system Th-B have been calculated i.e. phase relations have been defined for Th-B up to the melting range.For all ternary Th-B-C compounds, density functional theory calculations were made within the pseudopotential approach of VASP utilizing a general gradient approximation for the exchange correlation functional. A metaGGA approach was then applied to all compounds in their equilibrium structures, which significantly lowered the enthalpies of formation for ThC and ThC2, whereas it had only a small impact on ThB4. Atomic volumes and charges were computed by the concept of Bader and distinctly ionic features are derived consisting of Th+2, B+1, B-½, C-1, C-2 and C-3 ions. All compounds are, however, of metallic character.

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