Thermodynamic stability and electronic structure of thorium boron carbides

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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 ThB 66 (O)” is found at ThB 60 , (iii) the B-rich boundary of the hexaboride Th 1-x □ x B 6 (□ denotes a vacancy) extends according to its homogeneity region 0≤x≤0.22, (iv) novel thorium diboride with AlB 2 -type (a = 0.310(1) nm, c = 0.402(1) nm) forms in a peritectic reaction at 2200±50°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 ThC 2 , whereas it had only a small impact on ThB 4 . 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.