Peter Rogl1, Raimund Podloucky2, Henri Noel3, Gerald Giester4
1Institute of Materials Chemistry & Research, University of Vienna, A-1090 Wien, Austria;
2Institute of Physical Chemistry, University of Vienna, A-1090 Wien, Austria;
3Laboratoire de Chimie du Solide et Materiaux, UMR-CNRS 6226, Université de Rennes I, F-35042 Rennes, France;
4Institute of Mineralogy and Crystallography, University of Vienna, A-1090 Vienna, Austria
As generation IV nuclear fuels include actinoid carbides in combination with B4C control rods, interest has been revived in the corresponding phase relations for the An-B-C systems. Preliminary studies of phase relations in the ternary systems {Th,U,Np,Pu}-B-C prompted the formation of two main isotypic series of ternary compounds {Th,U,Np,Pu}BC (UBC-type, space group Cmcm or ThBC-type, space group P4122) and {Th,U,Np,Pu}B2C (ThB2C-type, space group R-3m). Since then isothermal sections have been established for all four systems revealing further compounds, the crystal structures of which have recently been defined from single crystal and powder X-ray intensity data.
Phase equilibria in the system Th-B-C, established at 1400 ºC, reveal four ternary thorium boron carbides: ThBC, ThB2C, Th3B2C3, "ThBC2". Whereas the structures of the former are already established earlier, the crystal structure of the latter (hitherto described as "ThBC2"), has now been defined from single crystal data as a new and unique orthorhombic structure type with the proper formula Th2B2C3 (space group Pnnm, #58; a=1.30655(9) nm, b=0.39757(3) nm, c=0.36507(3) nm). The crystal structure of Th2B2C3 is characterized by C-branched infinite chains ...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. Bonding from carbon atoms to thorium appears to correspond to the sum of radii for C2, whereas distances dTh-B slightly exceed the sum of the radii. According to the formula Th2B2C3 (formerly "ThBC2"), the phase relations for the Th-B-C system have been revised.
Density functional theory calculations were made for all thorium boron carbides as well as for the homologous uranium boron carbides (including also isotypic CeB2C) within the pseudopotential approach of VASP utilizing the general gradient approximation for the exchange correlation functional. Structural parameters optimized were in good agreement with the experimental values. 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 and the charge transfer among the atoms. Particularly the analysis of the electronic structure for Th2B2C3 shows features of chains and corresponding structural subunits with б-like bonding. The DFT heat of formations were studied along the sections which involve all the ternary {Th,U} boron carbides: ThB4 - ThC2 and B - ThC as well as for the corresponding U-sections.
Physical properties have been elucidated for two series of compounds [Th,U,Np.Pu]BC and [Th,U,Np,Pu]B2C revealing interest ing magnetic and electrical behavior dependent on the amount of spdf hybridization.
