Phase Equilibria and Structural Chemistry within Ternary Systems: Actinide Metal-Boron-Carbon
Early interests in the constitution and thermodynamic equilibria of the high melting actinidemetal boride and carbide phases were stimulated by their possible use as nuclear fuel materials. Thus the existence of several ternary borocarbide compounds has been reported in the literature however only few were characterized then.
Meanwhile most of the phase equilibria and compounds have been confirmed in our laboratory and in particular their crystal structures have been established from X-ray single crystal counter data (ThBC, Th3B2C3, ThB2C).
A detailed reinvestigation of melting behavior as well as of thermodynamic equilibria within both systems Th-B-C, U-B-C revealed the formation of several new compounds, which are observed from isothermal sections at 1000°C, 1600°C, 1800°C: Th3B2C3 (unique structure type); U2BC2 and ℓ,h-UB2C (ThB2C-type, related to AlB2-type with complex B-C ordering; phase transition ∼ 1675°C).
From melting point analysis (Pirani-technique, Ar) congruent melting behavior with relatively high melting temperatures was determined for the ternary borocarbides: ThBC (2101 ± 22°C), ThB2C (2040 ± 14°C), UBC (2144 ± 23°C), UB2C (2282 ±28°C).
Using the clear-cross principle in studying possible phase reactions, the thermodynamic stabilities (Gibbs free energy of formation) have been estimated in case of the compounds UBC (-ΔGf ⩾ 157 kJ/mole) and UB2C (-ΔGf ⩾ 201 kJ/mole).
Owing to this relative high stability of the ternary actinidemetalborocarbides as compared to their binary boride and carbide systems, ternary phase equilibria are dominated by the (Th,U)BC- and (Th,U)B2C-phases. Subsequently no phase equilibria between boron or “B4C” on the one hand and actinide metal carbides on the other hand were found to exist.
According to the classification scheme of ternary metal borides, zig-zag boron-boron chains (B-B ∼1.76–1.82 Å) with adjacent carbon atoms (B-C ∼1.55 Å) appear to be the basic structural unit in borocarbides of composition MBC as has been shown for the structure types of ThBC, UBC and YBC. From geometrical as well as structural chemical arguments YBC can be regarded as a link between the structure types of A1B2 and α-ThSi2 based upon simple shift operations. Similarly the ThBC-type is generated by a double shift (vector:1/2,0,1/2) out of the UBC-type structure.
From the viewpoint of crystal symmetry the correspondence of the metal-boron sublattice in the structure types of ThBC (P4122), αMoB(141/a 2/m 2/d) and similarly for the pair UBC, CrB (both C2/m 2/c 21/m) can be expressed in a theoretical crystallographic group-subgroup relationship. Thus a hypothetical structure (14 22; ThBC-type with idealized point positions) is found intermediate between αMoB (low temperature form) and ThBC. In this respect the shift operation ThBC-UBC among actinide borocarbides is an interesting analogue of the αMoB-CrB correlation for transition metal monoborides.
Considering the boron-boron aggregation in borocarbides MBC a gradual replacement of boron chains by boron pair formation i.e. C-B-B-C groups is observed with increasing radius ratio RM/RB (M= U,Y, Th). The developed structural regularities are confirmed from the crystal structure of Th3B2C3 (P2/m). Again the stability of this borocarbide can be understood by a topochemical combination of structural units: ThC + 2ThBC = Th3B2C3.
KeywordsBoron Carbide Vanadium Carbide Basic Structural Unit Metal Boride Ternary Phase Equilibrium
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