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Ordering of carbon atoms in (V1 − xNbx)2C, (Nb1 − xTax)2C and (V1−xTax)2C has been studied by electron diffraction. In addition to three basic structures of V2C (ζ-Fe2N type), Nb2C and Ta2C [anti-Cd(OH)2 type], two superstructures are found at intermediate compositions of (V1 − xTax)2C. One of them appears also in (V1 − xNbx)2C at x = 0.2–0.5 and the other in (Nb1 − xTax)2C at x = 0.1–0.5. In these five structures, carbon atoms are aligned along the c axis occupying alternate octahedral interstices in the h.c.p. metal cell. The existence conditions of these structures are interpreted by a pairwise interaction model regarding the carbon atom-vacant interstice rows parallel to the c axis. The interaction energies for these rows up to the third-neighboring pairs are estimated from diffuse-scattering profiles observed on the specimens quenched from high temperatures. The results are discussed in comparison with the theoretical proposals for the ground-state structures based on a triangular-lattice gas model.

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