The structure of the non-stoichiometric trigonal (rhombohedral) carbide ζ-Ta4C3t-x formed in the Ta-C system was studied by neutron diffraction, X-ray diffraction and metallography. The unit-cell parameters of the trigonal (space group R¯3m) carbide ζ-Ta4C3−x (TaC0.67) were found to be ah = 0.3123nm and ch = 3.0053nm. It was shown that the metallic close-packed sub-lattice of the ζ-Ta4C3−x phase consisted of alternating blocks in which the metallic atoms were arranged in the same manner as on the face-centered cubic sub-lattice of the cubic carbide TaCy and the hexagonal close-packed sub-lattice of the hexagonal carbide Ta2C, respectively, and that the former sub-lattice was intermediate between the latter 2 sub-lattices. This was the first experimental proof that the distribution of C atoms and structural vacancies in the ζ-Ta4C3−x carbide was ordered, and the distribution function of C atoms over the non-metallic sub-lattice sites involved in the ordering was calculated. The results obtained for the ζ-Ta4C3−x phase were used to refine the phase diagram of the Ta-C system.

Atomic-Vacancy Ordering in the Carbide Phase ζ-Ta4C3t-x. V.N.Lipatnikov, A.I.Gusev: Physics of the Solid State, 2006, 48[9], 1634-45