Atomic-vacancy ordering in the carbide phase ζ-Ta4C3t-x

The structure of the nonstoichiometric trigonal (rhombohedral) carbide ζ-Ta₄C_3t-x forming in the tantalum-carbon system is studied by neutron diffraction, x-ray diffraction, and metallography. The unit cell parameters of the trigonal (space group R-3m) carbide ζ-Ta₄C_3?x (TaC_0.67) are found to be a _h = 0.3123 nm and c _h = 3.0053 nm. It is shown that the metallic close-packed sublattice of the ζ-Ta₄C_3?x phase consists of alternating blocks in which the metallic atoms are arranged in the same manner as on the fcc sublattice of the cubic carbide TaC_y and the hcp sublattice of the hexagonal carbide Ta₂C, respectively, and that the former sublattice is intermediate between the latter two sublattices. We are the first to establish experimentally that the distribution of carbon atoms and structural vacancies in the ζ-Ta₄C_3?x carbide is ordered and to calculate the distribution function of carbon atoms over the nonmetallic-sublattice sites involved in the ordering. The results obtained for the ζ-Ta₄C_3?x phase are used to refine the phase diagram of the Ta-C system.