Ordering in the ζ-Ta4C3−x carbide phase

The structure of the ζ-Ta₄C_3−x nonstoichiometric trigonal (rhombohedral) carbide, which is formed in the tantalum-carbon system, has been analyzed by neutron diffraction, x-ray diffraction, and metallography. The ordered distributions of carbon atoms and structure vacancies have been experimentally determined and the distribution function of carbon atoms over the nonmetallic-sublattice sites, where ordering occurs, has been calculated. The parameters of the unit cell of the ζ-Ta₄C_3−x (TaC_0.67) trigonal (space group R[`3] mR\overline 3 m ) carbide are found to be a _H = 0.3123 nm and c _H = 3.0053 nm. It has been shown that the metallic close packed sublattice of the ζ-Ta₄C_3−x carbide is formed by alternating blocks, where metal atoms are located both in the fcc and hcp sublattices of the TaC_y cubic and Ta₂C hexagonal carbides, respectively.