The structure of non-stoichiometric carbide phase ζ-Ta4C3−x formed in the Ta–C system was studied by X-ray and neutron powder diffraction and metallography. Investigated carbide ζ-TaC0.67 crystallizes in a trigonal (rhombohedral) space group R3¯m with cell parameters atr=1.0180nm, αtr=17.64° (or ah=0.31216nm, ch=3.0058nm in hexagonal axes). The closely packed metal sub-lattice in carbide ζ-Ta4C3−x consists of alternating blocks where metal atoms were located 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 Ta2Cy. This metal sub-lattice represents a transition sub-lattice between these face-centered cubic and hexagonal close-packed sub-lattices. An ordered distribution of atoms C and structural vacancies in carbide ζ-Ta4C3−x was revealed and the distribution function of atoms C was calculated for non-metal sub-lattice sites, on which ordering takes place. It was shown that one long-range order parameter η describes the ordering of ζ-carbide and the η value in the investigated ζ-Ta4C3−x phase does not exceed 0.7. Carbide ζ-Ta4C3−x was stable in bulk and powdered states over a wide temperature interval of 300 to about 2400K and had a narrow homogeneity interval from TaC0.65 to TaC0.68. Microhardness of disordered and ordered tantalum carbide TaCy with the basic B1 structure and trigonal carbide ζ-Ta4C3−x was measured. The phase diagram of the Ta–C system was refined considering data obtained for the ζ-Ta4C3−x phase.

Atomic and Vacancy Ordering in Carbide ζ-Ta4C3−x (0.28≤x≤0.40) and Phase Equilibria in the Ta–C System. A.I.Gusev, A.S.Kurlov, V.N.Lipatnikov: Journal of Solid State Chemistry, 2007, 180[11], 3234-46