Atomic and vacancy ordering in carbide {zeta}-Ta{sub 4}C{sub 3-x} (0.28{<=}x{<=}0.40) and phase equilibria in the Ta-C system
- Institute of Solid State Chemistry, Ural Division of the Russian Academy of Sciences, 620041 Yekaterinburg (Russian Federation)
The structure of nonstoichiometric carbide phase {zeta}-Ta{sub 4}C{sub 3-x} formed in the tantalum-carbon (Ta-C) system is studied by X-ray and neutron powder diffraction and metallography. Investigated carbide {zeta}-TaC{sub 0.67} crystallizes in a trigonal (rhombohedral) space group R3-barm with cell parameters a{sub tr}=1.0180(1) nm, {alpha}{sub tr}=17.64 deg. (or a{sub h}=0.31216(2) nm, c{sub h}=3.0058(1) nm in hexagonal axes). The closely packed metal sublattice in carbide {zeta}-Ta{sub 4}C{sub 3-x} consists of alternating blocks where metal atoms are located in the same manner as on the FCC sublattice of the cubic carbide TaC{sub y} and the HCP sublattice of the hexagonal carbide Ta{sub 2}C{sub y}. This metal sublattice represents a transition sublattice between these FCC and HCP sublattices. An ordered distribution of atoms C and structural vacancies in carbide {zeta}-Ta{sub 4}C{sub 3-x} is revealed and the distribution function of atoms C is calculated for nonmetal sublattice sites, on which ordering takes place. It is shown that one long-range order parameter {eta} describes the ordering of {zeta}-carbide and the {eta} value in the investigated {zeta}-Ta{sub 4}C{sub 3-x} phase does not exceed 0.7. Carbide {zeta}-Ta{sub 4}C{sub 3-x} is stable in bulk and powdered states over a wide temperature interval of 300 to {approx}2400 K and has a narrow homogeneity interval from TaC{sub 0.65} to TaC{sub 0.68}. Microhardness of disordered and ordered tantalum carbide TaC{sub y} with the basic B1 structure and trigonal carbide {zeta}-Ta{sub 4}C{sub 3-x} is measured. The phase diagram of the Ta-C system is refined considering data obtained for the {zeta}-Ta{sub 4}C{sub 3-x} phase. - Graphical abstract: Ordered distribution of carbon atoms C and structural vacancies in a unit cell of the trigonal (space group R3-barm) {zeta}-Ta{sub 4}C{sub 3-x} phase. The closely packed metal sublattice in carbide {zeta}-Ta{sub 4}C{sub 3-x} consists of alternating blocks where Ta atoms are located in the same manner as on the FCC sublattice of the cubic carbide TaC{sub y} and the HCP sublattice of the hexagonal carbide Ta{sub 2}C{sub y}.
- OSTI ID:
- 21043748
- Journal Information:
- Journal of Solid State Chemistry, Journal Name: Journal of Solid State Chemistry Journal Issue: 11 Vol. 180; ISSN 0022-4596; ISSN JSSCBI
- Country of Publication:
- United States
- Language:
- English
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