Neutron and x-ray diffraction study and symmetry analysis of phase transformations in lower tungsten carbide W2C

The literature data on the crystal structure of disordered and different ordered phases of the lower tungsten carbide W2C are contradictory. In this context, the symmetry analysis of all possible superstructures of the carbide W2C is performed and the physically possible sequence of phase transformations in W2C carbide is established. Atomic and vacancy ordering in the lower tungsten carbide W2C with the basic hexagonal structure of the L(')3 type is studied by the neutron and x-ray diffraction methods. It is found that the trigonal epsilon-W2C phase (space group P (3) over bar 1m) is the only ordered phase of the lower tungsten carbide over a wide temperature interval of similar to 2300-1370 K. Trigonal phase epsilon-W2C is formed at a temperature of similar to 1900-2300 K most likely by the mechanism of the second-order phase transition. The disorder-order phase transition channel and the structure of the trigonal carbide epsilon-W2C are determined. The carbon atom distribution function is calculated for the trigonal epsilon-W2C superstructure. The distribution of carbon atoms in the trigonal epsilon-W2C phase is described by two long-range order parameters eta(15) and eta(17). It is shown that the lower tungsten carbide does not undergo solid-phase decomposition to W and WC over the investigated temperature interval from 2300 to 1370 K. The phase diagram of the W-C system is refined considering data obtained for the epsilon-W2C phase.