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The crystallographic structure of vanadium carbide having a carbon-to-metal atom ratio close to 0.83 has been determined from electron diffraction and nuclear magnetic resonance (NMR) studies. Material of this composition occurs within the nominally cubic (rocksalt) phase field of the vanadium-carbon phase diagram, but in this investigation it is shown that the structure is modified substantially by an unusual type of ordering that is associated with the carbon sublattice. The ordered structure is based on two interpenetrating fcc lattices, only one of which (the metal lattice) is completely occupied. The other is only 83% filled, but the carbon atoms (and carbon vacancies) are distributed in an ordered manner on the available lattice sites. As a consequence of ordering, the electron diffraction patterns exhibit supplementary spots which may be analyzed to obtain information regarding the symmetry and size of the superlattice unit cell. Moreover, the (NMR) of the principal vanadium isotope, V51, exhibit spectra which provide information about the disposition of carbon atoms within the unit cell. In the proposed structure, which belongs to the trigonal space group F31, or its enantiomorph F32, the carbon atoms and carbon vacancies are arranged so that all the vanadium atoms have exactly five nearest neighbour carbon atoms. The observation that ordering in material of this composition leads to a distribution of atoms that is homogeneous on an atomic scale is consistent with the electronic structure of vanadium carbide as it is currently understood, and suggests that it is appropriate to refer to the ordered compound as V6C5 When the ordered compound is examined for extended periods of time in a 100 kV electron microscope, it is observed that the material becomes slowly disordered. Several possible explanations were considered for this effect, but it has been concluded that the disordering results from the displacement of carbon atoms by impinging electrons (i.e., by radiation damage). To determine the magnitude of the displacement threshold, studies have been made of the disordering rate under electron bombardment at energies from 33 keV to 100 keV, using a Faraday cup to measure superlattice spot intensities. The results have been compared with a theory for the damage process, from which it is concluded that the displacement energy of carbon atoms in V6C5 is 5.4 ev. This value is unexpectedly low in comparison with the values that have been reported for most other materials, but it is consistent with a simple model for the displacement mechanism which appears to account for the threshold energy in this, and a number of other solids also. According to this model, observable damage will occur in a solid only if the displaced atom has sufficient energy to (1) create a vacancy, (2) permit transferring the energy pulse for n interatomic distances through the lattice to a position at which the vacancy-interstitial pair is stable, and (3) form an interstitial at the terminal point of the disturbance. calculations show that for most solids, the threshold energy is determined principally by mechanisms (2) and (3), but for V6C5 only the first process is important since a large number of vacant sites are available in the carbon sublattice to accept a displaced atom with the expenditure of very little energy. The fact that V6C5 can be disordered in situ by electron microscope beam bombardment has presented an unusual opportunity to study the effect of electron channelling on radiation damage rates. It has long been a question whether significantly less damage occurs when a sample to be irradiated by energetic electrons is oriented for "anomalous transmission," a phenomenon which is similar to the Borrmann effect for x-rays and to channelling for ion beams, Experiments with ion beams have demonstrated that the damage rate in channelling orientations can be 10 times less than in -random orientations, but no analogous experiments have been performed previously for electron damage because of the small angular separation between the channelling and de-channelling orientations. The small divergence of the electron microscope beam, the ability to orient the crystal with great precision, and the fact that the full analytical capabilities of the electron microscope can be used to monitor the damage process in V6C5 have now made such an experiment possible. Finally, studies have been made of the re-ordering kinetics of radiation damaged V6C5 to determine the activation energy for carbon diffusion. These measurements, which have been made in a temperature range about 1000 o C below that for which previous values have been obtained, have important implications for the mechanical properties of V6C5 and provide an explanation for the extremely short time constant associated with the order-disorder transformation in this material. |
