The electron energy-loss near-edge structure (ELNES) on the N K-edges from the transition metal mononitrides with the rock-salt structure and its comparison with that on the C K-edges from the corresponding transition metal monocarbides

This paper presents the shapes of the electron energy-loss near-edges structure (ELNES) on the N K-edge of the group IV_A (Ti, Zr, Hf) and group V_A (V, Nb, Ta) transition metal mononitrides close to stoichiometry. With the exceptions of NbN and TaN, these compounds have the rock-salt (B1) structure when close to stoichiometry. NbN exists with both the rock-salt structure and a hexagonal structure. Two distinct ELNES shapes were observed from it, one of which corresponds closely with previously published data from the rock-salt structure. Under normal conditions, TaN is considered to exist only in the hexagonal form, the rock-salt form being a high-temperature/high-pressure phase although it has been reported as the result of plasma jet heating of the hexagonal form. Again two distinct ELNES shapes were observed, one of which appeared to fit into the pattern of the shapes from the other compounds with the rock-salt structure. The systematic changes of shape observed are very similar to those observed in the equivalent carbides and qualitatively follow the behaviour expected from theoretical band structures. The change in the chemical shift of the N K-edge on going from a group IV_A nitride to a group V_A nitride is ～-0·8 eV while that on going from a group IV_A carbide to a group V_A carbide is ～+0·8 eV. This difference in behaviour is explained as the result of differences in the densities of states at the Fermi levels of the compounds. The position of the first peak in the ELNES also shows a systematic change in its energy relative to the core state as the number of valence electrons in the compound increases and also as the transition series of the metal species changes. The energies, E_r, of the peaks in the ELNES relative to the threshold follow a relationship similar to that predicted by Natoli, i.e. (E_r - V)a = const. where V is the ‘muffin tin’ potential and a is the lattice parameter. The first peak gives a negative constant in the relationship. The value of constant increases for each subsequent peak up to the sixth becoming positive for the fourth and higher peaks but drops slightly on going from the sixth to the seventh peak. Each peak gives a different value of V in the relationship. The data sets for the carbides and the nitrides are systematically different in a similar way for each peak and there are deviations from linearity within each set. The systematic difference is minimized and the linearity significantly improved if the difference in the energies of two prominent peaks is used instead of E_r. This systematic variation of peak energy with lattice parameter can be used to predict the lattice parameter. If both the nitride and the carbide data for the energy of a prominent peak relative to the threshold are used, this results in a maximum deviation of 4% (or ～0·02 nm). However, if the differences in the energies of two prominent peaks are used and the data for the carbides and the nitrides are treated independently, the maximum deviation drops to 0·4% (or ～0·002 nm). At this level, uncertainties in the lattice parameters themselves come into play and better characterized materials are required to set true limits to the accuracy of the predictions. Finally some applications in the microanalysis of materials are outlined briefly.