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Many carbides, nitrides, and oxides of the Group IVa and Va transition metals show large deviations from stoichiometric compositions. These deviations are accomplished by the subtraction of the interstitial nonmetal atoms, or in some cases the metal atoms, from their lattice sites. The formation of superlattice structures resulting from the ordering of the remaining atoms and vacancies is then possible. The structures and modes of formation of these superlattices have been examined for a number of compounds using transmission electron microscopy and electron diffraction. The nominally NaCl-type titanium and niobium monocarbides were examined within the composition limits TiC0.45 to TiC0.65 and NbCO.70 to NbC0.95 respectively. In the first system the cubic superlattice based on the composition Ti2C was found, while niobium carbide showed similarities to vanadium carbide in the formation of ordered structures based on the composition Nb6C5. Three possible structures for Nb6C5, two with monoclinic and one with trigonal symmetry, are discussed in terms of periodic low energy planar faults within the carbon-atom sublattice. Although the two hexagonal close-packed hemicarbides, V2C and Nb2C, show very small deviations from the stoichiometric composition, ordering is possible in the carbon-atom sublattice since this contains 50% vacancies. In both hemicarbides two ordered structures were found, stable at high and low temperatures. The high-temperature modifications of both V2C and Nb2C show similarities to the ε-Fe2N structure, though the exact nature of the ordered distribution could not be determined. The low-temperature modifications, although slightly different, both possess orthorhombic symmetry. Titanium and vanadium mononitrides (NaCl-type) were examined within the ranges of composition TiN0.50 to TiN1.0 and VN0.72 to VN0.95. In neither case was the presence of long-range order detected. However the hcp heminitride, V2N, exhibited nitrogen-atom order giving an ε-Fe2N-type structure. The NaCl-type monoxides of titanium and vanadium were examined within the ranges of composition TiO0.7 to TiO1.25 and VO0.70 to V01.35. In vanadium monoxide a tetragonal superlattice based on the unit cell composition V244O320 was found. The complex structure of this super-lattice, which results from vanadium-atom order, contains vanadium atoms in interstitial (tetrahedrally coordinated) positions, surrounded by four vanadium vacancies, as well as helices of vanadium vacancies. Isothermal heat-treatments enabled a partial phase diagram to be constructed showing the conditions for the formation of the V244O320 superlattice. Electrical resistivity measurements showed that vanadium monoxide was semiconducting down to liquid-nitrogen temperature. In titanium monoxide the structures of the two superlattices with compositions TiO1.0 and TiO1.25 were confirmed. Examination of quenched specimens in the region TiO0.7 to TiO0.9 showed a metastable ordered domain structure formed by the segregation of oxygen vacancies onto every third (110) plane of the cubic matrix. Specimens with compositions near TiO1.20 contained a small fraction of an orthorhombic superlattice formed by titanium vacancy segregation onto every third (110) plane. Two further investigations were made which were not directly concerned with superlattice structure. Examination of vanadium and niobium carbides within the nominal hemicarbide and monocarbide two-phase region showed the occurrence of compounds with unit cells based on long repeat sequences in the stacking of close-packed metal-atom planes. In both carbides the ζ-phase, containing twelve planes, and also in niobium carbide the ε-phase, containing nine planes, were found. The accommodation of carbon atoms in the ζ- and ε-phases resulting in possible ordered structures are discussed. Also, the modes of transformation between structures with different stacking sequences are critically examined. The second investigation was an in situ study in the electron microscope of the effect of electron irradiation on the superlattice V244O320. Disordering was observed, resulting from atomic displacements caused by collisions with electrons. The disordering process was found to proceed in two stages. Certain displacements in the V244O320 superlattice led to the formation of another superlattice with a smaller unit cell, V52O64, but with a structure closely related to that of V244O320. Further irradiation then causes the destruction of all long-range order. A value of 7.8ev was found for the vanadium-atom displacement energy, and numerical calculations based on a theoretical model for the disordering process gave good agreement with the observed time for disordering. Finally, the factors that control the formation of the observed superlattices are discussed. The various structures adopted by the different superlattices are explained in terms of the type of atomic bonding and the electronic structure in these transition-metal compounds. |
