Recent high-pressure experiments on iridium show a transition to a 14 atomic layer superlattice structure. Since iridium has a high bulk modulus, it is used in many high-pressure applications, for instance as a gasket for high-temperature, high-pressure diamond anvil cell experiments. The effects of pressure on this material are hence of interest. Of the transition metals, the iridium surface has been one of the most extensively studied surfaces experimentally. The field ion microscope has made it possible to observe in detail the behaviour of adatoms on the surface, and has led to interesting discoveries of the nature of atomic adsorption on the lr(111) surface. A number of theoretical and semi-empirical studies have been made on this topic. However, none of these studies take atomic relaxations into account in a satisfactory manner, and therefore do not give a complete understanding of the process of incorporation of adatoms onto the surface. In the present work, first-principles total energy calculations based on the plane wave pseudopotential method within the framework of the density functional theory are employed in the study of the bulk properties of iridium, and the crystal phases and defect structures of iridium under pressure. The bond-orientation model is extended to include the effects of pressure, and used to compute all of the ~2n defect structures of iridium as a function of atomic volume. Allowance for full atomic relaxations is made in computing the ideal and relaxed surface formation energies of the three low-index surfaces of iridium, and in investigating the nature of adsorption of single adatoms on the lr(111) surface. The formation energy of a vacancy on the Ir(111) surface is also computed. This is the first time such a calculation has been made.

Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 2003.