Plates 8-11] A systematic field-ion microscopy study of several iridium-based dilute substitutional solid solutions has been performed. Alloys of iridium with zirconium, tungsten, rhodium, platinum and ruthenium, having solute concentrations of between 0.1 and 5 at. %, have been examined by directscreen methods in order to establish the viability of differentiating atomic species solely by image contrast features arising from their differing response to the field-ionization-evaporation

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... . Reliable species recognition in this way allows the structural and crystallographic relations between the solvent and solute species to be determined collectively from careful field-evaporation sequences. A method of manual numerical analysis of micrographs has been de veloped which allows the salient contrast features from any one alloy system to be deduced. Further, the order of priority of various experimentally observed contrast features has been determined as a function of several parameters. The reliability of such investigations is discussed in terms of both experimental techniques used and the presence of artefact contrast events (e.g. field-induced surface vacancies) created by the technique itself. As a direct means of investigating such structures a t the atomic level, it is found th a t field-ion microscopy can supply information concerning the location of particular species and may also reveal some aspects of the surface electronic state of each atom species with respect to its coordinating surroundings. f The phosphor used to record m ost of the images was W ilamite (H 918) in which iondamage is self-annealing. Some early images were recorded on less stable FF 12. 1 5 Vol. 339. A. 3 lO x X o X X o X X d X X o X X d © © © c3