Surface segregation and adsorption effects of iron-technetium alloys from first-principles

Surface properties of Tc-rich and Fe-rich portions of the Tc-Fe binary alloy phase diagram were computed in this work on the basis of density functional theory. Tc and Fe were found to have minimal degrees of mixing in the parent phases, consistent with the experimentally derived phase diagram. The influence of oxygen on surface phase stability was also studied, with no significant impact on surface segregation or degree of surface mixing. Oxygen adsorption was shown to change the ordering of surface facets in Tc, such that the pyramidal phase becomes lower in energy than the prismatic phase, even with low coverage of oxygen. No evidence for increased surface segregation upon oxidation was found for the solid-solution phases. A potential-pH surface Pourbaix diagram was derived for Tc and H, OH and O adsorbed sub-monolayers were shown to be precursors to oxide formation. While Tc and Fe have similar reactivities and properties in their parent phases, and hence, also in solid-solution, the properties of the intermetallic are expected to be significantly different due to the size-mismatch between the elements.