Systematic investigation of positron annihilation in transition metals from first principles

In this work, we present a systematic theoretical study on the positron annihilation characteristics (positron lifetimes and momentum distributions) in transition metals ($3d, 4d$, and $5d$ metals) and other elements (carbon, aluminum, silicon, and phosphorus). Our calculations agree well with available reference experimental and theoretical data. We show that clear patterns exist in the evolution of the positron annihilation characteristics in pure elements. The evolution of momentum distribution in one transition metal series is mainly contributed by the filling of $d$-band electrons. For the positron lifetimes, the lifetimes of transition metals evolve with their $d$-band filling in a similar behavior as their atomic volumes. A case study is performed to show qualitatively the effect of solute elements on the Doppler spectra of defects. It is demonstrated that vacancy-solute complexes depict similar annihilation characteristics as the corresponding pure solute elements, meaning that vacancy-solute complexes can be reliably identified if the Doppler spectra of the pure solute elements are known. For the positron lifetimes, we found that they have a linear relation with the atomic volumes of elements for the same transition metal series. This work is expected to improve understanding of the positron annihilation characteristics of transition metals. The results could be used to investigate and identify the microstructures in alloys and compounds, such as vacancy-solute complexes, solute clusters, precipitates, and vacancies in different sublattices of compounds.