Role of surface defects in trap of monoatomic polonium on Pd surface

Abstract 210Po is a radiotoxic element and will evaporate into the cover gas above the coolant in accelerator-drive systems or lead-bismuth fast reactor. In order to prevent the build-up of Po and ensure the safety of the reactors, reliable filter systems need to be developed and Pd has been proposed as candidate filter material. To acquire fundamental knowledge of surface defects trapping monoatomic Po, adsorption and diffusion of Po on vacancy-defective Pd(111) and stepped Pd(211) surfaces have been calculated by first-principles. Strong chemisorption was obtained for Po on vacancy-defective Pd(111) and stepped Pd(211), vacancy and hcp site at step edge are preferred sites for Po adsorption. The activation energies for Po jump from vacancy on Pd(111) surface and for Po diffusion on stepped Pd(211) surface are obviously larger than that for Po diffusion on other non-vacancy sites on Pd(111) surface. The results indicate that surface defects such as vacancy and step defect are preferred sites for Po adsorption and retention.