The migration of point defects on bcc surfaces using a metallic pair potential

Abstract

An ab initio ion-ion pair potential derived from the theory of metals for bulk Na is employed to investigate point defect jump dynamics on bcc (211), (110) and (100) surfaces using molecular dynamics. This pair potential produces bcc slabs that are stable to shear, as opposed to pair potentials modeling rare gas interactions. On the channeled bcc (211) surface adatoms undergo a cross-channel exchange mechanism akin to that observed on the fcc (110) surface of the Lennard-Jones (LJ) crystal. Adatom migration parallel to the channels is much enhanced if compared to the latter case, parallel jump events being more frequent than cross-channel exchanges. These results are in accord with experimental FIM findings in metallic crystals of the two structures. The consequences of the softer repulsion of the metallic interaction (i.e., versus LJ) on point defect properties are most apparent on the compact bcc (110) face. Results for this face are reminiscent of the loosely packed fcc (100) LJ surface rather than of the close packed fcc (111) one. The mobilities of adatoms on bcc (211) and (110) surfaces are comparable. On the bcc (100) the adatom mobility is much lower instead, in accord with experiment. On the latter face adatoms are seen to migrate by an exchange mechanism, in addition to performing direct jumps between lattice sites above the surface. This is the first observation of adatom migration via exchanges involving an atom of the surface layer on isotropic surfaces. Surface vacancies are mobile on all three faces. Their mobility on the surfaces of bcc Na is overall higher than on those of fcc rare gas (LJ) crystals. The reverse is true in general for adatoms, with the noteworthy exception of channel diffusion on bcc (211) versus fcc (110). This behaviour is interpreted in terms of structure, surface packing and repulsion stiffness in the interatomic potential.