Diffusion and formation energies of adatoms and vacancies on magnesium surfaces

This paper reports classical molecular statics calculations of magnesium {0 0 0 1}, { 1 1 ¯ 0 1 } , { 1 1 ¯ 0 0 } A , { 1 1 2 ¯ 0 } and { 1 1 ¯ 0 0 } B surfaces, specifically formation energies of defects (adatoms and surface vacancies) and flat surfaces and diffusion energy barriers of the defects. The formation energies show that the { 1 1 ¯ 0 1 } surface is thermodynamically more favorable than { 1 1 ¯ 0 0 } A , { 1 1 ¯ 2 0 } and { 1 1 ¯ 0 0 } B surfaces; in contrast, literature reports have often ignored the { 1 1 ¯ 0 1 } surface. The diffusion energy barriers of both adatoms and surface vacancies show strong diffusion anisotropy on { 1 1 ¯ 0 1 } , { 1 1 ¯ 0 0 } A , { 1 1 2 ¯ 0 } and { 1 1 ¯ 0 0 } B surfaces. Based on this anisotropy, the ratio of diffusion distances (of either adatoms or surface vacancies) along two orthogonal directions on { 1 1 ¯ 0 1 } is 37–55 at room temperature. Using the results of formation energies and diffusion energy barriers we develop a more complete understanding of surface orientations in Mg nanoblades synthesized by physical vapor deposition [F. Tang, T. Parker, H.-F. Li, G.-C. Wang, T.-M. Lu, J. Nanosci. Nanotechnol. 7 (2007) 3239]. In contrast to previous reports, we postulate that the side surfaces of Mg nanoblades are { 1 1 ¯ 0 1 } because (a) they have the second lowest surface formation energy and (b) the ratio of diffusion distances on them agrees with the experimental value of approximately 50.