The structure of a model surface: i.e., an Fe monolayer on W(110) was studied via nuclear resonant scattering of X-rays. It was concluded that, at room temperature, the structure was nearly perfect, whereas upon increasing the temperature to 770K an increasing fraction of the Fe atoms feels the appearance of defects manifested by a beat structure due to an electric field gradient. The field gradient was caused by an asymmetry of the neighbourhood of part of the atoms and attributable to an increasing fraction of vacancies in the monolayer. With increasing temperature the beat structure gets smeared due to the onset of dynamics in the surface, i.e., the motion of the defects and consequently of the atoms. From the temperature dependence of the amplitude of the beat, the vacancy concentration was deduced, and thus the vacancy formation energy (0.17eV) and from the broadening, with less accuracy, the vacancy migration energy (0.16eV); resulting in 0.33eV for the activation energy of diffusion.

Diffusion Jumps of Single Atoms into Vacancies in an Iron Monolayer Studied by Nuclear Resonant Scattering. G.Vogl, E.Partyka-Jankowska, M.Zając, A.I.Chumakov: Physical Review B, 2009, 80[11], 115406