Importance of long-range charge transfer shown by multiscale modeling at realistic conditions
Point defects such as vacancies, when an atom is missing from the regular crystal lattice, or impurities, when one atom is substituted by a different species, play an important role in many technological applications, and they are fascinating and challenging to study in experiment and theory.
While sometimes oxides of high purity are needed, often defects are indeed desirable. For example, when a metal oxide is used as base material for a catalyst, surface oxygen vacancies can enhance the catalytic selectivity, driving reactions towards the desired products. Therefore, understanding the defect physics in oxides is very valuable. What makes the task complex is the dependence of defect type, concentration, and charge state on outer conditions such as temperature, pressure, and on concentration of other defects. Experimental determination of point defect stabilities and concentrations is challenging, in particular at realistic conditions, since it is hard to reach thermodynamic equilibrium. In theoretical studies often only the limiting case of small defect concentrations and/or non-interacting defects are considered. However, metal-oxide samples are typically doped, either intentionally or unintentionally. In this work it is shown how charge state and concentration of oxygen vacancies in magnesium oxide are largely determined by electrostatic effects due to charge transfer between surface vacancies and bulk dopants. As a consequence of the charge transfer, a space-charge region is built up that reaches deep into the bulk. To provide a quantitative estimate of the surface defect concentration as a function of outer conditions and dopant concentration, we apply an ab initio theoretical approach that combines high-accuracy calculations of the electronic structure of the material with a thermodynamic model, thus relating microscopic with macroscopic material properties. The gained insights can be used to tune oxygen vacancy concentrations at the surface of metal oxides. Furthermore, the results suggest that experimental information on doping concentrations and profiles may provide new insights into catalytic activity of oxide surfaces.
Norina A. Richter, Sabrina Sicolo, Sergey V. Levchenko, Joachim Sauer, and Matthias Scheffler: Concentration of Vacancies at Metal-Oxide Surfaces: Case Study of MgO(100), Phys. Rev. Lett. 111, 045502 (2013) , doi:110.1103/PhysRevLett.111.045502. http://link.aps.org/doi/10.1103/PhysRevLett.111.045502