Stoichiometric and defective terminations of the β-MnO2 (110), (100), and (101) surfaces are investigated as a function of oxygen partial pressure and temperature using ab initio thermodynamics. In agreement with studies on other rutile-type minerals, the (110) surface is predicted to be the most stable surface, followed by the (100) surface and then the (101) surface. The (110) and (101) surfaces are found to oxidize by formation of stable manganyl groups (Mn=O) at high oxygen chemical potentials, whereas the (100) surface is not likely to be oxidized under experimentally relevant conditions. All of the β-MnO2 surfaces studied undergo reduction processes in UHV, resulting in many complicated structural changes. A number of the reduced surfaces exhibit new surface reconstructions not yet observed for any rutile-type mineral. Analysis of the dependence of manganese coordination geometry on oxidation state is carried out to facilitate understanding of the surface reconstructions. It is determined that the competition between optimizing d-orbital occupation and minimizing steric and repulsive electrostatic interactions drives the surface reconstructions observed for reduced β-MnO2 surfaces. Symmetry-breaking at the surface allows for Jahn-Teller distortion of the reduced MnIII coordination sphere for some surface reconstructions.
Citation: Journal of Physical Chemistry C
Pub Type: Journals
ab initio thermodynamics, density functional theory, manganese dioxide, molecular orbital analysis, surfaces