Where Δ is the termination energy, term the total energy of the terminated structure, ref the
energy of the original MnO2 structure, unit,guest the energy of a unit cell of the guest oxide in its bulk
form, and unit,MnO2 the energy of a unit cell of the MnO2 in its bulk form, which is substituted. The
energies are reported in eV/MO2 units, with M being a transition metal.
A negative Δ suggests there is a favorable driving force for the guest material to segregate to the
under-coordinated sites. An oxide with a very negative unit,guest is very stable in its pure bulk form.
Consequently, it should be unfavorable to terminate the MnO2 with an oxide that is more stable in
the bulk form.
Figure 2: a) A top view of the rutile (110) surface. Two types of metal sites are
available. The CUS sites are shown with purple spheres and the Bridge sites with dark
green spheres. b) A stepped rutile MnO2 structure based on the (120) surface used to
calculate termination energies. Here the slab is repeated 5 times in the x direction and
2 times in y. c) Kinked rutile MnO2 structure based on the (120) surface used to
calculate termination energies. Here the slab is repeated 2 times in the x direction and
2 times in y. In all three figures, the purple spheres represent manganese, red spheres
oxygen and grey spheres a terminated step or kink.
A (120) MnO2 surface was terminated with TiO2, GeO2, SnO2, PtO2, RuO2 and IrO2 at the step site. The
results are summarized in Figure 3, where the termination energy is plotted as function of the (110)
surface formation energy of the guest oxides. The plot shows that the two quantities are correlated: