surfaces with more positive values of the (110) surface formation energies have positive termination
energies, i.e. PtO2, SnO2, RuO2 and IrO2. Other factors influencing the termination energy are the
lattice mismatch between the MnO2 and the guest oxide. It turns out that all compounds plotted in
Figure 3, apart from SnO2, exhibit calculated lattice parameters, a, ranging from 4.5 Å to 4.725,
comparable to the 4.499 Å of MnO2. Of all the oxides, the lattice parameters of GeO2 are closest to
matching the lattice parameters of MnO2; this could explain why this compound has the most
negative termination energy. On the other hand, SnO2 has the largest value of a, which would make
its incorporation into MnO2 less favorable, leading to an anomalously positive termination energy.
The calculated lattice constants are all listed in the Supporting Information in Table S1. Out of the six
guest materials, TiO2 and GeO2 stand out as the only materials exhibiting negative termination
energies.
To further investigate the termination we modeled kinked (120) MnO2 surfaces. The kinked structures
were made twice as large as the stepped structures but repeated with a shift of roughly one lattice
constant in the direction orthogonal to the step edge. When further repeated the stepped edge now
contains kinks, as seen in Figure 2c, where the grey spheres indicate the exposed edges. Due to a
difference in oxygen coordination for the resulting kinked structure, two Mn atoms are replaced with
Ti, forming the kink as shown in Figure 2c. This resulted in an average oxygen coordination of two for
each Ti, which is consistent with the stepped surface model. The resulting termination energy for Ti is
-0.3 eV/AtomTi, indicating that there is a driving force for this oxide to be located at the kink sites.
Since it is more negative than for the stepped surface, the kinks are more likely to be occupied by Ti
than the step sites. We also simulated an Ir-terminated (120) MnO2 kink surface; its stability was +0.1
eV (MO2)-1, i.e. it is unstable. Consequently, the data for the kink-terminated Ir and Ti surfaces seem
to be consistent with the equivalent data for the stepped surfaces.
In order to explore the possibility that the guest oxide, in particular TiO2, could form undesired
structures with MnO2, we also simulated the incorporation of Ti into the bulk of MnO2, the
substitution of Ti into the (110) surfaces of MnO2 and the formation of separate MnO2 and TiO2
phases. It turned out that none of these undesired structures were stable, as described in the