  Density functional theory calculations are used to investigate a-MnO2, a
structure containing a framework of corner and edge sharing MnO6 octahedra with
tunnels in between. Placing K+ ions into the tunnels stabilizes a-MnO2 with
respect to the rutile-structure b-MnO2 phase, in agreement with experiment. The
computed magnetic structure has antiferromagnetic (ferromagnetic) Mn-Mn
interactions between corner-sharing (edge-sharing) octahedra. Pure a-MnO2 is
found to be a semiconductor with an indirect band gap of 1.3 eV. Water and
related hydrides (OH-; H3O+) can also be accommodated in the tunnels; the
equilibrium K-O distance increases with increasing oxygen hydride charge.
