First-Principles Studies of the Atomic, Electronic, and Magnetic   Structure of a-MnO2 (Cryptomelane)
  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. 
Eric Cockayne and Lan Li
cond-mat.mtrl-sci