First-principles DFT + U studies of the atomic, electronic, and magnetic structure of α-MnO2 (cryptomelane)

Density functional theory DFT + U calculations are used to investigate α-MnO2, a structure containing a framework of corner and edge sharing MnO6 octahedra with tunnels in between. Placing K+ ions into the tunnels stabilizes α-MnO2 with respect to the rutile-structure β-MnO2 phase, in agreement with experiment. The computed magnetic structure has antiferromagnetic (ferromagnetic) Mn–Mn interactions between corner-sharing (edge-sharing) octahedra. Pure α-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.