First-principles DFT + U studies of the atomic, electronic, and magnetic structure of α-MnO ₂ (cryptomelane)

Density functional theory DFT + U calculations are used to investigate α-MnO ₂, a structure containing a framework of corner and edge sharing MnO ₆ octahedra with tunnels in between. Placing K ⁺ ions into the tunnels stabilizes α-MnO ₂ with respect to the rutile-structure β-MnO ₂ phase, in agreement with experiment. The computed magnetic structure has antiferromagnetic (ferromagnetic) Mn-Mn interactions between corner-sharing (edge-sharing) octahedra. Pure α-MnO ₂ is found to be a semiconductor with an indirect band gap of 1.3 eV. Water and related hydrides (OH ^-; H ₃O ⁺) can also be accommodated in the tunnels; the equilibrium K-O distance increases with increasing oxygen hydride charge.