First-principles studies of the atomic, electronic and magnetic structure of alpha-MnO2 (cryptomelane)
Eric J. Cockayne, Lan (. Li
First-principles density functional theory calculations are used to investigate alpha-MnO2, a structure containing a framework of corner and edge sharing MnO6 octahedra with tunnels in between. Calculations show that placing additional species, such as K+ ions,into the tunnels can stabilize alpha-MnO2 with respect to the rutile-structure beta-MnO2 phase,in agreement with experiment. The determined magnetic structure has antiferromagnetic Mn-Mn interactions between corner-sharing octahedra and ferromagnetic Mn-Mninteractions between edge-sharing octahedra, but long-range magnetic order is only expected at cryogenic temperatures. Pure alpha-MnO2 is found to be a semiconductor with an indirect band gap of 1.3 eV. The main effect of placing ions such as K+ inside the tunnel is to shift the Fermi level. Water and related hydrides (OH-; H3O+) can also be accommodated in the tunnels. When both K+ and an oxygen hydride are present in the tunnel, the K-O distance increases with increasing oxygen hydride charge, as expected from electrostatics.
and Li, L.
First-principles studies of the atomic, electronic and magnetic structure of alpha-MnO2 (cryptomelane), Chemical Physics Letters, [online], https://doi.org/10.1016/j.cplett.2012.06.061
(Accessed May 30, 2023)