First principles study of structure, defects and proton insertion in MnO₂

Abstract

We present an extensive First Principles study of structure, defects and proton insertion in Mn02. It is shown that the paramagnetic extrapolations of spin-polarized results are essential to correctly reproduce pyrolusite as the ground state of Mn02. While many other structures are found to be near degenerate in energy with pyrolusite, no thermal disorder exists in the system up to several thousand degrees as the strong correlation of the Mn-vacancy order along the lines of face sharing octahedra removes any low-energy excitations from the system. Mn-vacancies compensated by protons, ubiquitously present in commercial Mn02 have a dramatic effect on phase stability and induce the formation of ramsdellite Mn02 and twinning defects. We believe these proton compensated Mn vacancies to be the source of the structural complexity of synthetic Mn02 produced either electrochemically or chemically. It is shown that protons are always covalently bonded to an oxygen atom in Mn02. In ramsdeHite, the proton prefers the pyramidal oxygen to the planar coordinated oxygen atom. In both pyrolusite and manganite, the protons may appear to be at an octahedral center in experiments as the activation barrier for hopping between the two stable sites on each side of the octahedral position is only about 25 meV. Introduction of die Wolff disorder and twinning defects is found to have a large adverse effect on the diffusivity of protons in [gamma]-Mn02. Protonation also increase barriers to proton migration due to Jahn-Teller distortion and H-H interactions. Results indicate that direct H-H interactions are not that significant compared to oxygen mediated indirect interactions, observed in manganite. Experimental and calculated ramsdellite discharge curves deviate significantly at the early stages of the reduction process. We believe that a significant source of this discrepancy is the presence of proton compensated Mn vacancies in real Mn02, which create local sites with higher discharge potential. Calculations also suggest that the ordered phase, observed in experiments at mid-reduction (groutellite, MnOOHo.5), may be due to lattice remaining coherent during intercalation.