When half these pyramidal channel sites are occupied, half the Mn^4+ are reduced to Mn^3+ and the surrounding octahedrae are strongly distorted by the Jahn-Teller effect. A network of hydrogen bonds distorts the channels, with a transition to a Groutite-type structure. The other half of the pyramidal oxygens are then hydroxylated at a different rate. This first stage electrochemical reduction should be completely reversible (as required for a battery), with no change in the topology of the lattice with the Ramsdellite-Groutite transformation.
However, further hydroxylation attacks the planar oxygen, leaving isolated layers of Mn^4+, which are immediately and irreversibly further reduced to Mn(OH)2 pyrochroite.
Irreversible changes, which are obviously not desirable in a rechargable battery, are observed in an earlier stage in MnO2 samples containing large amounts of rutile defects. In rutile, there are only single channels, and the planar oxygen is apparently attacked at an earlier stage, producing correspondingly large amounts of pyrochroite that can readily be seen in the neutron diffraction patterns.
Such experiments can help us understand the undesirable irreversible processes that occur, and may eventually help in the design of a rechargable MnO2 battery. Similar measurements have been attempted with lead acid batteries, where neutrons are even more interesting for locating the light oxygens in the presence of the very heavy lead atoms.