What is a tunnel? A comment on frequent imprecision when speaking about tunnels, at the example of two representative structures in the octahedral molecular sieve group

Abstract

A paper by Su et al (2013)[1] is focused on the presentation of \b{eta}-MnO2 (pyrolusite) as a new high-capacity electrode for Na batteries, arguing that there is plenty of place in the tunnels to host sodium, which is clearly contrary to general knowledge. Similar imprecisions are found in other work so that it might be useful to shortly address the issue of cavities and tunnels in a general way. Here, two structures of the octahedral molecular sieve group (\b{eta}-MnO2 pyrolusite and {\alpha}-MnO2 hollandite) are taken as an example and analysed at some depth regarding their tunnel structure using advanced software. Tunnels and their occupation are of fundamental importance for the application of such materials as electrodes or in strongly correlated electron systems.