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

TL;DR

This paper clarifies the imprecise terminology used for tunnels in molecular sieve structures, analyzing two specific MnO2 structures to improve understanding of their tunnel features relevant to electrode applications.

Contribution

It provides a detailed analysis of tunnel structures in ta-MnO2 and lpha-MnO2, addressing common misconceptions and emphasizing the importance of precise terminology.

Findings

Analysis of ta-MnO2 and lpha-MnO2 tunnel structures

Highlights the imprecision in describing tunnels in existing literature

Provides a clearer understanding of tunnel occupation in these structures

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.