Framework response to local order in hollandite

Hollandite-supergroup minerals can be described as tunnel oxides, characterized by double-chains of edge-sharing octahedra. The presence of tunnels has raised interest in these minerals and their synthetic analogs, which have been proposed as a host for radioactive waste disposal and electrode materials. An accurate description of their structures is not straightforward, even in the case of the eponymous member of the supergroup, hollandite ⁠. Various distortions have been identified in the structures, involving both tunnel and octahedral cations, possibly lowering the symmetry from ideal tetragonal to monoclinic. It is well known that such distortions can be related to the X–ray diffuse scattering features sometimes observed in hollandites. However, a thorough characterization is still lacking. Here, we studied the local structure of hollandite from the Towers (Nancy) mine, Luis Lopez Manganese District, Socorro County, New Mexico, U.S.A., combining X-ray single-crystal diffuse scattering and X-ray absorption spectroscopy. At the local level, a clear monoclinic distortion of the tunnels can be observed, and the extent of the flexibility of the octahedral framework can be related to the size of the cations occupying the tunnels. Additionally, it is evident that Ba atoms not only distribute between occupied and unoccupied positions along the tunnels but are also involved in local correlations between adjacent tunnels. The combination of the two techniques facilitates a comprehensive description of the local structure of hollandite, leading to a better understanding of the possible distortions in hollandite-supergroup phases. Such information allows for a more systematic structure-properties correlation and, consequently, finer tuning of the physical properties and applications of hollandite-type materials.