Intercluster charge ordering in monoclinic and triclinic Ba–Mo-based hollandite phases†

Reproducible solid-state synthesis methods are presented for the preparation of tetragonal Ba_1.1Mo₈O₁₆, monoclinic Na_0.325(5)Ba_1.006(18)Mo₈O₁₆, and triclinic Ba_1.12(3)Mo₈O₁₆ hollandite phases, with complete, high-resolution crystal structures of the monoclinic and triclinic phases reported for the first time. The similar synthetic conditions allow direct comparisons between phases; differences between structures are shown to be correlated to subtle changes in the metal–metal bonding of the Mo₄ cluster motif that is unique to the Mo-based hollandites. The trends in the local Mo valence, stoichiometry, and key Mo–Mo bond lengths of these and other reported Mo-based hollandite phases together support an interchain charge ordering model for this family of compounds, which has been previously suggested for the case of K₂Mo₈O₁₆. An alternate model, where Mott physics dominate the electronic structure near the Fermi level, is not supported by temperature-dependent magnetic susceptibility measurements, which are reported down to 2 K. The incorporation and homogeneity of Na in the monoclinic phase is verified using atom probe tomography.