Zero Indirect Band Gap and Flat Bands in a Niobium Oxyiodide Cluster Material
Explorative chemistry in a reaction system composed of NbI$_4$, Li$_2$(CN$_2$), and Li$_2$O has led to the discovery of a number of niobium oxyiodide cluster compounds. During this reaction, the formation of solid phases was detected alongside with gaseous phases, resulting in a range of products with cluster cores of varying shapes. After several niobium oxyiodide cluster compounds have already been identified within this reaction system, two additional compounds, Nb$_6$O$_3$I$_{15}$ and Nb$_{11}$O$_6$I$_{24}$, are discovered and structurally characterized by single-crystal X-ray diffraction. Both structures are based on the butterfly-shaped, oxygen-capped niobium cluster [Nb$_4$O], which is extended to larger cluster fragments. The [Nb$_4$O] cluster core in Nb$_6$O$_3$I$_{15}$is extended by two [NbO] units to form a three-dimensional framework, and Nb$_{11}$O$_6$I$_{24}$ contains two connected [Nb$_4$O] units, which form chiral units within an antiferrochiral hexagonal packing of strings. The striking string-like character of Nb$_{11}$O$_6$I$_{24}$ was investigated in terms of its electronic structure and properties. DFT calculations showed Nb$_{11}$O$_6$I$_{24}$ to possess a zero indirect band gap, with a pair of 3-dimensional flat bands surrounding the Fermi level. These unusual features of the electronic band structure suggest the presence of strongly correlated inter-cluster singlet electron states, arising from the helical shape of the clusters, the hexagonal packing of the strings, and the delocalized nature of cluster electron wavefunctions.