Zero Indirect Band Gap and Flat Bands in a Niobium Oxyiodide Cluster Material

Jan Beitlberger; Mario Martin; Marcus Scheele; Marek Matas; Carl P. Romao; Markus Ströbele; H. -Jürgen Meyer

doi:10.1021/jacs.6c03891

paper

Zero Indirect Band Gap and Flat Bands in a Niobium Oxyiodide Cluster Material

arXiv:2602.16011 · doi:10.1021/jacs.6c03891

Abstract

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.

19 pages, 21 figures

Zero Indirect Band Gap and Flat Bands in a Niobium Oxyiodide Cluster Material

Abstract

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