Quasi-One-Dimensional Transition-Metal Chalcogenide Semiconductor (Nb₄Se₁₅I₂)I₂

The discovery of new low-dimensional transition-metal chalcogenides is contributing to the already prosperous family of these materials. In this study, needle-shaped single crystals of a quasi-one-dimensional (1D) material, (Nb₄Se₁₅I₂)I₂, were grown by chemical vapor transport, and the structure was solved by single-crystal X-ray diffraction (XRD). The structure has 1D (Nb₄Se₁₅I₂)_n chains along the [101] direction, with two I^- ions per formula unit directly bonded to Nb⁵⁺. The other two I^- ions are loosely coordinated and intercalated between the chains. Individual chains are chiral and stack along the b axis in opposing directions, giving space group P2₁/c. The phase purity and crystal structure were verified by powder XRD. Density functional theory calculations show (Nb₄Se₁₅I₂)I₂ to be a semiconductor with a direct band gap of around 0.6 eV. Resistivity measurements of bulk crystals and micropatterned devices demonstrate that (Nb₄Se₁₅I₂)I₂ has an activation energy of around 0.1 eV, and no anomaly or transition was seen upon cooling. Low-temperature XRD shows that (Nb₄Se₁₅I₂)I₂ does not undergo a structural phase transformation from room temperature to 8.2 K, unlike related compounds (NbSe₄)_nI (n = 2, 3, or 3.33), which all exhibit charge-density waves. This compound represents a well-characterized and valence-precise member of a diverse family of anisotropic transition-metal chalcogenides.