Quasi-One-Dimensional Transition-Metal Chalcogenide Semiconductor (Nb 4 Se 15 I 2 )I 2 .

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, (Nb 4 Se 15 I 2 )I 2 , were grown by chemical vapor transport, and the structure was solved by single-crystal X-ray diffraction (XRD). The structure has 1D (Nb 4 Se 15 I 2 ) n chains along the [101] direction, with two I - ions per formula unit directly bonded to Nb 5+ . 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 P 2 1 / c . The phase purity and crystal structure were verified by powder XRD. Density functional theory calculations show (Nb 4 Se 15 I 2 )I 2 to be a semiconductor with a direct band gap of around 0.6 eV. Resistivity measurements of bulk crystals and micropatterned devices demonstrate that (Nb 4 Se 15 I 2 )I 2 has an activation energy of around 0.1 eV, and no anomaly or transition was seen upon cooling. Low-temperature XRD shows that (Nb 4 Se 15 I 2 )I 2 does not undergo a structural phase transformation from room temperature to 8.2 K, unlike related compounds (NbSe 4 ) n I ( 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.