Low dimensional van der Waals bonded materials display a fundamental material unit, usually depicted as the simplest molecular formula obeying stoichiometry. A series of such fundamental units align in the bulk material phase due to weak van der Waals interactions. Overall, key advantages conferred by the chemical structure are the ease to scale the materials down to nanostructures under simultaneous conservation of the bulk structure and the reduction in defects amount.
Belonging to the larger class of quasi 1-dimensional van der Waals bonded materials, β-BiI has been recently reported as a novel topological insulator. The binary bismuth-iodine family class includes the known bismuth(III) iodide along with additional representatives such as α-BiI, BiI, BiI, and BiI. Having the same stoichiometric chemical formula, α-BiI and β-BiI show similar solid-state structures yet critically different physicochemical properties. Specifically, α-BiI represents the trivial insulator phase, while stacking of the bismuth atoms along the b crystallographic axis in the β-BiI phase yield a different topological insulator phase. Both isoforms crystallyse in the C2/m space group, with α-BiI having a unit cell volume almost double of its topological insulator counterpart. The β crytallographic angle is higher in the β-BiI: 107.87 vs 92.96, making the β-BiI more tilted (see images above).