New Environment for a Two-Dimensional Topological Insulator with Hexagonal Channels Hosting Diiodido-bismuthate(I) Anions in a Singlet State

A honeycomb layer of transition metal-centered bismuth cubes has attracted attention as part of the weak topological insulator (TI) Bi 14 Rh 3 I 9 . The intermetallic layer itself is a two-dimensional TI and has been found in Bi 13 Pt 3 I 7 and Bi 12 Pt 3 I 5, as well as in the new material Bi 38 Pt 9 I 14, yet with different stacking sequences and types of insulating spacer layers in all cases. The arrangements strongly influence the electronic situation, especially the coupling between the TI layers. X-ray diffraction and electron microscopy show that in Bi 38 Pt 9 I 14 the intermetallic layers are exclusively separated by single layers of iodide ions. The eclipsed stacking of the honeycomb layers results in hexagonal channels. These host hitherto unknown linear [BiI 2 ] − anions with bismuth atoms in the uncommon +I oxidation state. Quantum chemical calculations as well as magnetic susceptibility data indicate a singlet state stabilized by strong spin–orbit coupling. Bi 38 Pt 9 I 14 is a semiconductor with a very narrow energy gap, as resistivity measurements show. The nontrivial topological invariants of the electronic bands at the Fermi energy and the coupling of TI layers throughout the crystal suggest even a strong TI.