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Abstract:
A honeycomb layer of transition metal-centered bismuth cubes has
attracted attention as part of the weak topological insulator (TI)
Bi14Rh3I9. The intermetallic layer itself is a two-dimensional TI and
has been found in Bi13Pt3I7 and Bi12Pt3I5, as well as in the new
material Bi38Pt9I14, 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
Bi38Pt9I14 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
[BiI2](-) 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.
Bi38Pt9I14 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.
