Synthesis, Crystal and Topological Electronic Structures of New Bismuth Tellurohalides Bi₂TeBr and Bi₃TeBr

Halogen substitution, that is, bromine for iodine, in the series of topological Bi_<i>n</i>TeI (<i>n</i> = 1, 2, 3) materials was conducted in order to explore the impact of anion exchange on topological electronic structure. In this proof-of-concept study, we demonstrate the applicability of the modular view on crystal and electronic structures of new Bi₂TeBr and Bi₃TeBr compounds. Along with the isostructural telluroiodides, they constitute a family of layered structures that are stacked from two basic building modules, _∞²[Bi₂] and _∞²[BiTeX] (X = I, Br). We present solid-state synthesis, thermochemical studies, crystal growth, and crystal-structure elucidation of Bi₂TeBr [space group <i>R</i>3̅<i>m</i> (no. 166), <i>a</i> = 433.04(2) pm, <i>c</i> = 5081.6(3) pm] and Bi₃TeBr [space group <i>R</i>3<i>m</i> (no. 160), <i>a</i> = 437.68(3) pm, <i>c</i> = 3122.9(3) pm]. First-principles calculations establish the topological nature of Bi₂TeBr and Bi₃TeBr. General aspects of chemical bonding appear to be similar for Bi_<i>n</i>TeX (X = I, Br) with the same <i>n</i>, so that alternation of the global gap size upon substitution is insignificant. The complex topological inversion proceeds between the states of two distinct modules, _∞²[Bi₂] and _∞²[BiTeBr]; thus, the title compounds can be seen as heterostructures built via a modular principle. Furthermore, highly disordered as well as incommensurately modulated ternary phase(s) are documented near the Bi₂TeBr composition. Single-crystal X-ray diffraction experiments on BiTeBr and Bi₂TeI resolve some discrepancies in prior published work

Modular Design with 2D Topological-Insulator Building Blocks: Optimized Synthesis and Crystal Growth and Crystal and Electronic Structures of Bi_<i>x</i>TeI (<i>x</i> = 2, 3)

Structural engineering of topological bulk materials is systematically explored with regard to the incorporation of the buckled bismuth layer [Bi₂], which is a 2D topological insulator per se, into the layered BiTeI host structure. The previously known bismuth telluride iodides, BiTeI and Bi₂TeI, offer physical properties relevant for spintronics. Herewith a new cousin, Bi₃TeI (sp.gr. <i>R</i>3<i>m</i>, <i>a</i> = 440.12(2) pm, <i>c</i> = 3223.1(2) pm), joins the ranks and expands this structural family. Bi₃TeI = [Bi₂]­[BiTeI] represents a stack with strictly alternating building blocks. Conditions for reproducible synthesis and crystal-growth of Bi₂TeI and Bi₃TeI are ascertained, thus yielding platelet-like crystals on the millimeter size scale and enabling direct measurements. The crystal structures of Bi₂TeI and Bi₃TeI are examined by X-ray diffraction and electron microscopy. DFT calculations predict metallic properties of Bi₃TeI and an unconventional surface state residing on various surface terminations. This state emerges as a result of complex hybridization of atomic states due to their strong intermixing. Our study does not support the existence of new stacking variants Bi_<i>x</i>TeI with <i>x</i> > 3; instead, it indicates a possible homogeneity range of Bi₃TeI. The series BiTeI–Bi₂TeI–Bi₃TeI illustrates the influence of structural modifications on topological properties