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)

Abstract

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. R3m, a = 440.12(2) pm, c = 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 BixTeI with x > 3; instead, it indicates a possible homogeneity range of Bi3TeI. The series BiTeI–Bi2TeI–Bi3TeI illustrates the influence of structural modifications on topological properties.