Modular design with 2D topological-insulator building blocks: Optimized synthesis and crystal growth and crystal and electronic structures of BixTeI (x = 2, 3)

Structural engineering of topological bulk materials is systematically explored with regard to the incorporation of the buckled bismuth layer [Bi], which is a 2D topological insulator per se, into the layered BiTeI host structure. The previously known bismuth telluride iodides, BiTeI and BiTeI, offer physical properties relevant for spintronics. Herewith a new cousin, BiTeI (sp.gr. R3m, a = 440.12(2) pm, c = 3223.1(2) pm), joins the ranks and expands this structural family. BiTeI = [Bi][BiTeI] represents a stack with strictly alternating building blocks. Conditions for reproducible synthesis and crystal-growth of BiTeI and BiTeI are ascertained, thus yielding platelet-like crystals on the millimeter size scale and enabling direct measurements. The crystal structures of BiTeI and BiTeI are examined by X-ray diffraction and electron microscopy. DFT calculations predict metallic properties of BiTeI 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 BiTeI with x > 3; instead, it indicates a possible homogeneity range of BiTeI. The series BiTeI-BiTeI-BiTeI illustrates the influence of structural modifications on topological properties.This work was supported by the German Research Foundation (DFG) in the framework of the Special Priority Program (SPP 1666) “Topological Insulators” and by the ERA-Chemistry Program. We acknowledge support by Academic D.I. Mendeleev Fund Program of Tomsk State University in 2015 (research grant No. 8.1.05.2015), by Saint Petersburg State University (project No. 15.61.202.2015), by the Spanish Ministry of Science and Innovation (grant nos. FIS2013- 48286-C02-02-P and FIS2013-48286-C02-01-P), and by the Basque Departamento de Educacion, UPV/EHU (grant IT756-13).Peer Reviewe