Investigation of novel crystal structures of Bi–Sb binaries predicted using the minima hopping method

Abstract

Semi-conducting alloys Bi_xSb_1−x have emerged as a potential candidate for topological insulators and are well known for their novel thermoelectric properties. In this work, we present a systematic study of the low-energy phases of 35 different compositions of Bi_xSb_1−x (0 < x < 1) at zero temperature and zero pressure. We explore the potential energy surface of Bi_xSb_1−x as a function of Sb concentration by using the ab initio minima hopping structural search method. Even though Bi and Sb crystallize in the same Rm space group, our calculations indicate that Bi_xSb_1−x alloys can have several other thermodynamically stable crystal structures. In addition to the configurations on the convex hull, we find a large number of metastable structures which are dynamically stable. The electronic band structure calculations of several stable phases reveal the presence of strong spin–orbit interaction leading to the Rashba–Dresselhaus spin-splitting of bands which is of great interest for spintronics applications. We also find an orthorhombic structure of BiSb in the Imm2 space group which exhibits signatures of type-II Weyl semimetal. Additionally, we have studied the thermoelectric properties of the selected structures. Regarding thermoelectric properties, we find that the compositions which crystallize in the rhombohedral structure exhibit values of the Seebeck coefficient and the power factor similar to that of Bi₂Te₃ at room temperature, while the theoretical maximum figure of merit (Z_eT) is smaller than that of Bi₂Te₃. We observe enhancement in the thermopower with the increase in the strength of the Rashba–Dresselhaus spin-splitting effect.