Evidences of inner Se ordering in topological insulator PbBi₂Te₄-PbBi₂Se₄-PbSb₂Se₄ solid solutions

In topological insulators (TIs), carriers originating from non-stoichiometric defects hamper bulk insulation. In (Bi,Sb)₂(Te,Se)₃ TIs (BSTS TIs), however, Se atoms strongly prefer specific atomic sites in the crystal structure (Se ordering), and this ordering structure suppresses the formation of point defects and contributes to bulk insulation. It has accelerated the understanding of TIs' surface electron properties and device application. In this study, we select Pb(Bi,Sb)₂(Te,Se)₄ (Pb-BSTS) TIs, which are reported to have larger bandgap compared to counterpart compound BSTS TIs. The Se ordering geometry was investigated by combining state-of-the-art scanning transmission electron microscopy and powder X-ray diffractometry. We demonstrated the existence of inner Se ordering in PbBi₂(Te,Se)₄ and also in Pb-BSTS TIs. Quantitative analysis of Se ordering and a qualitative view of atomic non-stoichiometry such as point defects are also presented. Pb-BSTS TIs' Se ordering structure and their large gap nature has the great potential to achieve more bulk insulation than conventional BSTS TIs.