Detailed comparative theoretical and experimental study of electronic properties and spin structure was carried out for a series of Pb-based quaternary compounds PbBi₂Te_4−xSe_x. For all values of x, these compounds are theoretically predicted to be topological insulators, possessing at high Se content a remarkably large band gap and a Dirac point isolated from bulk states. Using spin- and angle-resolved photoemission spectroscopy, it was shown that the PbBi₂Te₂Se₂ and PbBi₂Te_1.4Se_2.6 compounds are characterized by well-defined spin-polarized topological surface state in the bulk gap. To define the probable distribution of atoms over the atomic sites for these samples, we performed ab initio calculations in ordered and disordered configurations of the unit cell.
We found that theoretical calculations better reproduce photoemission data when Te atoms are placed in the outermost layers of the septuple layer block.

"Impact of stoichiometry and disorder on the electronic structure of the PbBi₂Te₄−_xSe_x topological insulator" I. A. Shvets, Klimovskikh, Z. S. Aliev, M. B. Babanly, Sánchez-Barriga, M. Krivenkov,6 A. M. Shikin, and E. V. Chulkov. PHYSICAL REVIEW B 96, 235124 (2017)