Geometric structures and electronic properties of the Bi2X2Y (X, Y = O, S, Se, and Te) ternary compound family: a systematic DFT study

Abstract

Bi₂X₃ (X = S, Se, and Te) binary compounds have been widely studied as thermoelectric materials and reference topological insulators. Replacing one of the X atoms with a different atom Y of the same group has resulted in a new family of ternary compounds Bi₂X₂Y (X, Y = O, S, Se, and Te), which have attracted increasing attention very recently. By means of first-principles calculations within density functional theory, we have systematically studied the geometric structures and electronic properties of the whole family of these ternary compounds. We find that these compounds can be divided into two major groups. The first is traditional semiconductors, which include (i) Bi₂O₂Y, (ii) Bi₂S₂Y, and (iii) Bi₂OY₂. The other is topological insulators, which are (i) Bi₂Se₂Y and (ii) Bi₂Te₂Y. The optimized lattice parameters, band gaps as well as effective masses are in excellent agreement with recently available experimental measurements. In particular, we have found a new semiconductor material Bi₂S₂Te, which possesses an orthorhombic structure in space group pnma with a moderate direct gap of 0.5 eV at the HSE06 level and a small hole effective mass (∼0.22m₀), which should favour fast hole transport and deserves further experimental studies. This work provides a road map for the exploration of the fundamental properties of the ternary compounds in this family as well as for their practical applications at the device level.