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

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.