First-Principles Study On Geometric Structures And Physical Properties Of Ternary Bismuth-Based Compounds Bi₂X₂Y?X,Y=O,S,Se,and Te?

In recent years,researchers have studied the thermoelectric,photoelectric and topological properties of some materials in ternary bismuth-based compounds Bi2X2Y(X,Y=O,S,Se,and Te).However,the basic properties of these materials,such as geometric structures,electronic structures,optical properties,have not been systematically studied in experiments or theories,as well as that of low-dimensional materials.Therefore,the geometric structures and basic properties of ternary bismuth-based compounds Bi2X2Y(X,Y = O,S,Se and Te)are systematically studied by the first-principle calculations in this paper.The details are as follows:1.All the structural models of bismuth-based compounds in the experiment are investigated.The formation energies of these structures are calculated by using the first principle calculations,and the structures with the lowest formation energy of individual maternals are obtained.Then,the stability of the structures with lower energy was verified by the phonon dispersion spectra.Based on these stable structures,the electronic structure of the materials can be divided into two categories:semiconductors and topological insulators:The first class is traditional semiconductors,including:(i)Bi2O2Y,(ii)Bi2S2Y and(iii)Bi2OY2(where Y = S,Se,and Te).With the increasing of Y atomic number,the band gap of each kind of material gradually decreases.The optical absorption coeflficient of each material can reach the order of 106,which is similar to that of perovskite solar cell,and it makes these materials have good application potential in optical field.In addition,a ne.w material Bi2S2Tc is predicted by the DFT in our paper.Bi2S2Te belongs to orthorhombic system with the pnma space group,which keeps the same as Bi2S2Se.The lattice parameters are as follows:a=12.06 A,b=4.11 A,c=11.24 A,and it is a semiconductor with a direct band gap of 0.5 eV.It has a small hole effective mass(about 0.22 m0),which is helpful to the fast hole transmission.The second type is topological insulators,including:(i)Bi2Se2Y and(ii)Bi2Te2Y(where Y = S,Se,and Te).Except for several topological materials in the experiment,it is proved that the rest materials Bi2Se2S and Bi2Se2Te have strong topological properties by calculating the topological invariants Z2 and the surface states of(111).2.In this paper,the low-dimensional properties of the materials with layered structure in Bi2X2Y(X,Y = O,S,Se,and Te)family are also studied.The geometric structure,electronic structure,effective mass and optical absorption coefficient of monolayer materials are investigated.The results are as follows:the single layer material is semiconductor material with an indirect band gap.Owing to the smaller effective mass,it is far-reaching significance in electron transport or hole transport.In addition,the optical absorption coefficients of several materials are all up to the order of 106,which can be studied deeply in solar cells.