Polytypism in LaOBiS2-type compounds based on different three-dimensional stacking sequences of two-dimensional BiS2 layers

LaOBiS2-type materials have drawn much attention recently because of various interesting physical properties, such as low-temperature superconductivity, hidden spin polarization, and electrically tunable Dirac cones. However, it was generally assumed that each LaOBiS2-type compound has a unique and specific crystallographic structure separated from other phases (with a space group P4/nmm). Using first-principles total energy and stability calculations we find that contrary to this view the three-dimensional structure of this important family of compounds represents instead a family of energetically closely spaced modifications differing by the layer sequences and orientations. We find that the local Bi-S distortion, mainly induced by the interaction between the medium LaO and the BiS2 layers and the electron hybridization in the BiS2 layer, leads to three polytypes of LaOBiS2 according to the different stacking pattern of distorted BiS2 layers. Although the energy difference between the polytypes of LaOBiS2 is relatively small (within several meV/f.u.), they have obvious impact on the presence of inversion symmetry, which is reflected by the band structure and spin polarization. In addition, different choices of the medium atoms (replacing La) or active atoms (BiS2) manifest different ground state polytypes. One can thus tune the distortion and the ground state by substituting covalence atoms in LaOBiS2-family.