Symmetry lowering in LaOBiS₂. A mother material for BiS₂-based layered superconductors

We report X-ray diffraction studies of single-crystal samples of LaOBiS₂ and CeOBiS₂, which are mother materials for BiS₂-based layered superconductors. The structural symmetry in LaOBiS₂ is lowered from tetragonal to monoclinic, whereas no such symmetry lowering occurs in CeOBiS₂. Crystal structure analysis demonstrates that the space group of LaOBiS₂ is monoclinic P2₁m. The in-plane S atoms uniformly shift along the a-axis in each BiS₂ layer. Because the directions of the shift are opposite in the two adjacent BiS₂ layers, the spatial inversion symmetry remains. The distortion is attributed to the in-plane lattice mismatch between the BiS₂ layers and the block layers. (author)