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.