  We have investigated low-temperature crystal structure of BiCh2-based
compounds LaO1-xFxBiSSe (x = 0, 0.01, 0.02, 0.03, and 0.5), in which anomalous
two-fold-symmetric in-plane anisotropy of superconducting states has been
observed for x = 0.5. From synchrotron X-ray diffraction experiments, a
structural transition from tetragonal to monoclinic was observed for x = 0 and
0.01 at 340 and 240 K, respectively. For x = 0.03, a structural transition and
broadening of the diffraction peak were not observed down to 100 K. These facts
suggest that the structural transition could be suppressed by 3% F substitution
in LaO1-xFxBiSSe. Furthermore, the crystal structure for x = 0.5 at 4 K was
examined by low-temperature (laboratory) X-ray diffraction, which confirmed
that the tetragonal structure is maintained at 4 K for x = 0.5. Our results
suggest that the two-fold-symmetric in-plane anisotropy of superconducting
states observed for LaO0.5F0.5BiSSe was not originated from structural symmetry
lowering.
