  The band gaps of isostructural Bi2OS2 and LaOBiS2 were examined using optical
absorption and discussed with the band structures calculated based on the
crystal structures determined using synchrotron X-ray diffraction. The Bi 6p
and S 3p orbitals in the Bi-S plane were computationally predicted to
constitute the bands near the Fermi level. The optical reflectance spectra of
Bi2OS2 and LaOBiS2 showed optical band gaps of ca. 1.0 eV, which were close to
the computationally calculated direct band gaps of ca. 0.8 eV. Our results show
that Bi2OS2 and LaOBiS2 are semiconductors containing direct band gaps of
0.8-1.0 eV, and they are suggested to be candidates for optoelectronic
materials in the near-infrared region without highly toxic elements.
