Optical properties and electronic structure of yttrium oxysulfide

Reflectivity spectra of single crystals of Y₂O₂S have been measured at 8 and 300 K in the energy range up to 30 eV using synchrotron radiation as a light source. The dielectric constants, absorption coefficient, and effective number of electrons per molecule are derived through a Kramers-Kronig analysis. The x-ray photoelectron spectroscopy has also been performed, in order to get information on the valence band and the outermost core band. With use of the discrete variational Xα (DV-Xα) method, the electronic structure of a model cluster consisting of an yttrium ion surrounded by three sulfur ions and four oxygen ions has been calculated. The calculation indicates that the exciton absorption peak observed at 6.33 eV is associated with the transition from the S $3p$ state to the Y $4d$ state. The band gap energy is estimated to be 6.77 eV. The electronic structure of Y₂O₂S is discussed on the basis of the present experimental and theoretical results, and is compared with that of Y₂O₃.