Optical anisotropy and electronic structures of CdMoO4 and CdWO4 crystals:: Polarized reflection measurements, x-ray photoelectron spectroscopy, and electronic structure calculations

The polarized reflectivity spectra of CdMoO4 single crystals with a scheelite structure and of CdWO4 with a wolframite structure are measured in the 3-30 eV range by using synchrotron radiation. The spectra of optical constants for the crystallographic axes are derived by a Kramers-Kronig analysis. X-ray photoelectron spectroscopy (XPS) and the calculation of the electronic structure by using a discrete variational X alpha method are performed for CdMoO4. The calculation shows that the Cd 4d state is localized at the bottom region of the O 2p valence band, and the Cd 5s state has a significant contribution to the bottom of the conduction band, composed of the Mo 4d state. The XPS spectrum of CdMoO4 resembles that of CdWO4. Unlike the XPS spectrum, the optical spectra of CdMoO4 resemble those of scheelite CaMoO4 rather than CdWO4. An excitonic transition is observed as a weak shoulderlike structure in CdWO4, while it is not appreciable in CdMoO4. These experimental results are discussed in comparison with the theoretical calculation of electronic structures. The remarkable anisotropy of the optical spectra in CdMoO4 is explained by taking into account the presence of the Cd 5s state at the bottom of the conduction band. The dichroism of CdWO4 is discussed in terms of the chain structure of WO66- octahedra in wolframite crystals. The present study indicates that the Cd metal states play a crucial role in the optical properties near the fundamental absorption edge in both CdMoO4 and CdWO4.