Comparative study of optical spectra and electronic structures of CaMoO4 and CaWO4 crystals

Polarized reflectivity spectra of CaMoO4 and CaWO4 single crystals have been measured up to 30 eV by using synchrotron radiation as a light source. The optical constants for a- and c-axes are derived from the reflectivity spectra by a Kramers-Kronig analysis. A clear correspondence is found in the main structures of the optical spectra between molybdate and tungstate. It is revealed that the lowest bands peaking at 5.0 eV in CaMoO4 and at 6.0 eV in CaWO4 consisit of some fine structures with distinct dichroism. They are attributed to the joint density-of-states; in other words, no structures indicative of exciton transition are found at the fundamental absorption edge. From the comparison of the spectral profiles in CaMoO4 and CaWO4, the structures below 10 eV are ascribed to the transitions from the O 2p valence band to the conduction band mainly made of Mo 4d (W 5d) states, while those above 10 eV are assigned to the transitions to the Ca conduction band. The polarization dependence of the structures below 10 eV is explained well in terms of the intramolecular transition in an oxyanion complex in the scheelite crystal. The energy band structure of CaWO4 is discussed taking into account of the previous results such as the thermostimulated luminescence and the two-photon excitation. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.