  We have carried out optical-absorption and reflectance measurements at room
temperature in single crystals of AWO4 tungstates (A = Ba, Ca, Cd, Cu, Pb, Sr,
and Zn). From the experimental results their band-gap energy has been
determined to be 5.26 eV (BaWO4), 5.08 eV (SrWO4), 4.94 eV (CaWO4), 4.15 eV
(CdWO4), 3.9-4.4 eV (ZnWO4), 3.8-4.2 eV (PbWO4), and 2.3 eV (CuWO4). The
results are discussed in terms of the electronic structure of the studied
tungstates. It has been found that those compounds where only the s electron
states of the A2+ cation hybridize with the O 2p and W 5d states (e.g BaWO4)
have larger band-gap energies than those where also p, d, and f states of the
A2+ cation contribute to the top of the valence band and the bottom of the
conduction band (e.g. PbWO4). The results are of importance in view of the
large discrepancies existent in prevoiusly published data.
