Optical absorption of divalent metal tungstates: Correlation between the band-gap energy and the cation ionic radius

We have carried out optical-absorption and reflectance measurements at room temperature in single crystals of AWO 4 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 (BaWO 4 ), 5.08 eV (SrWO 4 ), 4.94 eV (CaWO 4 ), 4.15 eV (CdWO 4 ), 3.9–4.4 eV (ZnWO 4 ), 3.8–4.2 eV (PbWO 4 ), and 2.3 eV (CuWO 4 ). 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 A 2+ cation hybridize with the O 2p and W 5d states ( e.g. , BaWO 4 ) have larger band-gap energies than those where also p, d, and f states of the A 2+ cation contribute to the top of the valence band and the bottom of the conduction band ( e.g. , PbWO 4 ). The results are of importance in view of the large discrepancies existent in prevoiusly published data.