Optical absorption of divalent metal tungstates: Correlation between the band-gap energy and the cation ionic radiusR. Lacomba-Perales, J. Ruiz-Fuertes, D. Errandonea, D. Martínez-García and A. Segura
MALTA Consolider Team, Departamento de Física Aplicada-ICMUV, Universitat de València, Edificio de Investigación - C/Dr. Moliner 50, 46100 Burjassot (Valencia), Spain, EU
received 19 May 2008; accepted in final form 18 June 2008; published August 2008
published online 1 July 2008
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 5.26eV (BaWO4), 5.08eV (SrWO4), 4.94eV (CaWO4), 4.15eV (CdWO4), 3.9-4.4eV (ZnWO4), 3.8-4.2eV (PbW4), and 2.3eV (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.
71.20.Nr - Semiconductor compounds.
78.40.Fy - Semiconductors.
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