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

MALTA Consolider Team, Departamento de Física Aplicada-ICMUV, Universitat de València, Edificio de Investigación - C/Dr. Moliner 50, 46100 Burjassot (Valencia), Spain, EU

Corresponding author: daniel.errandonea@uv.es

Received: 19 May 2008
Accepted: 18 June 2008

Abstract

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.