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

TL;DR

This study measures the optical band-gap energies of various AWO4 tungstates and explores how these energies correlate with the ionic radius of the A cation, revealing electronic structure influences.

Contribution

It provides new experimental data on band-gap energies of AWO4 tungstates and analyzes their dependence on cation ionic radius and electronic structure contributions.

Findings

Band-gap energies range from 2.3 eV to 5.26 eV across studied tungstates.

Hybridization of s electron states correlates with larger band-gap energies.

Electronic structure differences explain discrepancies in previous data.

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 5.26 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.