Oxidation states of Fe and Ti in blue sapphire
Abstract
X-ray absorption near-edge spectroscopy (XANES) can be used to study the oxidation state of a dilute system such as transition metal defects in solid-state samples. In blue sapphire, Fe and Ti are defects that cause the blue color. Inter-valence charge transfer (IVCT) between Fe2+ and Ti4+ has been proposed to describe the optical color’s origin. However, the existence of divalent iron cations has not been thoroughly investigated. Fluorescent XANES is therefore employed to study K-edge absorptions of Fe and Ti cations in various blue sapphire samples including natural, synthetic, diffused and heat-treated sapphires. All the samples showed an Fe absorption edge at 7124 eV, corresponding to the Fe3+ state; and Ti at 4984 eV, corresponding to Ti4+. From these results, we propose Fe3+-Ti4+ mixed acceptor states located at 1.75 eV and 2.14 eV above the valence band of corundum, that correspond to 710 nm and 580 nm bands of UV-vis absorption spectra, to describe the cause of the color of blue sapphire.
- Publication:
-
Materials Research Express
- Pub Date:
- February 2016
- DOI:
- 10.1088/2053-1591/3/2/026201
- Bibcode:
- 2016MRE.....3b6201W