Pornsawat Wathanakul

Raman spectroscopic investigation on high refractive index glasses prepared from local quartz sand

High refractive index (RI) glasses prepared from local quartz sand and compounds of heavy elements, such as, barium carbonate, lead oxide, and bismuth oxide as major ingredients were investigated using Raman spectroscopy. The results showed changes in glass structures of different doping elements, namely, Ba, Pb, and Bi. Refractive indices, densities, and UV-vis-NIR spectra of the glass samples were also measured. The Raman spectroscopy can be used to investigate and/or identify heavy glasses, local ancient glasses as well as glass jewelry.

Oxidation states of Fe and Ti in blue sapphire

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 colors 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.

X-ray absorption near-edge structure of chromium ions in α-Al2O3

Both synthetic and natural α-Al2O3 samples with different Cr concentrations were investigated by XANES. The Cr contents were analyzed using LA-ICP-MS technique prior to this experiment. XANES spectra combined with first principle calculations showed the transformation from α-Al2O3:Cr3+ to α-Al2O3:Cr3+,Cr3+ or α-Cr2O3 as the Cr-Cr content increased. The absorption transformation could be resulted from the change of environment around Cr3+ ions, i.e., the Al-O-Cr bonds decreased while the Cr-O-Cr bonds increased. Significant differences in XANES line shapes observed for synthetic and natural α-Al2O3 samples suggested the differences in local environments around Cr ions in both samples. The XANES line shape of α-Al2O3:Cr3+,Cr3+ or Cr ion pairs corresponded to the Cr content, which was also confirmed by the intensity ratios between N2-line and sideband S of R-lines of photoluminescence spectra.

Defect-Induced Properties of Doped Alpha Alumina Samples

The dielectric constant values of a set of synthetic alpha alumina samples doped with Cr and Be were determined using parallel plate method. The voltage frequency was selected at 1 MHz. When Cr was doped to the sample, the dielectric constant was increased due to the increase in polarizations of the sample, whereas the Be-doped one showed the decrease in dielectric constant cf. the undoped synthetic alpha alumina (reference sample). Experiments on doping alpha alumina with both elements showed increasing in dielectric constant compared to the undoped reference sample, i.e., Cr has more influence to the dielectric constant value. Dielectric constants of a set of natural sapphires were also measured and compared to those obtained from the synthetic ones. The results of both natural and synthetic samples varied in the same direction. It suggests, however, that the geological origins of the samples are needed to be considered. This is due to the differences in physico-chemical conditions of their formations that would have influenced partitioning of trace elements as well as evolving of defects in natural samples. Increasing of the R1 and R2 photoluminescence wavelengths and decreasing of refractive indices of the doped samples cf. those of the reference sample were obtained from the experiments.

The Phase Transition of Corundum-Eskolaite Mineral Series Probed by X-Ray Absorption Spectroscopy

The red color of ruby is caused by Cr3+ impurities substituting Al3+ in Al2O3 structure. The complete substitution of Al3+ by Cr3+ produces Cr2O3 that has the green color. In this study, we focused on the phase transition of solid solution between corundum (Al2O3) and eskolaite (Cr2O3) mineral series. A series of Al2O3-Cr2O3 samples were prepared by mixing fine powder of Al2O3 and Cr2O3 in various ratios. They were ground together and then recrystallized. The obtained crystals were ground again in powder form for X-ray absorption near edge structure (XANES) measurements to avoid polarization dependence. The measurements were carried out at the Cr K-edge absorption energy using a 13-channel array Ge detector in the fluorescence mode. The XANES spectra showed line-shape transitions for 0.125 to 100% variation of Cr2O3. Significant variations in transitions were found when the Cr2O3 content was between 20% and 80%. With Cr2O3 concentration increasing, the sample colors apparently turned from pink to dark green. CIELAB color index measurements were performed. The variations of line-shape spectra of the Al2O3-Cr2O3 samples were found to be consistent with the CIELAB color index results. This Cr K-edge XANES study could be useful for the research in mineral sciences.

Dielectric Property of Barium-Based Glasses Fabricated Using Thailand Resource

Barium-based glasses were fabricated using raw materials from the beneficiation of Thailand resources. Dopants of selected transition and rare-earth element oxides were added as coloring agents in order to produce materials of desired commercial applicability. Dielectric constants, refractive index and absorption spectra of the glass samples were measured. It was found that both dielectric constants and refractive indices increased when the dopants were added. Colorless barium-based glass samples were visually changed to green by additions of copper oxide, to yellow by iron oxide, to pinkish brown by manganese oxide, to pink by erbium oxide, and to lavender blue by neodymium. The fabrication of barium-based glass based on local raw materials is more economical compared to those using imported raw materials.