The aluminum centers in α-quartz

Effect of Al(3+) co-doping on the luminescence properties of Cu doped Na2SiF6

Studies were carried out to assess the correlation between thermoluminescence (TL) and optically stimulated luminescence (OSL) of this phosphor. It was observed that the OSL and TL glow curve consists of a wide distribution of traps having different photo-ionization cross-sections, trap depths and frequency factors. In case of Al doped sample, some of the traps up to 200°C are assumed to act as a source traps for the observance of OSL due to thermal transfer of charge carriers into the deep traps beyond 480°C. This suggests that Al impurities play an important role in the thermal transfer OSL process. As most of the work on this phenomenon is done on natural materials (mainly quartz) in which aluminum is a natural impurity, this study will explain the role of Al in this phenomenon.

Self-trapping of single and paired electrons in Ge2Se3

We report the theoretical prediction of single and paired electron self-trapping in Ge(2)Se(3). In finite atomic cluster, density functional calculations, we show that excess single electrons in Ge(2)Se(3) are strongly localized around single germanium dimers. We also find that two electrons prefer to trap around the same germanium dimer, rupturing a neighboring Ge-Se bond. Localization is less robust in periodic, density functional calculations. While paired electron self-trapping is present, as shown by wavefunction localization around a distorted Ge-Ge dimer, single-electron trapping is not. This discrepancy appears to depend only on the boundary conditions and not on the exchange-correlation potential or basis set. For single- and paired-electron trapping, we report the adiabatic barriers to motion and we estimate hopping rates and freeze-in temperatures. For the single trapped electron, we also predict the (73)Ge and (77)Se hyperfine coupling constants.

Hole localization in [AlO4]0 defects in silica materials

First-principles calculations based on cluster models have been performed to investigate the ground state and the optically excited states of the [AlO(4)](0) hole in alpha-quartz and in the siliceous zeolite ZSM-5. The structure and spectroscopic properties of this defect have been studied using the recently developed Becke88-Becke95 one-parameter model for kinetics (BB1K) functional of Zhao et al., [J. Phys. Chem. A 108, 2715 (2004)]. Our results show that the BB1K method is significantly more reliable and more accurate than the standard density-functional theory (DFT) functionals at reproducing the localized spin density on one oxygen atom and the hyperfine coupling constants associated with the hole. Furthermore, we find that the BB1K results are in close agreement with experiments, and with the self-interaction-free unrestricted Hartree-Fock (UHF) and unrestricted second-order Møller-Plesset perturbation theory (UMP2) calculations. For the first time, we present results of the ground-state paramagnetic properties of the Al defect in ZSM-5. Similar to the theoretical work for defective alpha-quartz, we find that the BB1K, UHF, UHFLee-Yang-Parr, and UMP2 calculations show a localized hole on one oxygen neighboring the Al, while even the best to date thermochemically derived hybrid generalized gradient approximation density-functional, B97-2, predicts a different model where the hole is distributed over two oxygen. We have further considered the optical transitions of the [AlO(4)](0) center in alpha-quartz and ZSM-5. In both systems, our BB1K time-dependent density-functional theory (TDDFT) and configuration interaction singles (CIS) calculations predict that the most likely transition involves electron transfer from the hole-bearing oxygen to other neighboring oxygen ions. This reinforces the experimental conclusions obtained for defective alpha-quartz. Notably, the two lowest, most dominant excitation energies calculated by BB1K-TDDFT (1.99 and 3.03 eV) show excellent agreement with experiment (1.96 and 2.85 eV [B. K. Meyer, J.M. Spaeth, and J.A. Weil, J. Phys. C: Solid State Phys. 17, L31 (1987)]) clearly outperforming the CIS method and other DFT calculations available in the literature.

Sweeping and irradiation effects on hydroxyl defects in crystalline natural quartz

A study of electrodiffusion (sweeping) and irradiation effects on hydroxyl defects has been carried out on good quality optically clear natural quartz crystals. The crystals were of Arkansas and Brazilian origin which are used as starting material in hydrothermal synthesis of high purity cultured quartz crystals. In particular, various OH(- ) related point defects have been monitored using infrared absorption measurements in the 3100-3700 cm(-1) range. Our estimates from sweeping and irradiation results show that the Brazilian crystal had a much higher concentration of hydrogen than the Arkansas crystal. We have therefore designated the Brazilian crystal as "high-H" and Arkansas stone as "low-H" quartz. High- and low-H samples of natural quartz in three different conditions, unswept, Na-swept, and H-swept, were subjected to a sequence of low- and room-temperature irradiations that separate proton and alkali motion. Irradiation at 77 K reduces the strength of almost all the IR bands except the short-bond Al-OH(-) center. Warm up to room temperature results in significant recovery (85-95%). Al-OH(-) centers in natural quartz were found to exhibit irradiation characteristics similar to those of cultured quartz. For similar radiation doses (2 Mrad) the prominent bands, Li and H dependent at 3476 cm(-1) (designated as Li-n3a) and Na and H dependent at 3451 cm (-1) (designated as Na-n2a) showed a larger decay at 300 K-irradiation than at 77 K. The prominent H-dependent OH(-) band at 3468 cm(-1) (designated as H-n2a) did not show any depletion of peaks after 300 K-irradiation while the 77 K-irradiation reduced their strength to about 10% of the as-H-swept strength. The results have been discussed in terms of protons and alkali ions motion to shallow and deep traps and compared with cultured quartz.

Electrodiffusion (sweeping) of ions in quartz-a review

Sweeping, a high-temperature process that selectively exchanges monovalent ions in quartz, is reviewed. Sweeping is used commercially to replace interstitial alkalis with hydrogen. Electrodiffusion improves the radiation hardness of quartz oscillator crystals and lowers the etch channel density. Sweeping affects the substitutional aluminum with its associated alkali and the extended dislocation networks with their precipitated impurities. At high temperatures the interstitials are freed and swept down the open Z-axis channels by an applied electric field. These interstitials can then be swept out at the negative electrode, provided replacement ions, usually hydrogen, are brought in at the positive electrode. Thus, the alkali associated with the aluminum is replaced by a hydrogen, and the precipitates in the dislocation networks are modified so that they are less reactive to the etchants. With the field applied at a room temperature, a peak or plateau in the sample current is observed during warm-up near 250-300 degrees C, followed by a decay at the 500 degrees C sweeping temperature. When the current becomes steady, the sweeping is thought to be complete. While infrared and high-temperature resistance measurements are useful, EPR techniques seem to provide the most reliable test for complete ion exchange at the aluminum site.