Theory ofAl2O3incorporation in

Abstract

Different possible forms of Al 2 O 3 units in a SiO 2 network are studied theoretically within the framework of density-functional theory. Total-energy differences between the various configurations are obtained, and simple thermodynamical arguments are used to provide an estimate of their relative importance at different Al concentrations. Some experimentally accessible quantities ͑hyperfine parameters, electric-field gradient tensors, and core-level chemical shifts͒ are derived, and compared with experimental data where available. The results show that Al ions at low Al 2 O 3 concentrations will be present as isolated impurities bound to three or four O atoms, whereas at higher concentrations clusters consisting of four Al ions will form. In these clusters both four-and five-coordinated Al can occur, but no energetic preference for the five-coordinated state is found.

Doping effects in amorphous oxides

“…Therefore, he pointed out that the contribution of entropy is inevitable to explain the ErAl codoping effect; i.e., the Al codoping increases ¦S mix . 76) This idea is compatible with the fact that the incorporated Al 3+ ion behaves as a network modifier; i.e., the coordination number is 6. It should be noted that the coordination number of Al in SiO 2 depends on its concentration; i.e., if the Al 2 O 3 content is high, the 6-fold coordination is dominant, but the 4-fold coordination is found in the very low concentration (ppm level).…”

“…Their results indicated that the oxygen coordination number of Al is primarily 6, and the number of AlO 6 unit surrounding an Er is 1. Laegsgaard 76) calculated the formation energy of ¡-quartz supercell composed of 24 TO 4 units (T=Al and Si) including M 3+ ions (M=Er or Al) at the center of a sixmembered ring. The formation energy of the supercell including an ErAl complex (ErAl 3 O 6 ) was larger than that including an ErEr cluster (Er 2 Si 2 O 7 ) and an AlAl cluster (Al 4 O 6 ).…”

Dissolution of rare-earth clusters inSiO2by Al codoping: A microscopic model

“…I shall denote this structure ͓Al 4 O 6 ͔. In calculations which will be detailed in a separate paper 7 I have found that this is indeed a likely geometry for Al ions in silica at concentrations above 0.1%. However, in the case of interstitial Al, the structures schematically depicted in Figs.…”

Upconversion assisted self-pulsing in a high-concentration erbium doped fibre laser

“…As follows from XAFS experiments, the pair-correlation function has maxima at R 1 = 3 5 Å and R 2 = 3 9 Å, whereas it takes approximately a constant value for R > 4 5 Å [10]. It was found also that suppression of the short-range order leads to improved characteristics of high concentration erbium doped fibre amplifiers and lasers and can be realized by increasing the solubility of erbium in host matrix (codoping by Al [11] or using phosphate glass [12]) or by modification of deposition process (Direct Nanoparticle Deposition [13]). Application of such fibres as an active media for HC EDFLs resulted in auto-oscillations with two characteristic frequencies of about 10 kHz and 100 kHz and two threshold pump powers.…”