²⁷Al MAS NMR, EXAFS and X-ray induced absorption studies in Al-doped and Yb–Al-doped silica glasses fabricated by MCVD

Abstract

27Al MAS NMR, EXAFS, and X-ray induced absorption measurements have been carried out in Al-doped silica (AS: 0.4 wt% ≤ Al ≤ 6.3 wt%) and Yb–Al-doped silica (YAS: 0.02 wt% ≤ Yb ≤ 0.8 wt%, Al = 0.4 wt%) glasses fabricated by MCVD. AS and YAS glasses are found to contain the normal 4-, 5-, and 6-fold coordinated Al sites (Al(4), Al(5), and Al(6)). Furthermore, AS and YAS glasses have significant Al(4) intensities of 27Al NMR spectra in the range of chemical shift value at ∼38 ppm and ∼92 ppm, respectively. In AS glasses, Al(6) is dominant in Al < 1.5 wt%, but Al(4) rapidly increases with increasing Al concentration and its relative proportion becomes larger than that of Al(6). This behavior is contrary to the results in melt-quenching glasses. The Rayleigh scattering increases with increasing Al in AS glasses, indicating an onset of subnanometer scale clustering of Al. The mechanism of a pair generation of Al-OHC and Al-E’ center in AS glasses is proposed to be bond breaking of unstable triclusters composed of Al(n), Si(4) and Al(4) connected by O(3) with strained bonds. In YAS glasses, Al(4) increases with increasing Yb3+ concentration from 0 to 0.02 wt%, but Al(4) is almost constant with further increasing Yb. Yb–O coordination number, deduced from EXAFS, is ∼7, and it is almost independent of Yb concentration. These results suggest that neutralization of Yb3+ can be done not only Al(4), but also Al(5) and Al(6). A pair generation of Al-OHC and Yb2+ is more likely generated than a pair generation of Al-OHC and Al-E’ center in YAS glasses. This is caused by the fact that Yb2+ and/or Al-OHC in YAS glasses have deeper trapping levels than Al-E’ and/or Al-OHC in AS glasses , which are confirmed by annealing measurements. A plausible precursors of Yb2+ and Al-OHC is a tricluster composed of Al(n), Si(4) and Yb(7) connected by O(3).

Difference between X-ray-induced and 451 nm LD-induced photodarkening in DyAlGe-doped silica glasses

“…In order to investigate PD in Dy-doped silica glass in detail, defect formation induced by X-ray was investigated in the preforms. 7,8) Al-OHC [Al Oxygen Hole Center: ≡Al-O ̇-Si≡ (≡ denotes triple-fold coordination with O, and・denotes a paramagnetic electron)] 9,10) and Dy 2+ are pair-generated. 8) Figure 1(a) shows our proposed model of local structure before and after X-ray-induced-PD (X-PD).…”

Ge-codoping effect on X-ray-induced photodarkening in Al-doped and Dy-Al-doped silica glass

“…In Al-doped silica glass, pair generation of Al-OHC (oxygen hole center ºAl-O  -Siº(º denotes triple-fold coordination with O, and•denotes a paramagnetic electron)) and Al-E′ center (ºAl•) occurs. 21,22) According to 27 Al MAS NMR measurements, 20) we proposed that the pair-generation of these defects is caused by bond breaking of unstable triclusters composed of Al (4) , Si (4) and Al (4) connected by O (3) with a strained bond, as shown in Fig. 1(a).…”

“…1(a). From the results of the Yb-Al-doped silica glass, [22][23][24] it is presumed that pairs of Dy 2+ and Al-OHC are generated in the Dy-Al-doped silica glass, as shown in Fig. 1(b).…”

The difference in the coordination environment around Yb3+ ions between Yb–Al-doped and Yb–P-doped silica glasses

“…To elucidate the mechanism systematically, we have prepared Yb-, Yb-P-, Yb-Al-, and Yb-Al-P-doped silica glasses fabricated by modified chemical vapor deposition (MCVD) method, and have been investigating the local structure around the Yb 3+ ion by using extended X-ray absorption fine structure (EXAFS) and various optical measurements. [12][13][14][15] In Yb-doped silica (YS) glasses with Yb ⩽ ∼0.15 wt%, the Yb-O coordination number is ∼3, and Yb 3+ ions do not need to be clustered since a Yb 3+ is chargecompensated with three oxygen ions bonding to Si ions, i.e. [YbO 3/2 ] 0 .…”