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Article Cite This: J. Phys. Chem. B 2018, 122, 2809−2820

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Origin of Radiation-Induced Darkening in Yb3+/Al3+/P5+-Doped Silica Glasses: Effect of the P/Al Ratio Chongyun Shao,†,‡ Jinjun Ren,† Fan Wang,†,‡ Nadege Ollier,§ Fenghou Xie,†,‡ Xuyang Zhang,†,‡ Lei Zhang,*,† Chunlei Yu,*,† and Lili Hu*,† †

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Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § Laboratoire des Solides Irradiés, UMR 7642 CEA-CNRS-Ecole Polytechnique, 91128 Palaiseau, France ABSTRACT: Yb3+/Al3+/P5+-co-doped silica glasses with different P/Al ratios were prepared using the sol−gel method combined with high-temperature sintering. The evolution of composition-dependent color centers caused by X-ray irradiation in these glasses was correlated with their structural changes, which are controlled by the P/Al ratio. Nuclear magnetic resonance (NMR) and Raman spectra have been used to characterize the glass network structure, and advanced pulse electron paramagnetic resonance (EPR) has been employed to study the local coordination atomic structures of Yb3+ ions in pristine glasses as a function of the P/Al ratio. Si- (Si-E′), Al- (Al-E′, Al-ODC, AlOHC), P- (P1, P2, POHC), and Yb-related (Yb2+) color centers in irradiated glasses have been observed and explained by optical absorption and continuous wave-EPR spectroscopies. The formation mechanisms of these centers, the structural models of glasses, and the relationship between them were proposed. Direct evidence confirms that the formation of Yb2+ ions induced by radiation is highly dependent on the coordination environment of Yb3+ ions in glasses. In addition, the glass network structure significantly affects the generation of oxygen hole color centers (AlOHCs/POHCs) caused by radiation. These results are useful in understanding the microstructural origin and the suppression mechanism of the radiodarkening effect by phosphorus co-doping in Yb3+-doped silica fibers. solubility). Deschamps et al.13 found that when phosphorus is introduced in more amount than aluminum (P > Al) in Yb3+/ Al3+/P5+-doped glass, nearly no RD is observed. In contrast, when P < Al, a strong absorption band is induced by γ-ray radiation. By using optical absorption and electron paramagnetic resonance (EPR) spectroscopies, they demonstrated that Al-related color centers (AlOHCs) are primarily responsible for radiation-induced darkening, whereas Yb- and P-related color centers were not observed. Ollier et al.15 provided evidence of Yb2+ formation upon electron irradiation by in situ observation of Yb2+ emission, and they observed that Yb2+ formation is very sensitive to the P/Al ratio. Griscom et al.16 systematically studied P-related color centers (P1, P2, P4, lPOHC, and r-POHC) in γ-irradiated P5+ single-doped silica glasses by EPR spectroscopy. The structural models proposed for these five centers can be denoted (O)3−P• (P1), (O)4−P• (P2), (O)2−P• (P4), (O)3−P−O° (l-POHC), and (O)2−P− (O)2° (r-POHC). Likhachev et al.17 observed that the P1 color center, which absorbs around 1.6 μm, appears to be reduced by

1. INTRODUCTION Yb3+-doped silica fibers (YDFs) serve to design efficient, highoutput power laser sources that are used for industrial processing and biomedical applications.1,2 In addition, owing to their reduced weight, size, and high peak power combined with narrow pulse width, YDF-based laser is a very appealing technology to implement deep-space optical communication.3 However, YDFs suffer from photodarkening (PD) effects, which are induced by pump photons, and radiodarkening (RD) effects, which are induced by ionizing radiation, when operated under amplifying conditions and harsh radiative environments (such as space).4 These two darkening effects, which are due to the formation of color centers, can cause an obvious increase in fiber loss and a drastic decrease in laser slope efficiency.5 The dynamics, mitigation, and mechanism of the PD effect were systematically studied during the last decade.6−12 Compared with the PD effect, the RD effect of YDF has been less studied.4,13−15 Girard et al.14 showed that the radiation-induced attenuation of a rare-earth (RE)-doped active fiber is approximately 3 orders of magnitude larger than that of a passive (RE-free) fiber, which is presumed to be relevant to the formation of color centers associated with REs and dopants (such as Al and P, which are incorporated to increase RE ions’ © 2018 American Chemical Society

Received: December 21, 2017 Revised: February 12, 2018 Published: February 12, 2018 2809

DOI: 10.1021/acs.jpcb.7b12587 J. Phys. Chem. B 2018, 122, 2809−2820

Article

The Journal of Physical Chemistry B Al3+ and P5+ co-doping in comparison to the radiation-induced attenuation in Er3+/P5+- and Er3+/Al3+/P5+-co-doped fibers. These results suggest that the P/Al ratio plays an important role in the RD process of Yb3+/Al3+/P5+-doped glasses or fibers. However, the origins of these phenomena at the microscopic scale still remain unclear. Nuclear magnetic resonance (NMR) results illustrate that the network structure of Al2O3−P2O5−SiO2 glasses largely depends on the P/Al ratio, especially the coordination number of aluminum (4, 5, 6) and bridge oxygen number (2,3,4) linked to phosphorus. However, the response of these aluminum and phosphorus species to radiation in Yb3+/Al3+/P5+-doped silica glasses has not been reported in the previous studies.18,19 The effect of co-doping with aluminum and/or phosphorus on the local environment of Yb3+ ions in silica glasses has been examined using pulse EPR spectroscopy.20,21 Results show that phosphorus forms a solvation shell surrounding Yb3+ ions when P/Al > 1, that is, Yb3+ ions with an oxygen as the nearest neighbor and a P as the next-nearest neighbor, denoted Yb− O−P.13,20 However, coordination information of Yb3+ in the glass with P/Al = 1 is still unclear and there are ongoing studies to determine whether aluminum also facilitates a complete solvation shell21,22 or it only modifies the local structure of RE sites when P/Al < 1.20 Furthermore, there is no report available on the effect of the local Yb3+ structure on RD in Yb3+/Al3+/ P5+-doped silica glasses. In this work, a sol−gel method combined with hightemperature sintering was used to prepare Yb3+/Al3+/P5+doped silica glasses with different P/Al ratios. Compared with the traditional modified chemical vapor deposition method, this method can be used to prepare large-sized glass samples with a broad composition region. Yb2O3, Al2O3, and P2O5 can be easily and homogeneously doped into SiO2 glasses with this method.23 The radiation-induced color centers in Al3+ singledoped, P5+ single-doped, and Yb3+/Al3+/P5+-co-doped glasses with various P/Al ratios were comparatively studied using continuous wave EPR (CW-EPR) and optical absorption spectroscopies. Solid-state NMR and Raman spectra have been used to characterize the glass network structure of pristine samples, whereas advanced pulse EPR has been employed to study the local environments of Yb3+ ions as a function of the P/Al ratio. On the basis of the experiments mentioned above, the nature, formation mechanisms, and precursors of color centers responsible for RD are proposed. We demonstrated that the radiation-induced darkening in Yb3+/Al3+/P5+-doped silica glasses mainly originates from Yb2+ ions and oxygen hole centers (AlOHCs and POHCs). The generation of these centers is highly dependent on the local environment of Yb3+ ions and the network structure of glasses, which is controlled by the P/Al ratio.

Table 1. Mean Compositions and Tested P/Al Ratios in Glasses sample

Yb2O3 (mol %)

Al2O3 (mol %)

P2O5 (mol %)

theoretical P/Al ratio

tested P/Al ratio

Al/SiO2 YAP0 YAP0.25 YAP1 YAP1.5 YAP2 P/SiO2

0 0.1 0.1 0.1 0.1 0.1 0

4 4 4 4 4 4 0

0 0 1 4 6 8 5

0 0 0.25 1 1.5 2

0 0 0.23 0.91 1.34 1.88

single-doped (named Al/SiO2) and P5+ single-doped (named P/SiO2) samples were also prepared using the sol−gel method. To compare the absorption spectra of Yb2+ ions, two identical Yb3+/Al3+-co-doped samples (YAP0) were annealed in hydrogen and oxygen atmospheres at 1000 °C for 10 h, respectively. The results obtained from inductively coupled plasma (ICP) analysis show that the Yb2O3 and Al2O3 contents in silica glasses are close to the theoretical values, whereas the P2O5 content is slightly less than its theoretical value owing to the volatility of phosphorus. Bulk glasses were irradiated using an X-ray irradiation system (MultiRad 160) up to total accumulated doses of 1, 3, and 18 kGy at a dose rate of 10 Gy/min. Bulk glasses were cut and polished to 2 mm thick chips (Ø 15 mm) for the spectroscopic property tests. Powder samples of approximately 100 mg weight were used for the EPR and NMR tests. The absorption spectra were recorded using a Lambda 950 UV−vis−NIR spectrophotometer in the range of 200−1100 nm. Raman spectroscopy was conducted by a Renishaw inVia Raman microscope using a 488 nm argon-ion laser as an excitation source. All of the solid-state EPR experiments were performed on an E-580 BRUKER ELEXSYS X-band EPR spectrometer. The CW-EPR spectra were recorded at room temperature for paramagnetic point defects and at 10 K for Yb3+ ions. The microwave frequency was 9.38 GHz. The two-pulsed (π/ 2−τ−π−τ echo) echo-detected field-swept EPR (EDEPR) spectra were recorded at 4 K, and the π/2 and π pulse lengths were 16 and 32 ns, respectively. The time delay between the pulses was set to τ = 136 ns. To compare the local environments of Yb3+ as a function of the P/Al ratio, fourpulsed two-dimensional hyperfine sublevel correlation (2DHYSCORE) EPR spectra were recorded at 4 K using the pulse sequence π/2−τ−π/2−T1−π−T2−π/2−τ-echo with a time delay of τ = 136 ns under an external magnetic field of 350 mT. The τ value (136 ns) has been optimized by performing threepulsed electron spin echo envelope modulation (3P-ESEEM). All of the solid-state NMR experiments were performed on a Bruker Avance III HD 500 M spectrometer (11.7 T). 27Al magic angle spinning (MAS) was acquired with short pulses of 1 μs length and a relaxation delay of 1 s at a resonance frequency of 130.3 MHz (27Al) using a 4 mm MAS-NMR probe operated at a rotor frequency of 12.0 kHz. Static 31P wide-line NMR spectra were recorded using the Hahn spin echo method, recorded with 180° pulses of 5.8 μs length, a 50 μs interpulse delay, and a relaxation delay of 300 s. The 27Al and 31P chemical shifts were referenced to an aqueous solution of 1 M Al(NO3)3 and to 85% H3PO4, respectively. Line shape analysis and deconvolutions were performed using DMFIT software.25

2. EXPERIMENTAL SECTION Tetraethoxysilane, C2H5OH, AlCl3·6H2O, H3PO4, and YbCl3· 6H2O were used as precursors. Deionized water was added to sustain the hydrolysis reaction. Pure analytical-grade chemical reagents were weighed according to the molar composition of the samples listed in Table 1. The preparation process of Yb3+doped silica glasses using the sol−gel method was described in detail in ref 24. Fixed contents of Yb2O3 (0.1 mol %) and Al2O3 (4 mol %), as well as varying contents of P2O5 from 0 to 8 mol %, were used in the series of YAP glasses. The resultant samples were named YAP0, YAP0.25, YAP1, YAP1.5, and YAP2, with the lowest to highest P/Al molar ratio. For comparison, Al3+ 2810


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Figure 1. (a) Absorption spectra of pristine samples and (b) radiation-induced absorption spectra (RIA) obtained by subtracting the absorption spectra of pristine samples from those of 3 kGy X-ray-irradiated samples. The difference between the absorption spectra of reduced (red.) and oxidized (oxi.) YAP0 samples is used as a reference (the dotted curve). The inset photographs in (b) show the irradiated YAP0, YAP1, YAP1.5, and YAP2 samples as well as the reduced (red.) and oxidized (oxi.) YAP0 samples. (c) Magnified view of (b) in the NIR region.

Figure 2. (a) Deconvolution of the Yb2+ absorption spectrum obtained by subtracting the absorption spectra of the reduced and oxidized YAP0 samples and (b) CW-EPR spectra (10 K) of pristine samples as a function of the P/Al ratio. The inset in (b) shows a magnified view at a near-zero magnetic field. CW-EPR spectra (10 K) of pristine and 18 kGy-irradiated YAP0 (c) and YAP2 (d).

decrease in Yb3+ absorption intensity is detected as the P/Al ratio increases from 0 to 1. There is a rapid decrease when the P/Al ratio changes from 1 to 1.5, and then a slight decline is also observed as the P/Al ratio increases from 1.5 to 2. The effect of the P/Al ratio on spectroscopic properties of Yb3+ ions has been discussed in detail in our previous work.26 The UV absorption edges of the samples show a slow blue shift as the P/Al ratio increases from 0 to 1, and a rapid blue shift occurs when it changes from 1 to 1.5, after which a slight red shift is observed as it increases from 1.5 to 2. The effect of the P/Al ratio on the UV absorption edge of the samples is associated

3. RESULTS AND DISCUSSION 3.1. Radiation-Induced Color Centers Identified by Absorption and CW-EPR. Figure 1a,b shows the absorption spectra of pristine samples and the changes in the absorption spectra (also called radiation-induced absorption, RIA) caused by X-ray irradiation. Figure 1c shows a magnified view of Figure 1b in the NIR region. From Figure 1a, it is clearly observed that the P/Al ratio has a significant effect on the absorption coefficients of Yb3+ ions (850−1050 nm) and the UV absorption band (200−500 nm) of the samples. A slow 2811


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Figure 3. Gaussian decomposition of RIA spectra of Al/SiO2 (a) and P/SiO2 (b) samples, and RIA spectra of YAP0 in (a) and YAP2 in (b) samples are used as a reference (the dashed curve); (c) CW-EPR spectra (300 K) of 3 kGy X-ray-irradiated samples; and (d) magnified view of the central part of CW-EPR spectra in (c).

samples (not shown). In addition, the depth of this “hole” decreases as the P/Al ratio increases. This phenomenon suggests that a small quantity of Yb3+ ions is reduced to Yb2+ ions upon irradiation and the amount of radiation-induced Yb2+ ions decreases as the P/Al ratio increases. A detailed discussion of the reduction of Yb3+ to Yb2+ caused by radiation via low temperature (10 K) CW-EPR is provided and is shown in Figure 2. Figure 2a shows the absorption spectrum of Yb2+ ions obtained by subtracting the absorption spectra of the reduced and oxidized YAP0 samples. This spectrum was decomposed into six Gaussian components with peaks at 3.1, 3.7, 4.1, 4.7, 5.4, and 6.1 eV and with full width at half-maximum (FWHM) values of 0.49, 0.42, 0.6, 1.0, 0.4, and 1.1 eV, respectively. This result is in quantitative agreement with the work of Kirchhof et al.,28 and the cumulative fit peak is in good agreement with the observed absorption spectrum of Yb2+ ions. Figure 2b shows the low-temperature (10 K) CW-EPR spectra of pristine samples as a function of the P/Al ratio. Because of the fast spin lattice relaxation time of RE ions, no EPR signal of Yb3+ was observed at temperatures above 100 K.29 The CW-EPR spectra of Yb3+ ions were recorded at 10 K. In comparison to that in Yb3+ ions in crystals,29 it is difficult to describe the EPR spectra of Yb3+ in glasses with an isotropic g tensor because of the inhomogeneous broadening. Several studies30−32 have shown that the broad line over the entire range correlates with the Yb3+ ions diluted in glass, as observed in Figure 2b. Two different types of broad spectra can be observed from 100 to 600 mT, depending on the P/Al ratio: CW-EPR spectra of glasses with P/Al ≤ 1 have a maximum at

with the local environmental change of Yb3+ ions. This interpretation is supported by the work reported by Engholm et al.,11,27 in which the strong charge-transfer absorption band near 230 nm in Yb3+-doped aluminosilicate glass is shifted to shorter wavelengths (192 nm) in Yb3+-doped phosphosilicate glass owing to the change of the coordination shell around the Yb3+ ion, that is, Yb−O−P instead of Yb−O−Al. The effect of the P/Al ratio on the local environment of Yb3+ ions was revealed using advanced pulse EPR, as described in Section 3.2. RIA spectra were obtained by subtracting the absorption spectra of the pristine samples from those of the irradiated samples, as shown in Figure 1b. It is clearly observed that the RIA intensity decreases significantly as the P/Al ratio increases from 0 to 1.5, whereas no obvious change in RIA is observed as it increases from 1.5 to 2. Photographs of irradiated YAP0, YAP1, YAP1.5, and YAP2 samples are presented in Figure 1b. It is shown that the P-free sample (irradiated YAP0) exhibits a strong darkening effect after exposure to X-rays and almost no coloration appears in P-containing samples with P/Al > 1 (irradiated YAP1.5 and YAP2). This behavior indicates that the radiation resistance of Yb3+/Al3+/P5+-co-doped glasses has been significantly improved by phosphorus co-doping. By subtracting the absorption spectra of the reduced and oxidized YAP0 samples, additional absorption bands in the 190−500 nm range are observed (see the dotted curve in Figure 1b), which can be attributed to 4f14 → 4f135d1 transitions of the Yb2+ ions28 and expected to appear in irradiated Yb-containing samples. As can be seen in Figure 1c, a “hole” is observed at the 975 nm Yb3+ absorption wavelength in the Yb-containing samples, whereas it is absent in the Yb-free 2812


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The Journal of Physical Chemistry B around 110 mT, whereas the maximum for CW-EPR spectra of glasses with P/Al > 1 is located at 160 mT. This behavior is correlated with the local environment of Yb3+ (see Section 3.2). In addition, a shoulder at 75 mT (g ≈ 9.8) can be observed, which is attributed to Yb3+ clusters.30 As can be seen, the intensity of this shoulder decreases as the P/Al ratio increases, which means that Yb3+ ion clusters decrease with the increasing P/Al ratio. Figure 2c,d shows the pristine and 18 kGy X-ray-irradiated low-temperature EPR spectra of YAP0 and YAP2 samples, respectively. After irradiation, the Yb3+ EPR intensity in the YAP0 sample clearly decreases; this behavior is due to the reduction of Yb3+ to Yb2+ ions upon X-ray irradiation (Yb2+ ions are diamagnetic species). This interpretation is supported by another study in which the same phenomenon was observed in Yb3+-doped alumina-borosilicate glass upon γ-irradiation.32 The Yb3+ EPR signal in the YAP2 sample decreases slightly after irradiation under the same measurement conditions, which indicates that fewer Yb3+ ions were reduced to Yb2+ ions in the YAP2 sample than in the YAP0 sample under the same irradiation dose. This finding is in good agreement with the observation that the reduction of the Yb3+ optical absorption band is less pronounced in the irradiated YAP2 sample compared with that in the irradiated YAP0 sample (see Figure 1c). In addition to the absorption of Yb2+ ions, the absorption of point defects also plays an important role in radiation-induced darkening. RIA and EPR spectra are helpful in identifying the nature of these defects. To rule out the effect of Yb3+ ions, the RIA spectra of defects were obtained in Al3+ single-doped (Al/ SiO2) and P5+ single-doped (P/SiO2) silica glasses; the EPR spectra of defects were recorded at room temperature (no EPR signal of Yb3+ ions can be observed at this temperature) in the representative Yb3+/Al3+/P5+-doped silica glasses. Figure 3a,b presents the RIA spectra of Al3+ single-doped (Al/SiO2) and P5+ single-doped (P/SiO2) silica glasses, respectively; RIA spectra of YAP0 and YAP2 samples are also shown in Figure 3a,b for comparison (the dashed curve). The RIA spectrum of the Al/SiO2 sample was decomposed into five Gaussian components peaking at 2.15, 2.96, 4.17, 4.94, and 5.78 eV whose FWHM values are 0.66, 1.15, 0.8, 0.64, and 0.78 eV, respectively. This result is similar to that reported by Hideo et al.,33 and it agrees well with that of our previous work31 (see Table 2 for more details). The RIA spectrum of the P/SiO2 sample was decomposed into six Gaussian components peaking at 2.2, 2.5, 3.1, 4.5, 5.3, and 5.8 eV, whose FWHM values are 0.35, 0.63, 0.73, 1.27, 1.11, and 0.8 eV, respectively. A similar deconvolution result has been reported by Griscom et al.16 (see Table 2 for more details). Compared with those of the Al/SiO2 samples, the RIA spectra of the YAP0 glasses are different in the UV range but similar in the visible range, which suggests that the absorption in the visible range may originate from the same type of color centers in these two glasses. A similar result is also observed in the P/SiO2 and YAP2 samples, as shown in Figure 3b. Figure 3c,d shows the room temperature (300 K) EPR spectra of the irradiated Al/SiO2, YAP0, YAP1, YAP1.5, YAP2, and P/SiO2 samples. From Figure 3c, it can be observed that a hyperfine doublet with a 112 mT separation is present in the irradiated P-containing samples but is absent in the irradiated P-free samples. This doublet signal is ascribed to the P-related P2 defect (an unpaired electron trapped in a P atom coordinated with four oxygen atoms, denoted (O)4−P•,

Table 2. Peak Positions (Abs) and Full Width at HalfMaximum (FWHM) Values of Radiation-Induced Absorption (RIA), g Values, and Hyperfine Parameters (A) of EPR of the Radiation-Induced Point Defects RIA defects

abs (eV)

FWHM (eV)

EPR A1, A2, A3 (mT)

g1, g2, g3 0

5.78

0.78

Si-E′

5.8

0.8

Al-ODC

4.94

0.64

1.9996, 2.0005, 2.0018a 1.9999, 2.0014, 2.0018a none

Si-ODC Ge-ODC Sn-ODC Al-E′

5.0 5.13 4.90 4.17

0.38 0.46 0.5 0.80

none none none not observed

none none none not observed

Al-E′ AlOHC

4.1 2.96, 2.15 3.2, 2.3 2.98, 2.14

1.02 1.15, 0.66

2.001, 2.014, 2.024a

7.3, 4.7, 13.2a

AlOHC AlOHC

P2

4.5

1.27

P2

4.5

1.27

l-POHC

3.1

0.73

l-POHC r-POHC

3.1 2.2, 2.5, 5.3 2.2, 2.5, 5.3

0.73 0.35, 0.63, 1.11 0.35, 0.63, 0.74

r-POHC P1 P1

0.79

0 none

0.90, 1.0 1.3, 0.79

AlOHC

0.29

2.002, 2.009, 1 (YAP1.5 and YAP2) samples, a hyperfine doublet exhibiting a substantially smaller separation (∼4.5 mT) is ascribed to POHC (i.e., a hole trapped in one or two nonbridging oxygen atoms bonded to P, denoted (O)3−P−O° or (O)2−P−(O)2°) and arises from the hyperfine interaction of the hole spin (S = 1/2) in one or two oxygen atoms bonded to P with the nuclear spin of 31P (I = 1/2, 100% abundant). The previous study suggests that POHC has two variants: one is stable at room temperature ((O)2−P−(O)2°, labeled as r-POHC), whereas the other is stable only at low temperature ((O)3−P−O°, labeled as l-POHC).16 However, it is worth noting that lPOHC can still be detected even if the EPR measurements were carried out on the samples placed in ambient conditions 3 days after the irradiation. This result confirms what was already reported by Origlio et al.38 regarding the fact that l-POHC defects are stable even at room temperature. Because Al-E′ centers (an unpaired electron trapped in an Al atom coordinated with three oxygen atoms, denoted (O)3−Al•) have longer spin lattice relaxation times than those of Si-E′ and AlOHC centers, the Al-E′ center will be detected only if the EPR measurements are performed in the dispersion mode (not the absorption mode) and at very low microwave (∼μw) powers.39 In this study, Al-E′ centers (4.17 eV band) could not be detected by EPR because of experimental limitations (absorption mode), even though they are present in irradiated Al-containing silica glasses, as reported by Hideo et al.33 The origin of the 4.94 eV band (FWHM = 0.64 eV) in this study is not very clear. Amossov et al.35 reviewed the properties and structures of Si-, Ge-, and Sn-related oxygen-deficient centers (ODCs) in pure, Ge-, and Sn-doped silica glasses. Their absorption bands are located at 5.035 eV (FWHM = 0.41 eV), 5.13 eV (FWHM = 0.46 eV), and 4.90 eV (FWHM = 0.5 eV), respectively.35 The ODC defect is an EPR-silent center, the existence of which can be confirmed by emission spectra.35,40 The photoluminescence (PL) bands of Si-, Ge-, and Sn-related ODCs are located at 4.4 and 2.8 eV (Si-ODC, also called the B2 center), 4.2 and 3.1 eV (Ge-ODC), and 4.0 and 3.2 eV (SnODC), respectively.35 In this study, two PL bands at 2.6 and 3.4 eV can be observed upon excitation with a 250 nm (4.94 eV) xenon lamp in the irradiated Al/SiO2 sample (not shown). On the basis of the discussion above, the 4.94 eV band is temporarily ascribed to the Al-related oxygen-deficient center (Al-ODC). Table 2 summarizes the characteristics of point defects identified by optical absorption and CW-EPR tests. The RIA and EPR results reported by other researchers are also presented in Table 2 for comparison. By using RIA and EPR spectra, the nature of color centers responsible for RD has been identified. 3.2. Glass Structure Studied by Pulse EPR, NMR, and Raman Spectra. Figure 4 shows the two-pulsed echo-detected field-swept EPR (EDEPR) spectra of pristine samples as a function of the P/Al ratio. Similar to that of the CW-EPR (see Figure 2b), two distinct types of EDEPRs can be observed from 0 to 1200 mT, depending on the P/Al ratio: EDEPRs of glasses with P/Al > 1 have a maximum at around 430 mT, whereas the maximum in glasses with P/Al ≤ 1 is located at 550 mT. The CW-EPR and EDEPR spectra together illustrate that Yb3+ ions occupy two distinct sites depending on the P/Al ratio. In addition, the EDEPR intensity at a near-zero magnetic field decreases with the increasing P/Al ratio, as shown in the inset

Figure 4. Echo-detected field-swept EPR (EDEPR) spectra of pristine samples as a function of the P/Al ratio; the inset shows a magnified view of EDEPR at a near-zero magnetic field.

of Figure 4. This result also means that Yb3+ ion clusters decrease as the P/Al ratio increases.41 To probe the local environment of Yb3+ ions, an advanced pulsed EPR experiment (2D-HYSCORE) was performed. This type of experiment, which is based on the hyperfine interaction between the electronic spin and its surrounding nuclear spin, allows us to probe the local environment of electronic spin centers via the spin echo modulation at the Larmor frequency of the neighboring nuclei. As HYSCORE suffers from a blind spot effect, the τ value was previously optimized by performing 3P-ESEEM with varying τ values and the optimum value was found to be 136 ns. Figure 5a−d shows the 2D-HYSCORE spectra (4 K) recorded at a magnetic field of 350 mT for typical P/Al = 0 (YAP0), P/Al < 1 (YAP0.25), P/Al = 1 (YAP1), and P/Al > 1 (YAP2) samples, respectively. The three diagonal peaks located at 3.0, 3.9, and 6.0 MHz correspond to the Larmor frequencies of the nuclides of 29Si (natural abundance ∼4.68%), 27Al (natural abundance ∼100%), and 31P (natural abundance ∼100%), respectively. All of the resonances are observed in the (+, +) quadrant of the spectra, where the hyperfine coupling constant (A) is small compared with the nuclear Larmor frequency (ν) (A < 2ν).42 Moreover, there are no visible offdiagonal cross peaks. These results indicate that the hyperfine interaction between three types of nuclei and unpaired electrons of Yb3+ ions satisfies the weak coupling conditions. Therefore, we conclude that these resonance signals belong to the nuclei in the second- or higher-order Yb3+ coordination spheres (distance range 4−8 Å). Oxygen atoms that fully occupy the first coordination sphere of Yb3+ are not observed, owing to the extremely low natural abundance (∼0.038%) of 17 O. The HYSCORE spectra shown in Figure 5 were measured under identical conditions, and the relative peak intensities of the observed resonances can be compared. It is obvious that the intensities of 29Si, 31P, and 27Al vary with the P/Al ratio in glasses. When P/Al ≤ 1 (YAP0, YAP0.25, and YAP1), the 29Si peak amplitude decreases, whereas the 27Al and 31P peak amplitudes increase with the increasing P/Al ratio. When P/Al > 1 (YAP2), the 29Si peak completely collapses and a strong and extended 31P peak, as well as a weak and concentrated 27Al peak, can be observed. This behavior is related to the roles of phosphorus and aluminum in the dissolution process of the 2814


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Figure 5. Two-dimensional-HYSCORE spectra (4 K) of pristine YAP0 (a), YAP0.25 (b), YAP1(c), and YAP2 (d) samples recorded at 4 K and a magnetic field of 350 mT.

Figure 6. 27Al MAS-NMR (a), 31P static NMR (b), and normalized Raman spectra (c) for pristine samples.

Yb3+ ion cluster, as proposed by Deschamps et al.20 It is well known that the solubility of RE ions is very low in pure amorphous silica but very high in phosphate glass. The strong 29 Si peak and the weak 27Al peak in the HYSCORE, as well as the strong echo signal at a zero magnetic field in the EDEPR spectra of the YAP0 and YAP0.25 glasses, indicate that the low efficiency of Al to dissolve Yb3+ clusters in these glasses, that is, Yb−O−Yb and Yb−O−Si linkages, cannot be completely replaced by Yb−O−Al. The absence of peak corresponding to 29 Si nuclei and the very strong 31P peak in the HYSCORE spectrum as well as the absence of echo signal at a zero magnetic field in the EDEPR spectrum of the YAP2 glass indicates that P can efficiently dissolve Yb3+ clusters, that is, the almost complete replacement of Yb−O−Yb and Yb−O−Si linkages with Yb−O−P linkages.

To study the local environment evolution of Al3+ and P5+ with the increasing P/Al ratio, 27Al MAS-NMR and 31P static NMR spectra were recorded. Figure 6a shows the 27Al MASNMR spectra obtained from typical P/Al = 0 (YAP0), P/Al < 1 (YAP0.25), P/Al = 1 (YAP1), and P/Al > 1 (YAP2) samples. For the YAP2 sample, the 27Al MAS-NMR spectrum shows a dominant signal from 4-fold-coordinated aluminum (AlIV) at about 38.5 ppm and other two weak signals from 5- and 6-foldcoordinated aluminum sites located at about 0 and −21 ppm, respectively. With a decreasing P/Al ratio, the 27Al spectra become broad and poorly resolved. Such broadening of the spectra is believed to be caused by the multiple environments as well as other effects such as the paramagnetic effect of the Yb3+ ions. For the YAP1 sample, the spectra indicate the dominance or exclusive presence of 4-fold-coordinated aluminum (peak maximum near 38 ppm), as expected in AlPO4−SiO2 glasses.43 2815


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Figure 7. (a) Raman spectra of YAP2 as a function of the radiation dose. (b) Corrected Raman spectra obtained by subtracting the contribution at 2300 cm−1 from the overall spectra in (a), and all Raman spectra are normalized to the same intensity at 800 cm−1.

doped (P/SiO2) silica glasses are also presented in Figure 6c. The Raman spectra display a strong broad band at around 440 cm−1 and weaker features at 490, 600, and 800 cm−1. All of these vibrational modes originate from a fully polymerized [SiO4/2] tetrahedron network structure, as shown in the Raman spectrum of the pure SiO2 glass. Compared with those of pure SiO2 glass, weak new bands appearing near 1050 cm−1 in the YAP0 sample are assigned to stretching vibrations of tetrahedral [SiO4/2] groups bound to an Al atom.47 The sharp band at 1330 cm−1 in P/SiO2 is derived from the PO stretching vibration of OP(OSi)3 groups.19 The PO stretching vibration band in YAP2 sample shifts to 1326 cm−1, suggesting the presence of OP(OSi)2(OP) groups. Compared with those of the P5+ single-doped (P/SiO2) and Yb3+/Al3+-codoped (YAP0) samples, the broad band at 1000−1250 cm−1 in Yb 3+/Al3+/P 5+-co-doped samples (YAP1 and YAP2) is primarily ascribed to the contribution from AlPO4-like units.19 All of these Raman results are well consistent with the NMR results. Figure 7a shows the Raman spectra evolution of YAP2 as a function of the radiation dose. A broad band at around 2500 cm−1 becomes stronger as the radiation dose increases. The broad band may originate from the P-related defects that are produced upon X-ray irradiation. Origlio et al.48 reported an emission center originating from the P-related point defect in amorphous silica; this center features a broad fluorescence band centered at 3.0 eV (413 nm) and a long lifetime (5−6 ms) upon 4.8 eV (258 nm) laser excitation. Unlike that in the report from Origlio et al.,48 using a 488 nm argon-ion laser as an excitation source, we observe a broad fluorescence band centered at 550 nm whose intensity increases with the increasing radiation dose. In addition, we can also observe this luminescence band upon 488 nm excitation from an 18 kGy X-ray-irradiated P single-doped (P/SiO2) sample, as shown in the inset of Figure 7a. A deeper investigation on the origin of this fluorescence band is beyond the scope of this article. Except for this fluorescence band, the changes caused by irradiation in the Raman signal will be more clearly visible if we subtract this fluorescence band from the overall spectrum. The corrected Raman spectra, all normalized to the same intensity at 800 cm−1, are shown in Figure 7b. The most obvious difference between the various spectra is that the intensity of the 1326 cm−1 band assigned to the P(3) unit (PO) decreases

For the YAP0 and YAP0.25 samples, the broad and asymmetric spectra indicate that the aluminum species coexist in the coordination states of 4, 5, and 6. In addition, the average isotropic chemical shift (δiso) of AlIV species, as determined from the maximum peak positions, displays an obvious dependence on the P/Al ratio: for P/Al ≤ 1, δiso of AlIV decreases monotonically from 45 ppm in YAP0 to 38 ppm in YAP1, indicating that the Al−O−Si linkage is gradually replaced by the Al−O−P linkage with the increasing P/Al ratio; for P/Al ≥ 1, no obvious change in δiso of AlIV is observed with the increasing P/Al ratio, indicating that the second coordination sphere of these aluminum species is dominated by phosphorus.18,44 Figure 6b shows the 31P static NMR spectra for typical P/Al < 1 (YAP0.25), P/Al = 1 (YAP1), and P/Al > 1 (YAP2) samples. For the samples with P/Al ≤ 1 (YAP0.25 and YAP1), the 31P static NMR spectra show no detectable deviation from a Gaussian line shape, indicating that the phosphorus site has a nearly tetrahedral symmetry; in addition, their average isotropic chemical shifts (−30 ppm) are close to where they were previously located (at −28 ppm) in AlPO445 and AlPO4− SiO243 glasses. This line shape is assigned to tetrahedral PO4/2 units that interact with AlIV, as with the situation in AlPO4. As these P atoms are linked to four bridging oxygen atoms, they are hereafter denoted P(4) units. (With this P(n) nomenclature, the superscript denotes the number of bridging oxygen atoms attached to a P atom.) For the sample with P/Al > 1 (YAP2), the 31P static NMR spectrum was deconvoluted into four distinct line shape components. With the exception of the isotropic Gauss peak due to the P(4) unit mentioned above, a nonaxially symmetric component and two axially symmetric shielding tensors were observed. On the basis of the spectra reported previously in the literature,18 the nonaxially symmetric component was assigned to an anionic P(2) unit, which interacts with the higher-coordinated aluminum species (see Figure 6a) and is analogous to the situation in crystalline and glassy aluminum metaphosphate. The two axially symmetric shielding tensors are tentatively attributed to two types of P(3) units involved in linkages to silicon (such as OP(OSi)3 group) and phosphorus (such as OP(OSi)2(OP) group) species.19,46 To study the global glass network structure, the Raman spectra of typical glasses were recorded, as shown in Figure 6c. For comparison, the Raman spectra of pure silica and P single2816


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Figure 8. Schematic structures for samples YAP0 (a), YAP0.25 (b), YAP1 (c), and YAP2 (d).

as the radiation dose increases; this result indicates that PO bonding is gradually broken by X-ray irradiation. According to the results from pulse EPR, NMR, and Raman spectroscopies, the schematic structures of Yb3+/Al3+/P5+doped silica glasses with varying P/Al ratios (YAP0, YAP0.25, YAP1, and YAP2) are deduced and are presented in Figure 8a− d. 3.3. Correlation between Radiation-Induced Color Centers and the Glass Structure. The radiation-induced color center evolution in Yb3+/Al3+/P5+-doped silica glasses, as described in Section 3.1, is highly dependent on the P/Al ratio, which should be correlated to the structural information of these glasses, as described in Section 3.2. For the sample with P/Al = 0 (YAP0), the secondneighboring shell of Yb3+ is primarily a mixture of Al and Si atoms (Figure 5a). Our earlier study shows that trapped electron centers (TECs) and trapped hole centers (THCs) are always generated in pairs:31 In Al3+ single-doped silica glasses, TECs (Si-E′/Al-E′) and THCs (AlOHCs) are observed, as shown in Figure 3a; and in Yb3+/Al3+-co-doped glasses (YAP0), Yb2+ ions, which are formed by electron trapping in Yb3+ ions, dominate the TECs. Compared to that in the Al3+ single-doped sample, the clear decline in the absorption and EPR intensities of Si-E′/Al-E′ is observed in the Yb3+/Al3+-co-doped sample (YAP0); however, there is little change in the intensities of AlOHCs (Figure 3a,d). Their formations are described as follows

Although the silicon−oxygen-trapped hole center (Si−O° or NBOHC) is expected to form upon irradiation, neither the emission nor EPR signals support the existence of the NBOHC defect. This may be because negatively charged tetrahedral [AlO4/2]− has a priority to trap a positively charged hole. The presence of tetrahedral [AlO4/2]− (AlIV) has been revealed by NMR (Figure 6a); both tetrahedral AlIV and octahedral AlVI units coexist around RE ions as ligands, as proved by Funabiki et al.,49 in Nd3+/Al3+-doped silica glasses. For the samples with P/Al ≤ 1 (YAP0.25 and YAP1), equimolar Al and P atoms preferentially form AlPO4 units (see Figure 6a−c), which have a priority over Si atom to coordinate to the Yb3+ ions. Therefore, an obvious increase in 27Al and 31P peak amplitudes with the increasing P/Al ratio and the opposite trend for the 29Si peak are clearly observed in HYSCORE (Figure 5b,c). This indicates that the second-neighboring Si atoms of Yb3+ ions are gradually replaced by Al and P atoms (AlPO4 units) with the increasing P/Al ratio. The AlPO4 unit can be considered as a SiO2-type network in which two Si atoms are replaced by one Al and one P atom.19 Raman data on crystalline AlPO4 and SiO2 indicate that there is a large chargecompensating effect in AlPO4, with Al and P being closer to 4+ than to 3+, and 5+, respectively.19 Therefore, the formation of the Yb2+ and AlOHC pair may be inhibited to a certain degree by the electroneutral [AlPO4]0 unit (Figures 1c and 3d). However, the actual P/Al ratio (∼0.91 determined by ICPoptical emission spectroscopy) in YAP1 is less than its theoretical value (=1) owing to the volatility of phosphorus. The excess aluminum (especially AlIV unit), which is not located in AlPO4 units, is primarily responsible for the formation of AlOHCs according to reaction 3. For the samples with P/Al > 1 (YAP1.5 and YAP2), AlOHC has almost been inhibited because all of the AlIV units are located in AlPO4. With the exception for the AlPO4 unit, the excess of phosphorus forms P(3) and P(2) units as well as the P− O−P linkage, which may favor a solvation shell surrounding Yb3+ ions, and the local environmental changes (Yb−O−Al → Yb−O−P) of Yb3+ ions play an important role in their valence change. For example, the positively charged tetrahedral [PO4/2]+ coordinated to Yb3+ has a priority to trap an electron

hυ

[AlO3/2 ]0 + e− → [AlO3/2 + e−]− (Al ‐ E ′)

(1)

hυ

[SiO4/2 ]0 + e− → [SiO3/2 + e−]0 + O− (Si ‐ E ′)

(2)

hυ

[AlO4/2 ]− + h+ → [AlO4/2 + h+]0 (AlOHC)

(3)

hυ

[AlO4/2 ]− + Yb3 + → [AlO4/2 + h+]0 + Yb2 + (AlOHC)

(4) 2817


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Figure 9. Model for the formation of the P-related defects caused by X-ray radiation.

Table 3. Major Trapped Electron Centers (TECs) and Trapped Hole Centers (THCs) with Their Absorption Peak Positions (abs) in Irradiated Glass Samples Doped with Different Elements samples

Al

TECs abs (eV) THCs abs (eV)

Yb/Al/P (Al > P)

Si-E′; Al-E′ 5.78; 4.17 AlOHC 2.15/2.96

2+

Yb 3.1/3.7/4.1/4.7/5.4/6.1 AlOHC 2.15/2.96

Yb/Al/P (Al < P)

P

P2 4.5 AlOHC 2.15/2.96

P2 4.5 POHC 2.2/2.5/3.1/5.3

P2 4.5 POHC 2.2/2.5/3.1/5.3

hυ

to form P2 defects (see Figure 9a). This process may be described as follows

hυ

[PO5/2 ]0 → [PO3/2 + 2 × e−]0 + h+ → [PO3/2 + e−]+ (intermediate)

(P1)

(8)

− hυ

[PO4/2 ]+ + e → [PO4/2 + e−]0

This result is supported by the EPR and Raman spectra. In Figure 3c, the EPR signals of P1 and P2 increase with the increasing P/Al ratio for the samples with P/Al > 1 (YAP1.5 and YAP2). In Figure 7b, the Raman vibration peak (1326 cm−1) of the YAP2 sample assigned to the P(3) unit (PO bonding) decreases with the increasing radiation dose. This result is also supported by Likhachev et al.’s work,17 in which it was reported that the loss around 0.79 eV (1.6 μm) due to the P1 center seems to be reduced by Al3+ and P5+ co-doping in comparison with the radiation-induced attenuation in Er3+/P5+ and Er3+/Al3+/P5+-co-doped fibers. Our NMR and Raman measurements indicate that the P(3) unit can be completely replaced by the AlPO4 unit for Al3+- and P5+-co-doped samples with P/Al ≤ 1. Table 3 lists the major color centers (and their absorption peaks) in irradiated silica glasses doped with different elements. The absorption of TECs (Yb2+/Si-E′/Al-E′/P2) is primarily responsible for the absorption in the UV and visible regions, and the absorption of THCs (AlOHC/POHC/P1) is primarily responsible for the absorption in the visible and NIR regions. Under identical irradiation conditions, the CW-EPR measurement indicates that the number of radiation-induced AlOHCs decreases from 1.4 × 1017 spin/g in YAP0 to 4.8 × 1016 spin/g in YAP1 as the P/Al ratio increases for the samples with P/Al ≤ 1, whereas the number of radiation-induced POHC defects slightly increases from 6.5 × 1015 spin/g in YAP1.5 to 7.2 × 1015 spin/g in YAP2 as the P/Al ratio increases for the samples with P/Al > 1. This explains why RIA is obvious for samples with P/Al ≤ 1 but not significant for the samples with P/Al > 1 (Figure 1b,c). However, a high P/Al ratio (e.g., P/Al ≫ 1) is always accompanied by low absorption and emission cross sections in the Yb3+/Al3+/P5+-co-doped fiber26 and a radiationinduced P1 center that absorbs around 1.6 μm,16,51 which may be very detrimental for the Er3+/Al3+/P5+-co-doped fiber, which operates in a harsh radiation environment. Therefore, a P/Al

(5)

(P2)

Therefore, the radiation-induced Yb2+ ions decrease as the P/Al ratio increases (see Figure 1c). In particular, the radiationinduced Yb2+ ions can be effectively inhibited for the P/Al > 1 sample, as revealed by the low-temperature CW-EPR measurement (Figure 2d). At the same time, the number of radiationinduced P2 centers increases as the P/Al ratio increases (Figure 3c). Figure 9 shows the model describing the formation of the Prelated defects caused by X-ray irradiation in P-containing silica glasses. Upon irradiation, the negatively charged [(O)2−P− O2/2]− (P(2) unit), which is charge-compensated by highercoordinated aluminum species (see Figure 6a,b), traps a hole to form r-POHC (see Figure 9b). The formation of r-POHC can be expressed as follows hυ

[PO6/2 ]− + h+ → [PO6/2 + h+]0

(6)

(r ‐ POHC)

This result is supported by Pukhkaya et al.’s work,50 in which it was reported that the EPR intensities of r-POHC of various phosphate glasses are positively related to their P(2) amount. This result is also consistent with the structural model of rPOHC defect proposed by Griscom et al.16 in P-doped silica glasses. The r-POHC defect is a hole trapped in a pair of nonbridging oxygens bonded to the same phosphorus linked with two bridging oxygens (see Figure 9b). In addition, the electroneutral [OP−O3/2]0 (P(3) unit) is responsible for the formation of the P2 and l-POHC pair, as well as the P1 defect, via ionizing PO bonding (see Figure 9c). The formation process is described as follows hυ

2 × [PO5/2 ]0 → [PO5/2 + h+]+ + [PO5/2 + e−]− (l ‐ POHC)

Yb/Al/P (Al = P)

(P2)

(7) 2818



■

ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (NSFC) (Grant Nos. 61775224 and 61505232) and the National High Technology Research and Development Program of China (2016YFB0402201). We are very grateful to Profs. Shikai Wang, Qinling Zhou, Danping Chen and Jihu Su for their helpful discussions.

ratio slightly larger than 1 is suggested as optimal for improving the radiation resistance of Yb3+- or Er3+-doped silica fibers.

4. CONCLUSIONS In this work, Yb3+/Al3+/P5+-doped silica glasses with different P/Al ratios ranging from 0 to 2 were prepared using the sol−gel method combined with high-temperature sintering. By combining the optical absorption, Raman scattering, solidstate nuclear magnetic resonance (NMR), continuous wave and pulse electron paramagnetic resonance (EPR) methods, the effect of P/Al ratio on the radiation-induced color centers, the local environment of Yb3+ ions, and the glass network structure in Yb3+/Al3+/P5+-doped silica glasses were studied systematically. The formation mechanisms of color centers, the structural models of glasses, and the relationship between them were proposed. The results obtained show that trapped electron centers (Yb2+/Si-E′/Al-E′/P2) and trapped hole centers (AlOHC/ POHC/P1) are always generated in pairs upon irradiation. Among them, Yb2+ ions and oxygen hole centers (AlOHC/ POHC) are primarily responsible for the radiodarkening (RD) effect. The generation of Yb2+ ions and oxygen hole centers is highly dependent on the local environment of Yb3+ ions and the network structure of glass, which is controlled by the P/Al ratio. For the sample with P/Al < 1, Yb3+ ions are primarily surrounded by electroneutral [SiO4/2]0 and electronegative [AlO4/2]− units, and this favors the generation of Yb2+ and AlOHC pairs through Yb3+ + [AlO4/2]− → Yb2+ + [AlO4/2 + h+]0 (AlOHC). For the sample with P/Al = 1, Yb3+ ions are primarily located in the electroneutral [AlPO4]0 unit, which is very similar to the SiO2-type network and can to a certain extent inhibit the generation of Yb2+ and AlOHC pairs. For P/ Al > 1, aluminum and phosphorus formed preferentially in the AlPO4 unit and the excess phosphorus formed electroneutral [PO5/2]0 (P(3)) and electronegative [PO6/2]− (P(2)) units as well as the P−O−P linkage, which formed a solvation shell surrounding Yb3+ ions. AlOHC and Yb2+ pairs can be effectively suppressed owing to the formation of the AlPO4 unit and the coordination shell around Yb3+ ions bonding via Yb−O−P. At the same time, radiation-induced P-related defects (P1/P2/ POHCs) were observed because of the formation of P(3) and P(2) units. This work is useful for understanding the microstructural origin and the suppression mechanism of the RD effect by phosphorus co-doping in Yb3+-doped silica glasses or fibers. Furthermore, it provides important information on the composition and design of radiation-resistive Yb3+-doped silica fibers.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.Z.). *E-mail: [email protected] (C.Y.). *E-mail: [email protected]. Tel: +86-21-5991-4297. Fax: +8621-5991-4516 (L.H.). ORCID

Chongyun Shao: 0000-0001-9329-5685 Jinjun Ren: 0000-0001-5292-086X Notes

The authors declare no competing financial interest. 2819


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