Characteristic Coordination Structure around Nd Ions in SolâGel

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Characteristic Coordination Structure around Nd Ions in Sol−GelDerived Nd−Al-Codoped Silica Glasses Fuji Funabiki,*,† Koichi Kajihara,*,§ Ken Kaneko,§ Kiyoshi Kanamura,§ and Hideo Hosono†,‡ †

Frontier Research Center and ‡Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8503, Japan § Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan ABSTRACT: Al codoping can improve the poor solubility of rare-earth ions in silica glasses. However, the mechanism is not well understood. The coordination structure around Nd ions in sol−gel-derived Nd−Al-codoped silica glasses with different Al content was investigated by optical and pulsed electron paramagnetic resonance spectroscopies. Both tetrahedral AlO4 and octahedral AlO6 units were observed around Nd ions as ligands. The average total number of these two types of ligands for each Nd3+ ion was ∼2 irrespective of Al content and was larger by 1−2 orders of magnitude than that calculated for a uniform distribution of codopant ions (∼0.08−0.25). With increasing Al content, AlO4 units disappeared and AlO6 units became dominant. The preferential coordination of AlOx (x = 4, 6) units to Nd ions enabled the amount of Al necessary to dissolve Nd ions uniformly in silica glass at a relatively low temperature of 1150−1200 °C to be minimized, and the conversion of AlO4 units to AlO6 units around Nd ions caused the asymmetry of the crystal field at the Nd sites to increase and the site-to-site distribution to decrease. the Gibbs energy of mixing (ΔG = ΔH − TΔS) in such glasses because the entropy of mixing (ΔS) is increased but the enthalpy of mixing (ΔH) remains constant.13 In contrast, a molecular dynamics study7 suggested that tetrahedral AlO4 units do not prevent the clustering of Er3+ ions, even though each Er3+ ion is preferentially coordinated by two AlO4 units. To investigate the effects of these AlOx polyhedra on the solubility of RE ions, optical and ESEEM spectroscopic analyses were performed on a series of Nd−Al-codoped silica glasses with different contents of Al. Wet-chemical methods may form a coordination structure different from that derived from melt quenching or vapor phase methods because the relatively low consolidation temperature may preserve metastable structures. Glasses used in this study were prepared by a sol−gel method we developed recently.11,12 This method enables the synthesis of monolithic RE−Alcodoped silica glasses via macroporous gels, which can be dried relatively easily, without requiring alcohols, organic solvents, or other special additives.

1. INTRODUCTION Doping of transparent materials with rare-earth (RE) ions has been realized for various types of solid-state laser materials. Silica glass is one of the best host materials for RE ions because of its excellent optical transparency, chemical durability, thermal stability, mechanical strength, and shape workability. Fortunately, the poor solubility of RE ions in silica glass can be improved by addition of aluminum (Al) ions.1−12 This Al codoping suppresses the concentration quenching caused by the clustering of RE ions, making it possible to develop silicabased photoluminescent (PL) devices including fiber lasers and amplifiers. However, the mechanism of the homogeneous dissolution of RE ions in RE−Al-codoped silica glasses is not well understood; in particular, the microscopic picture is rather vague. We have investigated the local structure around RE ions using pulsed electron paramagnetic resonance (EPR) spectroscopy.8 EPR spectroscopy is a powerful tool to investigate the intermediate-range structure around a paramagnetic center because it enables time-domain measurements of the interactions between an unpaired spin and surrounding nuclear spins, which are unresolved in conventional continuous-wave EPR spectroscopy. Electron spin echo envelope modulation (ESEEM) spectroscopic analysis has revealed that Al3+ ions are uniformly distributed in Er−Al-codoped silica glass, forming octahedral AlO6 units.8 This finding led us to propose that the incorporation of different polyhedra (AlO6 units) in the network of corner-shared tetrahedra (SiO4 units) decreases © 2014 American Chemical Society

2. EXPERIMENTAL SECTION A dilute aqueous solution of nitric acid was added to tetraethoxysilane (Si(OC2H5)4, TEOS, Shin-Etsu Chemical, Japan, 25 mmol, 5.2 g) with a TEOS:H2O:HNO3 molar ratio of Received: February 5, 2014 Revised: May 30, 2014 Published: June 29, 2014 8792

dx.doi.org/10.1021/jp501264h | J. Phys. Chem. B 2014, 118, 8792−8797

The Journal of Physical Chemistry B

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1:x1:0.002 and stirred for 120 min at 20 °C in a sealed plastic container. The clear solution was further mixed with an aqueous solution of imidazole (C3H4N2, Wako, Japan), aluminum lactate (Al(lac)3, Wako, Japan), and neodymium acetate monohydrate (Nd(Ac)3·H2O, Wako, Japan) to form a solution with an overall TEOS:H2O:HNO3:imidazole:Al(lac)3:Nd(Ac)3 molar ratio of 1:(x1 + x2):0.002:y:zAl:zNd, where x1 + x2 and zNd were fixed at 10 and 0.01, respectively. After the mixture was stirred for 1 min, the stirring bar was removed and the solution was allowed to gel at 20 °C for ∼0.5−1 h. The wet gel was aged for 24 h at 60 °C. The solvent phase was disposed by opening the container, and the gel was gently dried at 60 °C. The dried gel was sintered in a tube furnace heated at a rate of 200 °C h−1 and maintained at a target temperature (1150−1200 °C) within 0.5 h. The sintering atmosphere was changed from oxygen to helium at 600 °C. Optical absorption spectra were measured using a conventional spectrometer (U-4100, Hitachi, Japan). PL time decay profiles were detected by an InGaAs PIN photodiode (G8605-23, Hamamatsu, Japan) covered by a 1064 nm optical bandpass filter and recorded using an oscilloscope. The excitation light from a laser diode (∼1.5 W at 808 nm) was periodically cut by an optical chopper. ESEEM spectroscopic analysis was performed on a glass sample (100 mg) at a temperature of 5 K using an X-band pulsed EPR spectrometer (E580, Bruker, Germany). Two-pulse electron spin echo (ESE) spectra were recorded using the pulse sequence π/2−τ−π−τ−echo with π/2 pulse of length 16 ns and π-pulse of length 32 ns while sweeping the magnetic field B. Three-pulse ESE spectra were recorded using the pulse sequence π/2−τ−π/2−T−π/2−τ− echo with π/2 pulse of length 16 ns at τ = 120 ns and B = 400 mT while increasing the time interval T.

n

Vsim =

(2)

i=1

The square root of the residual sum of squares (δRSS) between the experimental and simulated spectra derived from eq 3, δ RSS2 =

∑ (Vexp − Vsim)2 /∑ (Vexp)2 T

(3)

T

is minimized when the number (n) of nuclear spins is optimized. In this study, we considered two types of atoms with nonzero nuclear spins, 29Si (I = 1/2 and natural abundance of 4.7%) and 27 Al (I = 5/2 and natural abundance of 100%), and three types of structural units, four-coordinate silicon (SiO4) and four- and six-coordinate aluminum (AlO4 and AlO6, respectively). To simplify our simulation, the interaction parameters of the nuclear species were fixed at the constant values listed in Table 1 as follows. The distance of the surrounding nuclei from the Table 1. Interaction Parameters of Nuclear Species for Nd3+ Ion Used in Simulation 29

SiO4 27 AlO4 27 AlO6

Aahf (MHz)

Cq (MHz)

0.367 0.481 0.481

0 6 3

central Nd3+ ion (r) was fixed at 0.35 nm by reference to crystal structures of the corresponding oxides:20,21 Nd2Si2O7 and NdAlO3. The isotropic hyperfine coupling constant (Aihf) was set to zero because Fermi contact between the Nd3+ ion and the surrounding nuclei is absent, and the anisotropic hyperfine coupling constant (Aahf) was calculated from their nuclear magnetic moments and r. The quadrupole coupling constant (Cq) was selected according to nuclear magnetic resonance (NMR) spectroscopic data22,23 reported for aluminosilicate glasses. The asymmetric parameter (η) was unknown but assumed to be zero. Given that typically η < 0.5, this assumption results in errors of less than 0.003% for δRSS and less than 0.03 for n. The spatially averaged ESEEM spectra were individually calculated for each nuclear species by integrating the Euler angles (θ and φ) between the Nd3+ ion and the nuclear species over their full ranges (0−π and 0−2π, respectively).

3. ANALYSES Theoretical PL decay constants of the samples were evaluated from optical absorption spectra by Judd−Ofelt analysis14,15 following a procedure described in the literature.11 ESEEM spectroscopic analysis16−18 measures the ESE emitted from paramagnetic species (e.g., RE ions) exposed to microwave pulses under a high magnetic field. The spin Hamiltonian of an electron spin (S) interacting with a surrounding nuclear spin (I) consists mainly of electronic and nuclear Zeeman, hyperfine, and nuclear quadrupole coupling terms as follows: H = gμB⃗ ·S ⃗ − gNμ N B⃗ ·I ⃗ + hS ⃗·A ·I ⃗ + hI ⃗·Q ·I ⃗

∏ Vi

(1)

4. RESULTS Our sol−gel route to synthesize monolithic RE−Al-codoped silica glasses originally used aluminum nitrate as an Al source.11 This process requires neutralization, which is achieved by forming an acetate buffer system upon addition of ammonium acetate (pKa = 4.8 for acetic acid) as a neutralizer (Brønsted base). However, the acidity of aluminum nitrate is so strong that such neutralization is insufficient. This prevented us from obtaining monolithic glasses at zAl/zNd > 1 at zNd = 0.01 because addition of a large amount of ammonium acetate makes gels brittle, so it is difficult for them to maintain a monolithic form during drying. Thus, we used a less acidic Al source (aluminum lactate, pKa = 3.9 for lactic acid) and a more neutral base (imidazole, pKa = 7.0)24 to reduce the ratio of base to Al, y/zAl. This modification enabled us to increase zAl/zNd up to 2.5 at zNd = 0.01 in this process.

where g and gN are the electronic and nuclear g-factors, respectively, μ and μN are the electronic and nuclear Bohr magnetons, respectively, B is the magnetic field, A is the hyperfine coupling tensor, and Q is the quadrupole coupling tensor. Such electron spin−nuclear spin interactions split the energy levels of up and down electron-spin states and modulate the ESE intensity. Standard three-pulse ESEEM experiments record it as a function of the time interval (T) between the second and third pulses. When the hyperfine coupling frequency is much smaller than the Larmor frequency of nuclear spin, the ESEEM amplitude (V) due to the nuclear spins involved is approximately expressed as a product of all Vi due to the ith nuclear spin.18 The Vi may be calculated by the software EasySpin.19 The simulated ESEEM spectrum is obtained by using eq 2, 8793



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Figure 1 shows a photograph and optical absorption spectra of the prepared Al-free and Al-codoped silica glasses. The low

Figure 2. Photoluminescence decay curves of the 4F3/2 → 4I11/2 transition of Nd3+ ions in Nd−Al-codoped silica glasses prepared at zAl/zNd of 0, 0.8, 1.7, and 2.5. Figure 1. Photograph and optical absorption spectra of Nd−Alcodoped silica glasses prepared at zAl/zNd of 0, 0.8, 1.7, and 2.5.

The local structure around Nd3+ ions (S = 3/2) in the samples was examined by ESEEM spectroscopic analysis. Figure 3 shows the filed-swept two-pulse ESE spectra at

transparency of the Al-free glass is attributable to the light scattering from amorphous Nd-rich phases precipitated from silica-rich glass matrix.11 Al codoping improved the optical transparency of the samples considerably. In addition, the UV absorption edge gradually shifted to the shorter wavelength side as the Al content (zAl/zNd) increased. The optical absorption bands of Nd3+ ions between 400 and 920 nm were used to calculate intensity parameters Ω2, Ω4, and Ω6 and theoretical PL decay constants τJO using the Judd−Ofelt theory; the results are summarized in Table 2. The quality of the calculation, defined by root-mean-square (RMS) deviations between the observed and calculated line strengths normalized to RMS of the observed ones, was ∼5−6%. τJO increased with zAl/zNd. Table 2 also lists the concentrations of SiOH groups evaluated from the peak absorption cross section of the first overtone band of the SiO−H stretching mode at ∼7250 cm−1 (∼1.7 × 10−21 cm2 11). Figure 2 depicts the infrared PL decay curves of the 4F3/2 → 4 I11/2 transition of Nd3+ ions in the glasses shown in Figure 1. The PL decay of the Al-codoped silica glasses was markedly slower than that of the Al-free silica glass and became slower with increasing zAl/zNd. The decay curves could not be described by a single exponential. The PL decay constant τexp was evaluated by fitting the time (t) decay of the normalized PL intensity (I/I0) with the stretched exponential function, I/I0 = exp[−(t/τexp)β]. The stretched exponent β (0 < β ≤ 1) represents the extent of the distribution in τexp; there is no distribution when β = 1, and the distribution becomes large as β decreases. The τexp and β values evaluated in this way are listed in Table 2. The τexp was small when zAl/zNd = 0 and increased with zAl/zNd.

Figure 3. Field-swept two-pulse electron spin echo spectra at various τ of Nd−Al-codoped silica glasses prepared at zAl/zNd of (a) 0, (b) 0.8, (c) 1.7, and (d) 2.5.

Table 2. Judd−Ofelt and Photoluminescence Decay Parameters and SiOH Concentration of Nd−Al-Codoped Silica Glasses Prepared at zAl/zNd of 0, 0.8, 1.7, and 2.5 intensity parameters (10−20 cm2)

lifetime (μs)

zAl/zNd

Ω2

Ω4

Ω6

τexp

τJO

τexp/τJO

β

SiOH concn (1020 cm−3)

0 0.8 1.7 2.5

2.9 4.8 6.1 6.9

3.5 3.1 3.2 2.9

4.0 3.0 2.9 2.7

5 209 240 263

557 688 695 754

0.01 0.30 0.35 0.35

0.52 0.76 0.80 0.82

2.2 3.0 2.2 0.8

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various τ of the samples. The ESE intensity of the Al-free silica glass was much weaker than those of the Al-codoped silica glasses because clusters of Nd3+ ions, which do not give ESE signals because of very fast spin−spin relaxation, were formed.25 The spectra are very broad in all the samples. The ESE peak of the Al-free silica glass was located at a low magnetic field (∼200 mT) because of the clustering of Nd3+ ions, whereas the ESE peaks of the Al-codoped glasses were located at a high magnetic field (∼700 mT) because of the asymmetry of the RE sites.8 Additionally, the peak shapes depended on τ because of the spin−spin interaction between Nd3+ ions and surrounding 27Al and 29Si nuclei and the spin−spin relaxation. Figure 4a shows three-pulse ESE spectra of the samples measured at τ = 120 ns and B = 400 mT. The ESE intensity

Figure 5. (a) Electron spin echo envelope modulation spectra individually calculated for each nuclear species: 29SiO4, 27AlO4, and 27 AlO6, and experimental and simulated spectra of Nd−Al-codoped silica glasses prepared at zAl/zNd of (b) 0, (c) 0.8, (d) 1.7, and (e) 2.5.

Figure 4. (a) Three-pulse electron spin echo spectra of Nd−Alcodoped silica glasses prepared at zAl/zNd of 0, 0.8, 1.7, and 2.5. (b) Spectra divided by an exponential decay function. (c) Fourier transform spectra. The arrows in (c) indicate the Larmor frequencies of 29Si and 27Al at B = 400 mT.

decayed with increasing T through spin−lattice relaxation. Figure 4b shows the ESE spectra divided by an exponential decay function. The amplitude modulation observed for the Alfree and Al-codoped silica glasses were distinctly different. Figure 4c shows the spectra following Fourier transformation, which indicate that 27Al exists close to Nd3+ ions in all Alcodoped silica glasses. Figure 5a shows the ESEEM spectra individually calculated for each nuclear species at a distance of 0.35 nm from the Nd3+ ion. Figure 5b−e compares the experimental ESEEM spectra with the simulated ones derived from eqs 2 and 3. The experimental and simulated spectra show reasonably good fits with δRSS less than 0.3% for all samples. The estimated numbers of 29SiO4, 27AlO4, and 27AlO6 nuclei for each Nd3+ ion are presented in Figure 6. In the Al-codoped silica glasses, both four- and six-coordinate aluminum were found around Nd3+ ions as ligands, in addition to four-

Figure 6. Estimated numbers of 29SiO4, 27AlO4, and 27AlO6 for each Nd3+ ion in Nd−Al-codoped silica glasses prepared at zAl/zNd of 0, 0.8, 1.7, 2.5.

coordinate silicon. The total number of AlO4 and AlO6 units for each Nd3+ ion remained nearly constant at ∼2 irrespective of zAl/zNd, whereas the fraction of AlO4 units decreased considerably with increasing zAl/zNd.

5. DISCUSSION Table 2 summarizes the results of the Judd−Ofelt analysis of the sol−gel-derived Nd−Al-codoped silica glasses. The increase of Ω2 with zAl/zNd suggests that the asymmetry of the crystal 8795



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field at the Nd3+ sites and the covalency of the Nd−O bonds increase with zAl/zNd.26,27 A simultaneous reduction of Ω4 and Ω6 also suggests an increase in the covalency of the Nd−O bonds.28 The observed changes in Ω values are consistent with results of previous studies.2−4,9,11 The observed PL decay constant of the 4F3/2 → 4I11/2 transition of Nd3+ ions, τexp, increases with zAl/zNd. τJO for this transition evaluated by Judd−Ofelt analysis increases as well. As a consequence, the PL quantum yield, defined by τexp/ τJO, shows a weak dependence on zAl/zNd at zAl/zNd ≥ 0.8. The large increase in τexp/τJO observed for Al codoping of zAl/zNd ≤0.8 evidently originates from the suppression of the clustering of Nd3+ ions. However, the τexp/τJO values are not high enough; it may be partly because of the high concentration of SiOH groups (up to ∼1020 cm−3), which commonly accelerate nonradiative transitions in PL centers. Table 2 also shows that β increases with zAl/zNd, indicating a decrease in the site-to-site distribution of the local environment of Nd3+ ions. The larger site-to-site distribution at small zAl/zNd may be explained by the coexistence of AlO4 and AlO6 units (Figure 6). Such mixed coordination increases the variation of the local environment of Nd3+ ions. The ESEEM experiment of this study estimates the average coordination structure around Nd3+ ions due to the following reason: the broadening of the ESE spectra (Figure 3) originates from the strong hyperfine coupling between S = 3/2 and I = 7/2 of Nd3+ ion,29 orientational averaging of g-factor and hyperfine anisotropies, and inhomogeneous g-factor broadening in glass.8 Thus, even if there is a variety of coordination structures around Nd, each piece of structural information is spread and overlapped with the others at a selected magnetic field. In addition, the frequency bandwidth of π/2 pulse of length 16 ns is relatively wide (∼80 MHz). Therefore, partial excitation (measurement at 400 mT) collects nearly averaged structural information. As shown in Figure 6, each Nd3+ ion in the Alcodoped silica glasses is typically coordinated by ∼2 AlOx units irrespective of the Al content (0.8 ≤ zAl/zNd ≤ 2.5), where the formation of 2Nd + 2Al or similar small clusters is supposed to satisfy the glass compositions. The average coordination number of ∼2 and the presence of small RE clusters are consistent with the results of a theoretical study.7 The fraction of AlO4 units decreases with increasing zAl/zNd. This result is consistent with those of extended X-ray absorption fine structure (EXAFS) studies2,6 on RE−Al-codoped silica glasses and NMR spectroscopic studies on Sm−Al-codoped silica glass5 and Al2O3−SiO2 glasses,30 where AlO4 units are dominant at molar ratios of Al to Si below ∼0.01, whereas AlO5 and AlO6 units appear in samples with a higher content of Al. AlO5 units are found only in glasses prepared by the splat quenching method because of the relatively high enthalpy of formation.30 Although the average total number of AlOx units for each Nd3+ ion is nearly constant at ∼2, the solubility of Nd3+ ions increases with Al content, as observed in Figures 1 and 2. These observations can be understood in terms of the increased number of AlO6 units around Nd3+ ions as follows. The AlOx units have negative charges, i.e., [AlO4]− and [AlO6]3−, if the structures consist only of bridging oxygen atoms. These units preferentially coordinate to RE3+ ions to compensate their negative charges. In the Al-free silica glass, Nd3+ ions are clustered together with nonbridging oxygen (NBO) ions in the SiO4 units. With increasing Al content, the clusters of Nd3+ ions are gradually decomposed by Al3+ ion that consume one or three NBO ions for formation of [AlO4]− or

[AlO6]3− units, respectively. The affinity of AlO6 units for Nd3+ ions should be larger than that between Nd3+ ions and AlO4 units because of their larger negative charge. Namely, AlO6 units must be more effective at increasing the solubility of Nd3+ ions than AlO4 units, where the formation of 2Nd + 2Al or similar small clusters is again supposed to satisfy the local charge neutrality. In addition, the larger negative charge of the AlO6 unit generates a steeper electric field gradient and lowers the symmetry of the crystal field at the Nd3+ sites, thereby increasing the transition probabilities of the nominally forbidden 4f−4f lines. This interpretation is consistent with the increase in Ω2 with zAl/zNd. This mechanism explains the observed increase in quantum yield (τexp/τJO) with increasing zAl/zNd. Consequently, this study finds a close relationship among the PL properties of Nd3+ ions, fractions of AlOx polyhedra, and Al content. Finally, we consider the advantages of wet-chemical methods over melt quenching or vapor phase methods to modify the coordination structure around RE ions. In our previous study,8 Er−Al-codoped silica glass containing 0.2 and 3.8 at % Er3+ and Al3+ ions vs Si, respectively, was prepared by chemical vapor deposition. Macroporous silica soot was dipped in methanol solutions of ErCl3 and AlCl3 and consolidated into a transparent glass rod at a relatively high temperature of 1600−1700 °C. In this procedure, larger TΔS improves the uniformity of the glass, eliminating metastable structures including clusters of RE ions. The average total number of AlOx units for each Er3+ ion was estimated to be 0.3, which is much smaller than in this study (∼2), even though the Al content is the same (0.8−2.5 at % vs Si). In this study, the preferential coordination of AlOx units to Nd3+ ions is clearly observed, suggesting the presence of attractive interactions between Nd3+ and Al3+ ions in the Si−O−Si network in silica gels. Sintering at relatively low temperature (≤1200 °C) has smaller TΔS but maintains the Nd−O−Al bonds, which may reduce ΔH, compared with sintering at higher temperature. We confirm that the sol−gel method is suitable to tailor the coordination structure around RE ions in silica glass.

6. CONCLUSIONS Nd−Al-codoped silica glasses with high optical transparency were prepared by a cosolvent-free sol−gel method at relatively small molar ratios of Al to Nd. The local structure around Nd3+ ions was examined by optical and ESEEM spectroscopic analyses. The obtained results are summarized below: (1) Tetrahedral AlO4 and octahedral AlO6 units coexist around Nd3+ ions as ligands. The average total number of AlOx (x = 4, 6) units for each Nd3+ ion is ∼2 irrespective of Al content and is much larger than that calculated for a uniform distribution of the codopants (∼0.08−0.25). (2) With increasing Al content, the number of AlO4 units around Nd3+ ions decreases to zero, whereas that of AlO6 units increases. (3) The transition probability of Nd3+ ions, PL lifetime, and stretched exponent of PL time decay distinctly increase with Al content. This is because the conversion of AlO4 units to AlO6 ones around Nd3+ ions increases the asymmetry of the crystal field at the Nd3+ sites and decreases the site-to-site distribution. These results indicate that the local environment around Nd3+ ions depends on the oxygen coordination number of Al3+ ions, which changes with Al content. These observations 8796



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(14) Judd, B. R. Optical Absorption Intensities of Rare-Earth Ions. Phys. Rev. 1962, 127, 750−761. (15) Ofelt, G. S. Intensities of Crystal Spectra of Rare-Earth Ions. J. Chem. Phys. 1962, 37, 511−520. (16) Dikanov, S. A.; Tsvetkov, Y. D. Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy; CRC Press: Boca Raton, FL, 1992. (17) Deligiannakis, Y.; Louloudi, M.; Hadjiliadis, N. Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy as a Tool To Investigate the Coordination Environment of Metal Centers. Coord. Chem. Rev. 2000, 204, 1−112. (18) Schweiger, A.; Jeschke, G. Principles of Pulse Electron Paramagnetic Resonance; Oxford University Press: New York, 2001. (19) Stoll, S.; Schweiger, A. EasySpin, a Comprehensive Software Package for Spectral Simulation and Analysis in EPR. J. Magn. Reson. 2006, 178, 42−55. (20) Smolin, Y. I.; Shepelev, Y. F. The Crystal Structures of the Rare Earth Pyrosilicates. Acta Crystallogr., Sect. B: Struct. Sci. 1970, 26, 484− 492. (21) Geller, S.; Bala, V. B. Crystallographic Studies of Perovskite-like Compounds. II. Rare Earth Aluminates. Acta Crystallogr. 1956, 9, 1019−1025. (22) Schaller, T.; Stebbins, J. F. The Structural Role of Lanthanum and Yttrium in Aluminosilicate Glasses: A 27Al and 17O MAS NMR Study. J. Phys. Chem. B 1998, 102, 10690−10697. (23) Leonova, E.; Hakeem, A. S.; Jansson, K.; Stevensson, B.; Shen, Z.; Grins, J.; Esmaeilzadeh, S.; Eden, M. Nitrogen-Rich La−Si−Al− O−N Oxynitride Glass Structures Probed by Solid State NMR. J. NonCryst. Solids 2008, 354, 49−60. (24) Kuwatani, S.; Maehana, R.; Kajihara, K.; Kanamura, K. AmineBuffered Phase Separating Tetraethoxysilane−Water Binary Mixture: A Simple Precursor of Sol−Gel Derived Monolithic Silica Gels and Glasses. Chem. Lett. 2010, 39, 712−713. (25) Funabiki, F.; Matsuishi, S.; Hosono, H. Local Structure around Rare-Earth Ions in B2O3 Glass at High Pressure. J. Appl. Phys. 2013, 113, 223508/1-6. (26) Reisfeld, R. Fluorescence and Nonradiative Relaxations of Rare Earths in Amorphous Media and on High Surface Area Supports: A Review. J. Electrochem. Soc. 1984, 131, 1360−1364. (27) Tanabe, S.; Ohyagi, T.; Soga, N.; Hanada, T. Compositional Dependence of Judd−Ofelt Parameters of Er3+ Ions in Alkali-Metal Borate Glasses. Phys. Rev. B 1992, 46, 3305−3310. (28) Tanabe, S.; Hanada, T.; Ohyagi, T.; Soga, N. Correlation between 151Eu Mössbauer Isomer Shift and Judd−Ofelt Ω6 Parameters of Nd3+ Ions in Phosphate and Silicate Laser Glasses. Phys. Rev. B 1993, 48, 10591−10594. (29) Prokhorov, A. D.; Borowiec, M. T.; Dyakonov, V. P.; Kamenev, V. I.; Prokhorov, A. A.; Aleshkevych, P.; Zayarnyuk, T.; Szymczak, H. The Ground State and EPR Spectrum in Monoclinic KY(WO4)2:Nd3+ Single Crystal. Physica B 2008, 403, 3174−3178. (30) Poe, B. T.; McMillan, P. F.; Angell, C. A.; Sato, R. K. A1 and Si Coordination in SiO2−Al2O3 Glasses and Liquids: A Study by NMR and IR Spectroscopy and MD Simulations. Chem. Geol. 1992, 96, 333−349.

demonstrate that the sol−gel method can readily tailor the coordination structure of RE-doped silica glasses because it allows low-temperature sintering.

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AUTHOR INFORMATION

Corresponding Authors

*F.F.: phone, +81-45-924-5127; fax, +81-45-924-5127; e-mail, [email protected]. *K.K.: phone, +81-42-677-2827; fax, +81-42-677-2827; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS ESEEM spectroscopic analysis was supported by a grant from the Advanced Photon Science Alliance Project of the Ministry of Education, Culture, Sports, Science and Technology of Japan. K.K. thanks the Nippon Sheet Glass Foundation for Materials Science and Engineering, and the Japan Society for the Promotion of Science (JSPS) for a Grant-in-Aid for Scientific Research (B) (Grant 24350109).

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REFERENCES

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Characteristic Coordination Structure around Nd Ions in SolâGel

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