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Structural Characterization of Rare-Earth Doped Yttrium Aluminoborate Laser Glasses Using Solid State NMR Heinz Deters,† Andrea S. S. de Camargo,†,‡ Cristiane N. Santos,‡,§ Cynthia R. Ferrari,‡,§ Antonio C. Hernandes,‡ Alain Ibanez,§ Matthias T. Rinke,† and Hellmut Eckert*,† Institut fu¨r Physikalische Chemie, WWU Mu¨nster, Corrensstrasse 30, D48149 Mu¨nster, Germany, Instituto de Fı´sica de Sa˜o Carlos, UniVersidade de Sa˜o Paulo, AV. Trabalhador Sa˜ocarlense 400, Sa˜o Carlos - SP, Brazil, and Institut Ne´el, CNRS and UniVersite´ Joseph Fourier, BP166, F-38042, Grenoble Cedex 9, France ReceiVed: April 9, 2009; ReVised Manuscript ReceiVed: July 10, 2009
The structure of laser glasses in the system (B2O3)0.6{(Al2O3)0.4-x(Y2O3)x} (0.1 e x e 0.25) has been investigated by means of 11B, 27Al, and 89Y solid state NMR as well as Y-3d core-level X-ray photoelectron spectroscopy. 11 B magic-angle spinning (MAS) NMR spectra reveal that the majority of the boron atoms are three-coordinated, and a slight increase of four-coordinated boron content with increasing x can be noticed. 27Al MAS NMR spectra show that the alumina species are present in the coordination states four, five and six. All of them are in intimate contact with both the three- and the four-coordinate boron species and vice versa, as indicated by 11 27 B/ Al rotational echo double resonance (REDOR) data. These results are consistent with the formation of a homogeneous, nonsegregated glass structure. For the first time, 89Y solid state NMR has been used to probe the local environment of Y3+ ions in a glass-forming system. The intrinsic sensitivity problem associated with 89Y NMR has been overcome by combining the benefits of paramagnetic doping with those of signal accumulation via Carr-Purcell spin echo trains. Both the 89Y chemical shifts and the Y-3d core level binding energies are found to be rather sensitive to the yttrium bonding state and reveal that the bonding properties of the yttrium atoms in these glasses are similar to those found in the model compounds YBO3 and YAl3(BO3)4. Based on charge balance considerations as well as 11B NMR line shape analyses, the dominant borate species are concluded to be meta- and pyroborate anions. Introduction During the past decade, rare-earth doped glasses and ceramics have been of great scientific and technological interest, as they present considerable compositional flexibility and ability to accommodate and disperse larger quantities of rare earth ions than single crystals.1-4 In the search for new and efficient hostion compositions that are insensitive to fluorescence quenching effects, the spectroscopic and photophysical properties of both the glassy frameworks as well as of the rare earth dopants in such materials are presently the focus of much attention. It is well-known that the emission properties, as well as lasing action, are critically influenced by ion-lattice and ion-ion interactions such as vibrational relaxation, energy migration and cross relaxation energy transfer processes.5 As these processes are known to be directly related to the local coordination of the rare-earth ions and to their interionic distance distributions, it is important to obtain detailed structural information about these issues. Glasses of the ternary system Y2O3-Al2O3-B2O3 have been introduced as excellent alternatives to single crystalline materials for special laser applications involving self-frequency doubling or self-sum frequency mixing.6 Although the preparation conditions, the extent of the region of glass formation, as well as some basic bulk and spectroscopic properties of these glasses have been published in the literature,7-11 no detailed discussion of the structure of these glasses as a function of composition is * Corresponding author. E-mail:
[email protected]. † WWU Mu¨nster. ‡ Universidade de Sa˜o Paulo. § CNRS and Universite´ Joseph Fourier.
yet available. Owing to its element-selectivity, its local selectivity, and its inherently quantitative character, solid state NMR is an ideally suited method for providing such kind of structural information in glasses.12,13 In the present study, we develop a description of the framework structure of these glasses based on high-resolution 11B and 27Al NMR spectra. While singlepulse magic-angle spinning (MAS) experiments are informative regarding the coordination states of the boron and aluminum atoms, complementary 11B/27Al double resonance NMR studies can give quantitative information regarding the boron-aluminum connectivities in the network.14-24 Finally, the local environment of the fluorescent rare earth ions in these (and other types of) glasses is completely unknown to date. The difficulty (and challenge) lies in the 4fnparamagnetism of these ions, which broadens their NMR signals beyond detectability. A potential solution to this problem is the study of diamagnetic mimics such as scandium, yttrium or lanthanum ions via NMR spectroscopy. In the present contribution we introduce high resolution 89Y solid state NMR as a new tool to investigate rare earth ion coordination and distribution in glassy optical and laser materials. Despite its 100% natural abundance and spin 1/2 character, the 89Y isotope presents serious difficulties to solid state NMR studies owing to its low gyromagnetic ratio, resulting in low detection sensitivity and serious probe ringing effects. Additional difficulties arise from long spin-lattice relaxation times, requiring the use of long recycle delays during signal averaging. Although 89Y NMR has thus far been applied to numerous crystalline model compounds and ceramics,25-39 to the best of our knowledge, no structural studies of glasses have appeared in the literature to date. Here we show that static and MAS NMR spectra with excellent
10.1021/jp9032904 CCC: $40.75 2009 American Chemical Society Published on Web 08/13/2009
Characterization of Laser Glasses signal-to-noise ratios can be obtained by using a combination of paramagnetic doping and direct acquisition of Carr-Purcell spin echo trains.40 On the basis of such measurements, we discuss compositional trends in the local rare earth environments on the basis of chemical shift data and explore the effects of the paramagnetic dopants Nd3+ and Er3+ upon the NMR peak positions, linewidths, and relaxation behavior. Another potentially useful technique to reveal local structural information in glasses is Y-3d core level X-ray photoelectron spectroscopy. While a few papers characterizing yttrium-oxidebased solids have appeared in the literature,41-44 this technique seems to have never been used for the structural characterization of yttrium-containing glasses. In the present study we explore the potential of XPS for providing complementary information in the present yttrium aluminoborate glass system. Experimental Section Sample Preparation and Characterization. Glasses along the composition line (B2O3)0.6{(Al2O3)0.4-x(Y2O3)x} (0.1 e x e 0.25), referred to hereafter as “Yx” (x given in percent) were prepared by the conventional melt-quenching technique, using a resistive furnace open to atmosphere. Reagent grade H3BO3 (Mallinckrodt, 99%), Al2O3 (Alfa Aesar, 99,99%), and Y2O3 (Strem Chemicals, 99,99%) were dry mixed in a mechanical mortar for 1 h to obtain homogeneous mixtures and thermally treated in an uncovered platinum-gold crucible at 500 °C for 30 min, with a heating rate of 10 °C/min, for the decomposition of boric acid. The mixtures were then heated from 1000 to 1400-1520 °C and melted during 30-60 min, depending on composition. A 25 g melt of each composition was then poured into a stainless steel mold preheated at 500 °C. After this, the glasses were annealed at 500 °C for 6 h. The same procedure was employed for the rare-earth (RE) doped glasses using reagent grade Nd2O3 (Merck, 99.9%), Er2O3 (Alfa Aesar, 99.99%), or Yb2O3 (Alfa Aesar, 99.99%); in this case the annealing temperatures and times were 600 °C and 16 h, respectively. Glass transition temperatures were determined by differential thermal analysis (DTA) using a Netzsch STA 409 instrument operated at a heating rate of 10 °C/min (Table 1). Doped crystalline model compounds Nd0.01Y2.99Al5O12 (YAG:Nd), Yb0.01Y0.99AlO3 (YAP:Yb), Eu0.01Y0.99BO3 (YBO3: Eu), and Yb0.01Y0.99Al3(BO3)4 (YAB:Yb) were prepared using the Pecchini method, reacting the nitrate salts of yttrium, aluminum, and the rare-earth species with citric acid in the presence of sorbitol.10 The dried citrate polymerizate was heated to 400 °C for 24 h, followed by annealing at 600 °C (700 °C in the case of YAlO3) in an O2-rich atmosphere to eliminate organic residues. The crystalline phases were obtained in a third calcination step in an oxygen-rich atmosphere. Annealing temperatures/durations were 1300 °C/5 h for doped yttrium aluminate (YAP), 1150 °C/5 min for doped yttrium aluminum borate (YAB), and 1100 °C/3 h for doped yttrium borate (YBO3). A ceramic sample of Nd-doped yttrium aluminum garnet (YAG) was obtained commercially. Solid State NMR. 11B and 27Al solid state NMR spectra were acquired at 160.5 and 130.3 MHz, respectively, on a Bruker Avance DSX 500 FT-NMR spectrometer (11.74 T) equipped with various 4 mm MAS NMR probes operated at a typical spinning frequency of 13-14 kHz. Short excitation pulses (corresponding to solid flip angles below 30°) were used. 27Al triple-quantum (TQ) MAS NMR spectra45 were measured at 11.74 T, using the three-pulse sequence,46 at a spinning frequency of 14 kHz. The lengths of the strong preparation and reconversion pulses were 3.0 and 1.0 µs, respectively, and the
J. Phys. Chem. C, Vol. 113, No. 36, 2009 16217 TABLE 1: Compositions and Glass Transition Temperatures of the Undoped and Compositions of the Paramagnetically Doped Yttrium Aluminoborate Glasses sample (Tg/°C) Y10 (719) Y15 (715) Y20 (712) Y25 (710) Y20Nd0.10 Y20Nd0.20 Y20Nd0.35 Y20Nd0.50 Y20Nd0.75 Y10Er0.50 Y20Er0.10 Y20Er0.25 Y20Er0.35 Y20Er0.50 Y20Er0.75 Y15Yb0.50 Y20Yb0.10 Y20Yb0.20 Y20Yb0.35 Y20Yb0.50 Y20Yb0.75 Y20Yb5.00
RE2O3/mol % Y2O3/mol % Al2O3/mol % B2O3/mol %
0.10 0.20 0.35 0.50 0.75 0.50 0.10 0.25 0.35 0.50 0.75 0.50 0.10 0.20 0.35 0.50 0.75 5.00
10 15 20 25 19.90 19.80 19.65 19.50 19.25 9.50 19.90 19.75 19.65 19.50 19.25 14.50 19.90 19.80 19.65 19.50 19.25 15.00
30 25 20 15 20 20 20 20 20 30 20 20 20 20 20 25 20 20 20 20 20 20
60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60
single quantum signal was detected by a soft pulse of 10 µs length. The evolution time was incremented in steps of 8.9 µs, and 230 data sets were acquired in the t1 dimension, using 540 scans and a relaxation delay of 0.3 s. Isotropic chemical shifts δiso and the “second order quadrupolar effects” (SOQE ) CQ(1 + η2/3)1/2, with CQ and η being the nuclear electric quadrupolar coupling constant and the electric field gradient asymmetry parameter, respectively) were obtained by comparing the centers of gravity of the projections in the isotropic F1 and the anisotropic F2 dimensions. 11B TQ MAS spectra were obtained using a MAS rotation frequency of 25 kHz. TQ excitation and reconversion to zero-quantum coherence was effected at a nutation frequency of 192 kHz (for a liquid sample) with pulses of 2.1 and 0.8 µs length, respectively, and the signal was detected with a soft pulse of 10 µs length. The evolution time was incremented in steps of 10 µs, and 480 data sets acquired in the t1 dimension, using 24 scans at recycle delays of 4-10 s. 11B and 27Al isotropic chemical shifts δiso are externally referenced relative to BF3 · Et2O and a 1 M aqueous Al(NO3)3 solution, respectively. All line shape analyses were done using the DMFIT program.47 11B{27Al} and 27Al{11B} REDOR experiments were conducted with the Bruker Avance DSX 500 spectrometer at a spinning frequency of 14 kHz. In both cases the conventional REDOR sequence of Gullion and Schaefer was used.48,49 In the 11B{27Al} REDOR experiments the 180° pulse lengths for the 11B observe nuclei were 5.8 and 7.5 µs for the three- and four-coordinated boron species, respectively, and the relaxation delay was 20 s for the glasses and 40 s for the crystalline YAl3(BO3)4 (YAB) model compound. The 27Al 180° pulse length was 5.8 µs and the pulses on the 27 Al channel were phase cycled according to the XY-4 scheme.49 27Al{11B} REDOR experiments were done with a 180° pulse length of 5.8 µs for both 27Al and 11B, and the relaxation delay was 1 s for the 27Al observe nuclei. The pulses on the 11B channel in this experiment were also phase cycled according to the XY-4 scheme. 89 Y NMR spectra were measured at 24.5 MHz on a Bruker Avance DSX 500 FT-NMR spectrometer (11.74 T) equipped with a 7 mm probe. Spectra were obtained both under static
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Figure 1. 11B MAS NMR spectra of the yttrium aluminoborate glasses and the crystalline model compounds YBO3 and YAl3(BO3)4.
and magic angle spinning (MAS) conditions, using a spinning frequency of 5 kHz. Full spin echo trains were recorded, which were produced by a Carr-Purcell-Meiboom-Gill sequence (π/ 2)x-τ1-πy-τ1-τ-[τ1-(π)y-τ1-2τ]n with signal acquisition during the time period 2τ.40 The following acquisition parameters were used: 90° pulse length 9.0 µs, 180° pulse length 18.0 µs, time interval between the refocusing pulses 2τ ) 1.20 ms, echo preparation delay τ1 ) 91 µs, number of echoes n constituting the echo train: 40-80, relaxation delay 1-90 s (depending on the paramagnetic dopant level). The echo train was Fourier transformed, yielding the expected peak pattern spaced at a frequency interval corresponding to the inverse delay between subsequent refocusing pulses. Chemical shifts are externally referenced relative to 1 M aqueous YCl3 solution using the signal of an 8 M yttrium nitrate solution containing 0.25 M Fe(NO3)3 as a secondary standard. X-ray Photoelectron Spectroscopy. XP spectra were measured using a KRATOS Axis-Ultra spectrometer in ultrahigh vacuum (p < 10-7 Pa). Monochromatic Al KR radiation (hν ) 1486.6 eV) was used with a 15 kV accelerating voltage and 10 mA filament current. The charge neutralizer was run with a filament current of 1.8 A, a charge balance of 2.3 V and a filament bias of 1.0 V. For narrow scans with high-energy resolution, a pass energy of 20 eV was applied. Immediately before the measurements, the different glasses were powdered and pressed into small copper plates in order to get a smooth surface. The obtained spectra were analyzed by using the software CasaXPS V. 2.2.0. The absolute binding energies of the various peaks from the obtained XPS spectra were determined by referencing to the C 1s peak (arising from ubiquitous pump-oil impurities) whose binding energy is assumed to be 284.6 eV. Results, Data Analysis, and Interpretation Local Boron Environments Studied by 11B MAS NMR. The 11B MAS NMR spectra (intensity in arbitrary units as a function of resonance shift δ) of the crystalline model compounds YBO3, YAl3(BO3)4, and the undoped glasses are presented in Figure 1. For YBO3, the crystallographically unique boron atom is four-coordinated, giving rise to a sharp resonance line near 0.4 ppm, in agreement with the literature.50 In the case of YAl3(BO3)4 two crystallographically distinct orthoborate BO33(B(3)) groups are present in the crystal structure51 in a 3:1 ratio, which differ only with regard to their second coordination
Figure 2. (a) 11B TQ MAS spectrum obtained for crystalline YAl3(BO3)4. (b) 11B MAS NMR spectrum of crystalline YAl3(BO3)4. Red curve illustrates a simulation based on two distinct components in a 3:1 ratio (individually displayed as dotted curves). The small peak near 0 ppm is due to an impurity with four-coordinated boron.
spheres. While each B(3)-I site (population 1) is surrounded by six Al3+ ions, the B(3)-II site (population 3) is surrounded by four Al3+ and two Y3+ ions. Both sites contribute to a secondorder quadrupolar powder pattern suggesting close to axially symmetric electric field gradients as expected for the D3h local symmetry of the orthoborate species. Figure 2a shows that both boron sites can be discriminated by the TQ MAS experiment. The isotropic chemical shifts and nuclear electric quadrupolar coupling parameters obtained by fitting the individual cross sections of the 2D-plot along the F2 dimension are summarized in Table 2. With these parameters, a good fit of the MAS NMR spectrum was obtained on the basis of two signal components in the expected 3:1 ratio (see Figure 2b). The spectra of the glasses reveal the presence of three- and four-coordinated boron units. Simulation of these data with the DMFIT program47 yields the quantitative deconvolution summarized in Table 2. Due to the high local symmetry resulting in weak quadrupolar interactions the resonance attributed to the four-coordinated boron sites has an approximately Gaussian line
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sample Y10 Y15 Y20 Y25 YBO3 YAl3(BO3)4
B MAS NMR Analysis of the Yttrium Aluminoborate Glasses and Crystalline Model Compounds group
δiso ((0.2)/ppm
G/La
LBb((10)/Hz
B(4) B(3) B(4) B(3) B(4) B(3) B(4) B(3) B(4) B(3)-II B(3)-I
0.3 17.2 0.3 17.3 0.4 17.6 0.5 18.0 0.4 18.9 20.1
0.81
440 340 440 340 450 340 460 340 300 90 57
0.80 0.80 0.81 0.32
% ((3)c
CQ((0.10)/MHz
η((0.05)
2.70
0.32
2.70
0.28
2.70
0.30
2.70
0.35
16 84 18 82 20 80 21 79
2.90 3.00
0.07 0.08
73 27
a Fraction of Gaussian (vs Lorentzian) character of the fitting function used. b Line broadening parameter (full width at half-maximum) of the convolution function used. c Obtained by fitting the B(3) resonance to two components (see text).
Figure 3. 11B MAS NMR spectrum of glass Y25 along with two simulation attempts; left: one-component fit for the B(3) units; right: twocomponent fit for the B(3) units.
shape, whereas the three-coordinated boron units give rise to a second-order quadrupolar powder pattern. As illustrated in Figure 3, fitting the broad line shape component with just one set of parameters results in significant deviations between experiment and simulation, whereas an almost perfect fit can be obtained if one assumes a superposition of two second-order quadrupolar powder patterns. In spite of this good agreement between experimental and simulated lineshapes, such a twocomponent fit turns out to be inconsistent with TQ MAS data of these glasses, which are summarized for a representative sample in Figure 4. TQ MAS gives no evidence for distinguishable three-coordinated boron species. Thus, we conclude that the two-component fit is a purely artificial one without structural significance. Nevertheless it is useful for obtaining high-fidelity values N4 for the fractions of four-coordinated boron atoms in these glasses; values obtained from this analysis are listed in Table 2. Figure 1 and Table 2 reveal a slight compositional trend of increasing N4 with increasing Y content. Figure 5a-c shows the 11B MAS NMR spectra of glasses paramagnetically doped with Nd3+, Yb3+, and Er3+ ions. Clearly, the presence of the rare-earth dopants results in substantial loss in resolution of the spectra, suggesting that both the B(3) and the B(4) signals are significantly broadened by paramagnetic interactions of the 11 B nuclei with the rare earth’s electron spins. For a given dopant species, the broadening effects are of comparable magnitude for B(3) and B(4), as quantified by the line broadening parameter (LB) used in the simulation of the spectra. Figure 5, panels d and e, indicates that this parameter depends linearly on rare earth ion concentration, with a slope dependent on the ion type. To a first approximation, the different slopes observed in Figures 5d and e for the different ions can be attributed to differences in the sizes of the respective magnetic moments involved. Local Aluminum Environments Studied by 27Al MAS NMR. The 27Al MAS NMR spectra of the undoped yttrium aluminoborate glasses are shown in Figure 6 and reveal the presence of four-, five-, and 6-fold coordinated aluminum sites
Figure 4.
11
B TQ MAS spectrum obtained for glass Y25.
with resonance shifts near 50-60, 30, and 0 ppm, respectively, in agreement with literature data on other types of glasses.16-20 Owing to asymmetrically broadened resonances which are caused by strong second-order quadrupolar effects with wide distributions of electric field gradients, the spectral overlap of these individual resonances is considerable. This fact makes a reliable spectral deconvolution and, hence, a quantification of the three types of aluminum sites impossible. In contrast, the 27 Al TQ MAS NMR spectra, viewed along the isotropic F1 dimension show considerably improved resolution (see Figure 7). Table 3 summarizes the isotropic chemical shift and secondorder quadrupolar effect (SOQE) values obtained by comparing
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Figure 5. Top: 11B MAS NMR spectra of Y20 glasses doped with various levels of (a) Nd2O3, (b) Er2O3, and (c) Yb2O3. Bottom: Line broadening fit parameter LB as a function of doping level for (d) the B(3) units and (e) the B(4) units.
Figure 6. 27Al MAS NMR spectra of yttrium aluminoborate glasses and the model compound YAl3(BO3)4.
the centers of gravity of the signals along both dimensions of the 2D plot. Further included are approximate area fractions, which were obtained by deconvolutions of the isotropic lineshapes along the F1 dimension. It should be borne in mind that this approach is not rigorously quantitative, as the TQ-excitation and reconversion efficiencies depend on the magnitude of the nuclear electric quadrupolar coupling constant CQ in a complicated manner. In spite of this caveat, we suggest that a simple peak analysis of the data along the F1 dimension might yield reasonably accurate results, as the TQ MAS results suggest that the CQ values of the three different aluminum coordination environments are of comparable magnitudes. Even if the absolute
Figure 7. glass Y10.
27
Al TQ MAS NMR spectrum of yttrium aluminoborate
aluminum speciation determined in this manner might be subject to some systematic error, we believe that the procedure can reveal trends as a function of composition, because the CQ values associated with the three types of Al environments remain more or less constant within this glass series. Table 3 suggests that the fraction of four-coordinate aluminum tends to increase as the amount of alumina is (formally) replaced by yttria. Furthermore, an increase of yttrium oxide content also results in an increase of the isotropic chemical shift observed for the tetrahedral aluminum sites whereas the chemical shifts of Al(5) and Al(6) units remain approximately constant. 27Al NMR
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TABLE 3: 27Al NMR Parameters Extracted from TQ MAS NMR Spectra of Undoped Yttrium Aluminoborate Glasses and Crystalline Model Compounds sample Y10 Y15 Y20 Y25 YAl3(BO3)4
group
δiso((1)/ppm
amount ((4%)/%
SOQE ((0.2)/MHz
Al(4) Al(5) Al(6) Al(4) Al(5) Al(6) Al(4) Al(5) Al(6) Al(4) Al(5) Al(6) Al(6)
62.2 34.2 6.0 64.7 35.6 6.4 66.1 36.1 6.4 66.4 36.2 6.2 -1.2
27 37 36 33 34 33 34 32 34 35 30 35 100
4.9 4.8 4.2 5.0 4.7 4.3 4.7 4.4 4.0 4.8 4.4 4.0 1.8
spectra of the rare-earth doped glasses were found to be essentially identical to those of the undoped materials. However, the absence of peak broadening effects not necessarily implies a vanishing paramagnetic interaction; the data might simply indicate that the 27Al resonances in these glasses are dominated by other interactions, such as second-order quadrupolar broadening effects. Framework Connectivity Studied by 11B{27Al} and 27 Al{11B} REDOR. Figure 8 summarizes the 11B{27Al} REDOR results obtained on the glasses under study, as well as on the crystalline model compound YAl3(BO3)4 (YAB). Because of the quadrupolar nature of both nuclides and the multispin-
Figure 8. 11B{27Al} REDOR curves of yttrium aluminoborate glasses and the model compound YAl3(BO3)4. Top: three-coordinated boron units; bottom: four-coordinated boron units.
Figure 9. Dependence of M2(11B{27Al}) on Al/B ratio in yttrium aluminoborate glasses and the model compound YAl3(BO3)4.
TABLE 4: M2 Values ((10%) Obtained from 27Al{11B} and 11 B{27Al} REDOR Experiments on the Yttrium Aluminum Borate Glasses and Crystalline YAl3(BO3)4 M2/106 rad2 s-2 sample YAl3(BO3)4 Y10 Y15 Y20 Y25 a
Al(4) 25.8 27.1 25.5 32.6
Al(5) 31.4 32.4 31.5 33.6
M2/106 rad2 s-2
Al(6) 39.5 32.7 34.1 33.1 44.5
B(3) 69.2 48.3 37.3 28.4 20.9
B(4)
a
51.0 38.2 29.3 22.8
Weighed average for the two crystalline boron sites.
character of the dipolar interactions, no comparisons with theoretical calculations are possible in this case. Following our previous approach, the data were approximated as parabolae in the limit of short dipolar evolution times,52,53 and van Vleck second moments54 were extracted by comparison of their curvatures, measured within the data range 0 e ∆S/S0 e 0.2, with that measured for YAB, for which the second moment (average over the two boron sites) can be computed from the crystal structure.51 Figure 9 reveals that the M2 values vary linearly with the Al/B ratio of the glasses and are essentially identical within experimental error for the three- and the fourcoordinate species. The latter result is in sharp contrast with the situation in sodium aluminoborate glasses, where the B(4) units show substantially weaker dipolar couplings with 27Al than the trigonal boron species.17 Furthermore, the REDOR attenuation was found to be complete at longer dipolar evolution times (∆S/S0 ∼ 0 for NTr ∼ 30 ms, see Figure SI, Supporting Information), indicating that the glasses are not segregated into separate YAl3(BO3)4 and B2O3 domains. All these experiments are consistent with a random distribution of B-O-Al linkages in the glass structure and the absence of phase separation. Finally, Table 4 summarizes the M2 values extracted from siteresolved 27Al{11B} REDOR measurements (see Figure SII, Supporting Information), which were analyzed in the same manner as the 11B{27Al} REDOR data. The results indicate that generally all three types of Al species (Al(4), Al(5), and Al(6) units) are intimately interacting with borate units, and there is little dependence on glass composition. While for the Al(5) and Al(6) species the 27Al-11B dipolar coupling strengths are comparable in magnitude to that in YAB, the four-coordinate aluminum species appear to show distinctly weaker dipolar couplings with 11B. This observation may simply reflect the fact that only four Al-O-B linkages are possible for fourcoordinated aluminum and, further, that these anionic units experience some repulsion by the anionic borate species in the glass structure. Nevertheless, the fact that each aluminum species
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Figure 10. 89Y MAS NMR spectra of representative yttrium aluminoborate glasses and crystalline model compounds.
TABLE 5: 89Y Isotropic Chemical Shifts ((0.5 ppm unless Specified Otherwise) and MAS NMR Linewidths ((5 Hz unless Stated) of Yttrium Aluminoborate Glasses and Crystalline Model Compounds sample
δiso/ppm
LB/Hz
Nd0.01Y2.99Al5O12 Yb0.01Y0.99AlO3 Yb0.01Y0.99Al3(BO3)4 Eu0.01Y0.99BO3 Er0.004Y1.996O3
222.0 216.0 134.4 116.0 1. 272 2. 314 -66.0 100 ( 1 102 ( 1
213 460 457 218 1. 425 2. 469 114 3200 ( 100 5100 ( 100
Er0.01Y0.99VO4 Y20Nd0.75 Y20Yb5.00
shows a sizable dipolar interaction with each boron species is consistent with a homogeneous glass structure. Local Yttrium Environments Studied by 89Y NMR and Y-3d X-ray Photoelectron Spectroscopies. Figure 10 shows 89 Y MAS NMR spectra of some doped crystalline model compounds and representative glasses. Our results on the crystalline materials are in excellent agreement with literature data, regarding previous measurements on Y2O3,27,28 YAG,27,28 YVO4,36 YAlO3,27,28 YBO3,37 and YAB.38 The 89Y chemical shifts of the crystalline materials (Table 5) span the range from -66 ppm in doped YVO4 to +314 ppm in doped Y2O3. The latter compound reveals two distinct resonances, consistent with the existence of two crystallographically inequivalent sites. In all of the other compounds, the MAS NMR spectra are consistent with the presence of single yttrium sites. Figure 10 also includes results on some representative glassy materials; these are, to the best of our knowledge, the first 89Y spectra ever recorded in the glassy state. Linewidths on the order of 100 ppm (2.5 kHz) are observed for these samples, caused by wide distributions of isotropic chemical shifts. Because of this broadening, significant intensity losses are observed during the deadtime (typically 200 µs) in single-pulse experiments. Thus, 89 Y MAS NMR spectra of glasses are best recorded as rotorsynchronized spin echo experiments, and a further substantial increase in the signal-to-noise ratio is possible by acquiring full CPMG-type spin echo trains. As previously discussed, Fourier transformation of these echo trains results in “spikelet spectra”, the intensity profiles of which mark the envelope of the inhomogeneously broadened spectra.40 (See Figure 11 for an example). Our results indicate that paramagnetic doping up to levels of 5 mol % Yb2O3 does not lead to a significant reduction in T2 values, allowing the observation of CPMG spin echo trains
Figure 11. Fourier transforms of the CPMG trains of two representative glass samples under static and MAS (rotor synchronized Hahn spin echo) conditions. The arrow indicates the frequency of irradiation, where the spikelet is missing for experimental reasons.
over the time interval of several seconds; at the same time, spin-lattice relaxation times decrease from values near 100 s (in undoped samples) to below 1 s in the doped materials. Thus, as indicated by Figure 12, spectra with high signal-to-noise ratios are accessible, both under static and MAS conditions, within a few hours of signal averaging. An undesirable side-effect of too strong doping, observed particularly with Er3+ and Yb3+ at levels of or above 1 mol % is an overall broadening of the static powder patterns, resulting from the anisotropy of the paramagnetic interactions of the 89Y spins with the fluctuating magnetic moments associated with nearby rare-earth dopants. These “pseudo-contact interactions”,55 which lead to a line shape broadening that is linear (in Hz) as a function of magnetic field strength, increase with increasing doping level and increasing size of the rare-earth magnetic moment, as found in the present study. In MAS NMR spectra they produce wide spinning sideband patterns forming the envelope of the overall static line shape, leading to a gradual loss in resolution.56 Thus, there exists an optimum paramagnetic doping level, where the 89Y lineshapes are not yet significantly affected by pseudocontact broadening effects, while the spin-lattice relaxation rates are already significantly enhanced. Our results suggest that doping levels of ∼1 mol % Nd2O3 create such an optimum condition for observing 89Y MAS NMR spectra of these glasses. Table 6 summarizes the analysis of these spectra, referred to as f(δ) in the following, in terms of a moments approach: The first moment M1 ) Σδ f(δ)/(Σ f(δ)) specifies the signal center of gravity 〈δ0〉, while the second moment M2 ) Σ(δ - δ0)2 f(δ)/
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Figure 13. Y-3d core level photoelectron spectra of yttrium aluminoborate glasses and crystalline model compounds.
TABLE 7: Y-3d Core-Level Binding Energies Measured via X-ray Photoelectron Spectroscopy for Yttrium Aluminoborate Glasses and Crystalline Model Compounds BE/eV
Figure 12. 89Y CPMG Fourier transforms under MAS conditions obtained for representative yttrium aluminoborate glasses. The arrow indicates the frequency of irradiation.
TABLE 6: Average Isotropic 89Y Chemical Shifts 〈δ〉 ((5 ppm) and Line Broadening Parameter M2 ((10 ppm) Extracted via the Moments Analysis of the Spikelet Spectra of Doped and Undoped Yttrium Aluminoborate Glasses under MAS Conditions sample
〈δ〉/ppm
M2/ppm
Y10Er0.50 Y15Yb0.50 Y20Nd0.20 Y20Nd0.50 Y20Nd0.75 Y20Er0.75 Y20Yb5.00 Y25
69a 95 106 106 107 94a 124a 116
175 54 52 51 52 203 237 48
a Data with lower precision because of wider spread of spikelets, see text.
(Σ f(δ)) is a quantitative measure of the overall line broadening effect. Whereas paramagnetic doping does not seem to affect the signal center of gravity within experimental error, Er3+ and Yb3+ doping produces a large increase of the M2 values, thereby lowering the precision with which the 89Y signal center of gravity 〈δ0〉 can be determined. Despite the imprecisions involved, Table 6 indicates that 〈δ0〉 changes monotonically from around 70 ppm up to 120 ppm with increasing yttria content within the present glass series. These shifts are in very good agreement with those measured in doped yttrium borate and yttrium aluminoborate glasses and differ significantly from those measured in yttrium oxide or yttrium aluminates. Figure 13 shows Y-3d X-ray photoelectron spectra of the crystalline model substances and the glasses. The splitting observed in each spectrum arises from spin-orbit coupling (3d5/2
sample
Y 3d5/2
Y 3d3/2
Y3Al5O12 YAlO3 YBO3 YAl3(BO3)4 Y25 Y20 Y15 Y10
156.87 157.32 158.02 158.77 157.95 158.02 158.20 158.35
158.89 159.33 160.00 160.81 159.98 160.07 160.25 160.39
and 3d3/2 states). The binding energies of the crystalline yttrium aluminates (yttrium aluminum perowskite, YAlO3, YAP, and yttrium aluminum garnet, YAG) are shifted to significantly lower values compared to YBO3 and YAB (Table 7). The binding energies observed for the glassy materials are found close to those measured for YBO3 and YAB, thus confirming the conclusion from NMR spectroscopy that the bonding state of yttrium is dominated by borate, rather than aluminate ligation. This feature evidences that the yttrium cations act as glass modifiers. Furthermore, a monotonic shift toward lower binding energies can be observed as a function of yttria content, suggesting that the character of the yttrium binding becomes increasingly “YBO3-like” with increasing yttria content. The above conclusions are further supported by the fact that annealing these samples well above their crystallization temperatures (between 1050 and 1150 °C) results in YBO3 and YAB as the sole phases formed (data not shown). Discussion and Conclusions The results of the present study reveal that glasses in the system (B2O3)0.6{(Al2O3)0.4-x(Y2O3)x} (0.1 e x e 0.25) possess a homogeneous structure with strong boron/aluminum mixing in the network. The majority of the boron atoms are threecoordinate, while aluminum is present with coordination numbers four, five, and six. This result is in contrast to the situation observed in sodium aluminoborate glasses (NaBAl),16 where the dominant fraction of aluminum is four-coordinate and hence adopts a network former role. In the present system, the appearance of large amounts of boron-connected Al(5) and Al(6) units indicates that aluminum adopts in a large part a network modifier role, as do the yttrium ions. This result agrees with previous predictions57 and experimental findings18 on the influence of the ion potential on Al coordination in glass systems. Issues to be discussed in the following sections include (1) the local boron environments in relation to the overall charge compensation, (2) the compositional trends observed in the
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boron and aluminum speciations, and (3) the bonding state of the yttrium ions, in relation to the glass composition. Local Boron Environment and Global Charge Compensation. The network modifier role of Y3+ and of large amounts of Al3+ found in the present study immediately raises the question of how the positive charges of these ions are compensated by the framework. While it is difficult to answer this question on a local level, owing to the multitude of local oxygen configurations present in these glasses, some global considerations are useful. Based on the 89Y NMR as well as the Y-3d core level XPS data of the present study we know that this anionic charge compensation proceeds primarily via the borate species and only to a minor extent through the (fourcoordinated) aluminate ions in the network. Therefore we can rule out the presence of electrically neutral planar BO3/2 units in these glasses, which would result in far too few anions in the network for charge compensation. We can also rule out triply charged orthoborate (BO33-) units because these would result in far too many negative charges in the framework. Also, Figure 1 already reveals that the spectral parameters characterizing the boron states in the glasses are very different from those characterizing the orthoborate groups of YAB. Based on these considerations charge compensation in the present glasses is likely to proceed mostly by the formation of meta- or pyroborate units, which feature three-coordinated boron atoms having one and two negatively charged nonbridging oxygen atoms, respectively. As a matter of fact, if all the aluminum species (like the yttrium species) acted as network modifiers, the composition 40 M2O3 - 60 B2O3 (M ) Y, Al) implies an average charge of -2 per boron atom, corresponding to the stoichiometry of aluminum or yttrium pyroborate. In reality, the average charge per boron atom is somewhat lower, as the 27Al NMR results indicate that a substantial fraction (about 1/3) of the aluminum atoms do not require charge compensation but rather act as anionic charge compensators themselves. We can exemplify some semiquantitative considerations for the glass with x ) 0.2, having the composition 20 Al2O3 - 20 Y2O3 - 60 B2O3. As the 27Al NMR results indicate that 2/3 of the aluminum atoms act in a network modifier role, the total cationic charge to be compensated is (26.7 + 40) × 3 ) 200. 1/3 of the Al atoms form singly charged AlO4/2- units, resulting in 13.3 anionic charges. Thus, the balance amounts to 200 - 13.3 ) 186.7 negative charges to be distributed over the 120 boron atoms present in the glass. Of the latter, 20% are four-coordinated BO4/2- groups, leaving 186.7 - 0.2 × 120 ) 162.7 negative charges to be distributed over 96 three-coordinated boron atoms. Obviously this can be realized by forming a mixture of pyroand metaborate units, with the former being in the majority. Further evidence for such boron species comes from the comparatively large asymmetry parameters characterizing the 11 B resonances in Figure 1. These asymmetry parameters suggest that the electric field gradient deviates substantially from axial symmetry, as expected (and observed experimentally) if the local boron environment is made up by a combination of bridging and nonbridging oxygen atoms. Typical asymmetry parameters found for metaborates in the literature range from 0.60 in LiBO258 to 0.80 in NaBO2;59 in the case of pyroborates, values near 0.45 to 0.5 have been published for sodium and magnesium salts.58,59 In the present glasses, the η values appear somewhat lower, possibly reflecting the fact that the classical distinction between bridging and nonbridging oxygens starts to break down in systems with highly charged network modifier species. If a mixture of meta- and pyroborate environments is indeed formed, Figure 4 illustrates that these units cannot be discriminated by
Deters et al. distinct 11B chemical shifts. As a matter of fact, the literature data on crystalline meta- and pyroborates indicate that the chemical shift difference between these groups is rather small,58-60 and in glasses this difference is likely to be obscured by the continuous chemical shift and quadrupolar coupling parameter distribution that is typically present. As discussed by Brow et al. Raman spectroscopy can be useful for the detection of meta and pyroborate units in ternary borate glasses.61 Raman spectra recorded on our samples (data not shown) reveal poorly resolved bands with maxima near 750 and 850 cm-1, which can be attributed to the B-O-B stretching vibrations of the meta- and pyroborate units, respectively. In addition, a continuous broad band in the 1200-1600 cm-1 range can be attributed to B-O- stretching vibrations involving the terminal oxygen atoms of either groups. Altogether these Raman results are in support of the presence of both types of borate units in our glasses. Compositional Trends in the Boron and Aluminum Speciations. Another, more subtle structural issue to be discussed are the systematic changes in the boron and aluminum speciations indicated in Tables 2 and 3. With increasing x-value the fractions of both four-coordinated species B(4) and Al(4) increase. A trend of this kind has been previously noted in scandium-substituted sodium aluminophosphate glasses and reflects the fact that the rare-earth ion is structurally not equivalent to the alumina component.62 We speculate that these changes in speciation are related to the larger number of oxygen atoms required to ligate the Y3+ network modifier (coordination number six to eight) in relation to the Al3+ modifier (coordination number five or six). As a result of this increased demand, a larger fraction of the aluminum atoms may be pushed into the role as network formers, required to provide anionic charge compensation. In line with this interpretation, the 27Al chemical shift trend noted in Table 3 for the Al(4) species may reflect the changing ratio of Al3+ to Y3+ network modifier ions with which these anionic units are locally in contact. Bonding State of the Yttrium Ions. Regarding the bonding state of the yttrium ions, the present contribution reports the first 89Y NMR data ever obtained on a glassy system. This breakthrough is made possible by combining the advantages of paramagnetic doping (leading to accelerated spin-lattice relaxation) and the acquisition of full CPMG spin echo trains for signal amplification. Both the 89Y chemical shifts and the Y-3dcore level binding energies turn out to be parameters that respond rather sensitively to the yttrium bonding state. Comparison of the glass data with those of a number of crystalline reference compounds indicates that the yttrium bonding state in these glasses is similar to that in YBO3 and YAl3(BO3)4 and rather different from that in yttrium aluminate compounds. In YAl3(BO3)4 the six O(3) atoms coordinated to yttrium are all bonded to one three-coordinated boron and one six-coordinated aluminum atom each. However, because of the shorter B-O distance compared to the Al-O bond distance, and also because of the lower boron coordination number, the boron atoms exert a stronger influence on the local charges of these O atoms than the aluminum atoms do. This fact may explain why both the Y-3d binding energy and the 89Y chemical shift measured for this model compound is relatively close to that in YBO3, where the second coordination sphere of yttrium is entirely defined by boron atoms. The corresponding data obtained for the glasses are found close to those of YAl3(BO3)4 and YBO3, and the evident compositional trend toward YBO3 with increasing B/Al ratio indicates that in the present glasses the second coordination sphere of yttrium is affected by the composition. Thus, a
Characterization of Laser Glasses segregation of YAl3(BO3)4 type environments and a B2O3-like matrix is not an applicable structural model. Even though a clear distinction between bridging and nonbridging oxygen atoms in the present system is not as well-defined as in alkali borate glasses, global charge balance considerations indicate that the meta- and pyroborate units provide the primary anionic compensation. This conclusion can be tested locally, in a more quantitative fashion, by a detailed analysis of 89Y-11B and 89 Y-27Al distance correlations obtained by spin echo double resonance (SEDOR) experiments. Such experiments will be the subject of future work in our laboratories. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft and the Brazilian Agencies FAPESP (No. 04/00093-0), CAPES (No. 455/04-1) and CNPq. ASSC gratefully acknowledges a personal fellowship by the Alexandervon-Humboldt Foundation, and H.D. thanks for support by the NRW Forschungsschule “Molecules and Materials - A Common Design Principle”. A.I. thanks for support by the CAPESCOFECUB project. Supporting Information Available: Additional figures showing 11B{27Al} REDOR curves (Figure SI) and 27Al{11B} REDOR curves (Figure SII). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Jaque, D.; Lagomacini, J. C.; Jacinto, C.; Catunda, T. Appl. Phys. Lett. 2006, 89, 121101. (2) Jiang, S.; Myers, M.; Peyghambarian, N. J. Non-Cryst. Solids 1998, 239, 143. (3) de Camargo, A. S. S.; Jacinto, C.; Catunda, T.; Nunes, L. A. O. Appl. Phys. B: Laser Opt. 2006, 83, 565. (4) Ho¨nninger, C.; Paschotta, R.; Graf, M. Appl. Phys. Lett. B: Laser Opt. 1999, 69, 3. (5) de Camargo, A. S. S.; Terra, I. A. A.; Nunes, L. A. O.; Li, M. S. J. Phys. Condens. Matter. 2008, 20, 255240. (6) Santos, C. N.; Mohr, D.; Silva W. F.; de Camargo, A. S. S.; Eckert, H.; Li, M. S.; Vermelho, M. V. D.; Hernandes, A. C.; Ibanez, A.; Jacinto, C. J. Appl. Phys. 2009, 106, 023512(1-6). (7) Chakraborty, L. N.; Rutz, H. L.; Day, D. E. J. Non-Cryst. Solids 1986, 84, 86. (8) Rutz, H. L.; Day, D. E.; Spencer, C. F. J. Am. Ceram. Soc. 1990, 73, 1788. (9) Rocherulle, R. J. Mater. Sci. Lett. 2003, 22, 1127. (10) Hemono, N.; Rocherulle, R. J.; Le Floch, M.; Bureau, B. J. Mater. Sci. 2006, 41, 445. (11) Brow, R.; Tallant, D. R.; Turner, G. L. J. Am. Ceram. Soc. 1997, 80, 1239. (12) Eckert, H. Prog. NMR Spectrosc. 1992, 24, 159. (13) Bureau, B. Photonic Glasses 2006, 21. (14) Lee, S. K.; Deschamps, M.; Hiet, J.; Massiot, D.; Park, S. Y. J. Phys. Chem. 2009, 113, 5162. (15) Hoyer, L. P.; Helsch, G.; Frischat, G. H.; Zhang, L.; Eckert, H. Glass Sci. Technol. 2005, 78, 165. (16) Zu¨chner, L.; Chan, J. C. C.; Mu¨ller-Warmuth, W.; Eckert, H. J. Phys. Chem. B 1998, 102, 4495. (17) Bertmer, M.; Zu¨chner, L.; Chan, J. C. C.; Eckert, H. J. Phys. Chem. B 2000, 104, 6541. (18) Chan, J. C. C.; Bertmer, M.; Eckert, H. J. Am. Chem. Soc. 1999, 121, 5238. (19) Du, L. S.; Stebbins, J. F. Solid State Nucl. Magn. Reson. 2005, 27, 37. (20) Pantano, C. G.; Prabakar, S.; Mueller, K. T. Phys. Chem. Glasses 2003, 44, 125. (21) Hansen, M. R.; Jakobsen, H. J.; Skibsted, J. J. Phys. Chem. C 2009, 113, 2475.
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