Direct ¹⁷O NMR experimental evidence for Al–NBO bonds in Si-rich and highly polymerized aluminosilicate glasses

Abstract

By using solid-state ¹⁷O NMR spectroscopy, we provide the first direct experimental evidence for bonds between Al and non-bridging oxygen (NBO) ions in aluminosilicate glasses based on rare-earth (RE) elements, where RE = {Lu, Sc, Y}. The presence of ∼10% Al–NBO moieties out of all NBO species holds regardless of the precise glass composition, at odds with the conventional structural view that Al–NBO bonds are absent in highly polymerized and Si-rich aluminosilicate glass networks.

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Owing to their importance for both materials and earth sciences, vast efforts have been spent to improve the structural understanding of aluminosilicate (AS) glasses.¹ Ternary M₍₂₎O–Al₂O₃–SiO₂ glasses normally involve a monovalent alkali (M⁺) or divalent alkaline-earth (M²⁺) metal cation. The glass networks comprise SiO₄ and AlO₄ tetrahedra that are cornershared by bridging oxygen (BO) atoms. The additional negative charge of each AlO⁻₄ moiety (relative to SiO₄) requires nearby cations for attaining local charge-balance, while the remaining M^z+ cations depolymerize the glass network by converting BO (O^[2]) atoms into non-bridging oxygen (NBO; O^[1]) species.¹ The relative BO/NBO speciations in melts and glasses dictate many properties, such as viscosity, conductivity, and thermal expansion.^1,2

The following three features of the structural understanding of AS glasses have prevailed for decades, all building around the consequences of the excess negative charge of the AlO₄ groups:^1,2 (i) Both Si and Al are four-fold coordinated (Si^[4] and Al^[4]) by O, except if the network-modifier content is insufficient for balancing the entire Al speciation as Al^[4]; then, higher-coordination AlO₅ and AlO₆ polyhedra form whenever zn_M < n_Al or n_Si < n_Al,^1,2 where n_E denotes the stoichiometric amount of element E in the glass and z is the charge of M^z+. (ii) To avoid local negative charge-accumulation in the structure, there is a strong preference for Si^[4]–O–Al^[4] linkages, whereas those of Al^[4]–O–Al^[4] are absent (the “Loewenstein rule”³). (iii) Moreover, there is a dominance of Si–NBO contacts relative to Al–NBO, implying that all NBO species are accommodated by SiO₄ in silica-rich AS glasses.^1,4

However, over the past decade, violations of properties (i)–(ii) are well documented for AS glasses based on mono/di-valent cations, where several studies reveal minor fractional populations of AlO₅ groups (a few %) in fully charge-balanced (n_M = n_Al/z = n_Si/z) “tectosilicate” AS glasses^4c,5 (notwithstanding that glasses formed at high pressure reveal significant AlO₅/AlO₆ populations⁶). Moreover, while Loewenstein's rule holds strictly for crystalline AS phases featuring n_Al ≤ n_Si (such as zeolites and minerals^1a), minor deviations thereof are reported for M₍₂₎O–Al₂O₃–SiO₂ glasses.⁷ Yet, whereas the early literature identified the preference of Si–NBO over Al–NBO associations, the existence of the latter were often deduced from circumstantial evidence.² Nevertheless, property (iii) is nowadays assumed to apply universally for any SiO₂-dominated M₍₂₎O–Al₂O₃–SiO₂ glass. Direct experimental evidence for Al–NBO contacts only exist for amorphous M–Al–O aluminate phases, or AS glasses that are simultaneously rich in network-modifiers and Al₂O₃, while SiO₂ is a minor component (<40 mol%).⁴

However, the presence of trivalent rare-earth (RE³⁺) cations in RE₂O₃–Al₂O₃–SiO₂ glasses⁸ leads to markedly higher configurational and chemical disorder, as mirrored in the following structural features: (1) significant AlO₅/AlO₆ populations prevail throughout the entire range of RE AS compositions, i.e., not only for those featuring n_Al > n_Si and/or insufficient modifier contents.⁹ The relative amounts of higher-coordination polyhedra were demonstrated to grow for decreasing SiO₂ content,^9d–f and particularly when the RE³⁺ cation field-strength, CFS = z/R², is increased,^9b–e,g where R is the ionic radius. The markedly more cross-linked AS glass network stemming from the higher-coordination Al^[p] species was recently employed for explaining the progressively enhanced Vickers hardness observed for RE–Al–Si–O glasses with growing CFS according to La³⁺ < Y³⁺ < Lu³⁺ < Sc³⁺.^9e,10 (2) The high positive charge of the RE³⁺ cations implies clear violations of the Loewenstein rule, reflected in a pronounced Al/Si atomic disorder for RE AS glass networks, as demonstrated by ²⁹Si and ²⁷Al NMR,^9f,11 as well as by molecular dynamics (MD) simulations.^9f,h Noteworthy, the properties (1) and (2) apply generally to all AS glasses examined thus far from the RE = {La, Y, Lu, Sc} systems, regardless of the precise RE/Al/Si composition.^9–11

Regarding the potential presence of Al^[p]–NBO contacts, i.e., violation of property (iii) of the prevailing structural picture of (Si-rich) AS glasses, we have recently presented circumstantial experimental evidence by ²⁹Si NMR of a significant BO/NBO intermixing among SiO₄/AlO₄ groups in La₂O₃–Al₂O₃–SiO₂ structures,¹¹ whereas MD-simulations of Y and Lu bearing glasses reveal that significant fractions (20–50%) of all NBO species are accommodated by AlO_p groups.^9e Here we provide the first direct experimental proof of significant Al–NBO contacts in SiO₂-rich RE₂O₃–Al₂O₃–SiO₂ glasses with RE = {Y, Lu, Sc}, by utilizing magic-angle spinning (MAS) ¹⁷O NMR. Each specimen was prepared with ≈20% ¹⁷O-enrichment and is denoted RE^a_b(r), where a and b represent the nominal Al₂O₃ and SiO₂ contents in mol%, respectively, and r = n_O/(n_Si + n_Al) conveys the glass network polymerization degree.^1c All glasses feature 42–65 mol% SiO₂ and n(RE₂O₃) < n(Al₂O₃); see Table 1. The ESI† describes all sample preparation and basic characterization procedures, as well as the NMR experimentation discussed below.

Table 1 RE₂O₃–Al₂O₃–SiO₂ glass compositions and oxygen speciations

Glass compositions^a					Oxygen populations^b				NBO populations^c
Glass	aRE2O3 (mol%)	bAl2O3 (mol%)	cSiO2 (mol%)	n Al/nSi	x ^[0]	x ^[1]	x ^[2]	x ^[3]	x ^[1]	x ^[1]Si	x ^[1]Al
a Nominal aRE2O3–bAl2O3–cSiO2 glass composition with a + b + c = 100 mol%. b MD-derived fractional populations of oxygen coordinations {x^[p]} with p = {0, 1, 2, 3}, where only bonds to Si and Al are counted to define the coordination number p. c Fractional populations of NBO species (x^[1]) obtained by ^17O MAS NMR, shown together with the contributions from Si–NBO (x^[1]Si) and Al–NBO (x^[1]Al) species, where x^[1] = x^[1]Si + x^[1]Al. Values within parentheses are the corresponding MD-derived data. The uncertainties are ±0.015 and ±0.010 for the populations derived from NMR and MD simulations, respectively.
Y^2855(2.21)	17.05	27.70	55.25	1.00	0.010	0.226	0.653	0.109	0.221 (0.226)	0.206 (0.154)	0.015 (0.072)
Y^3742(2.21)	21.16	37.34	41.50	1.80	0.014	0.232	0.578	0.172	0.210 (0.232)	0.186 (0.119)	0.024 (0.113)
Lu^2165(2.21)	14.50	20.97	64.53	0.65	0.009	0.223	0.684	0.084	0.241 (0.223)	0.224 (0.166)	0.016 (0.056)
Lu^2855(2.21)	17.05	27.70	55.25	1.00	0.013	0.230	0.641	0.115	0.261 (0.230)	0.236 (0.154)	0.025 (0.076)
Lu^2551(2.45)	23.75	25.43	50.82	1.00	0.030	0.329	0.560	0.079	0.369 (0.329)	0.329 (0.212)	0.040 (0.117)
Sc^3649(2.07)	14.77	36.21	49.02	1.48	0.008	0.160	0.610	0.216	0.162 (0.160)	0.144 (0.091)	0.018 (0.069)
Sc^2955(2.21)	16.96	28.48	54.56	1.04	0.017	0.233	0.628	0.121	0.183 (0.233)	0.170 (0.151)	0.014 (0.082)

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Fig. 1 displays ¹⁷O MAS NMR spectra recorded from various RE–Al–Si–O glasses with RE = {Y, Lu, Sc} and variable cation compositions, as well as average network polymerization degrees. All NMR spectra manifest two main groups of resonances: one from ¹⁷O^[2] species—whose peak-maximum ranges between 31–72 ppm and depending primarily on the n_Al/n_Si molar ratio—and one from NBO ions located at the SiO₄ groups; Si–¹⁷O^[1] (∼137–158 ppm). While the ¹⁷O^[2] NMR signal dominates, that from Si–¹⁷O^[1] grows concurrently with r, i.e., when the glass-network polymerization decreases. Moreover, a weak but significant ¹⁷O resonance appears in the high-ppm region (∼175–250 ppm) of all NMR spectra in Fig. 1: it is assigned to Al–¹⁷O^[1] motifs. Incidentally, such a signal was previously reported by Schaller and Stebbins in the ¹⁷O MAS NMR spectrum from one Y₂O₃–Al₂O₃–SiO₂ glass.^9b However, despite noting that the “NBO peak may include oxygens bonded to AlO₄ or SiO₄ groups” (then referring to the peak herein assigned to Si–O^[1] groups), they tentatively attributed the high-ppm signal to “NBO species with more yttrium neighbors” than those contributing to the more intense ¹⁷O^[1] resonance. This ¹⁷O NMR peak appears to be a general feature of high-CFS RE-based AS glasses, but we did not detect it from La₂O₃–Al₂O₃–SiO₂ glasses (data not shown), in accordance with observations made in ref. 9b.

Fig. 1 ¹⁷O NMR spectra recorded from the as-indicated RE₂O₃–Al₂O₃–SiO₂ glasses at 14.1 T and 24.0 kHz MAS. The signals from BO and Si–NBO moieties are marked by dotted lines, whereas that from Al–NBO is highlighted by a grey rectangle. The relative amounts (in %) of the Al–NBO moieties out of the entire O speciation is indicated at the right portion of each spectrum.

Note that neither the ¹⁷O MAS NMR spectra (Fig. 1) nor their ²⁷Al counterparts may directly inform about the presence of Al–NBO contacts. The ²⁷Al NMR spectra were recently reported for Y,^9e Lu,^9e and Sc^9g AS glasses, all revealing coexisting AlO₄, AlO₅, and AlO₆ groups. Yet, unambiguous evidence for the assignment of the high-shift ¹⁷O resonance to Al–¹⁷O^[1] species is provided by the ¹⁷O{²⁷Al} TRAPDOR NMR¹² data presented in Fig. 2. Here the ¹⁷O–²⁷Al dipolar interaction is recoupled by applying a strong radio-frequency (rf) pulse for τ_rec = 2.5 ms. For all ¹⁷O sites in close spatial proximity to ²⁷Al, an attenuated integrated ¹⁷O NMR signal intensity [S(τ_rec)] results relative to that observed in the absence of ¹⁷O–²⁷Al recoupling by using a spin-echo [S₀(τ_rec)]. Indeed, due to the presence of Si–O–Al and Al–O–Al structural motifs, the BO-deriving ¹⁷O NMR signals manifest a significant signal dephasing. This also applies to the weak ¹⁷O resonance ∼175–250 ppm in Fig. 2 (assigned to Al–¹⁷O^[1] bonds), as is evidenced by its high dephasing ratio ΔS/S₀ = [S₀(τ_rec) – S(τ_rec)]/S₀(τ_rec) obtained by deconvoluting the net ¹⁷O NMR peakshapes, as exemplified for the S₀(τ_rec) spectra in Fig. 2(b, d and f).

Fig. 2 (a, c, e) ¹⁷O NMR spectra recorded at 9.4 T and 14.0 kHz MAS from the as-indicated Y and Lu bearing AS glasses by employing ¹⁷O{²⁷Al} TRAPDOR NMR.¹² The spectra labelled by S(τ_rec) and S₀(τ_rec) were obtained in the presence and absence of dipolar dephasing, respectively (τ_rec = 2.5 ms). The difference between the black and red traces reflects the degree of ¹⁷O–²⁷Al contacts among the BO, Al–NBO and Si–NBO species, with the number on top of each signal representing the dephasing degree, ΔS/S₀ (uncertainty ±0.03). (b, d, f) Experimental S₀(τ_rec) NMR spectra (black traces) displayed together with the component peaks (grey traces) obtained by spectra deconvolution. The curves beneath each NMR spectrum in (b, d, f) represents the difference between the experimental and best-fit results.

In contrast, the “primary” NBO-stemming resonance reveals no dephasing within the experimental/deconvolution uncertainties; the deconvolution results of Fig. 2(b, d and f) verify that the apparent reduction of this signal stems exclusively from its overlap with the (indeed dephasing) ¹⁷O^[2] resonances. This strongly suggests that the main ¹⁷O^[1] NMR peak originates exclusively from Si–NBO fragments, also verifying the absence of contributions from Al–NBO moieties to this signal, as corroborated further by the additional TRAPDOR NMR data shown in Fig. 3. Fig. 3 also includes ¹⁷O NMR spectra recorded by the ²⁷Al→¹⁷O RAPT-CP technique.¹³ In this experiment, solely ¹⁷O species in close proximity to ²⁷Al are detected. Indeed, while the ¹⁷O resonance-range stemming from BO structural sites is very similar to that observed directly by using central-transition (CT) selective single pulses, no significant NMR-signal intensity is observed in the region ≳125 ppm that is primarily associated with Si–¹⁷O^[1] moieties (note that the weak ²⁷Al→¹⁷O polarization-transfer efficiency coupled with the low abundance of Al–¹⁷O^[1] groups precludes their observation).

Fig. 3 ¹⁷O NMR spectra recorded at 14.1 T from the as-indicated RE AS glasses by employing (a, b) ¹⁷O{²⁷Al} TRAPDOR¹² and (c, d) ²⁷Al → ¹⁷O RAPT-CP¹³ NMR experiments, the latter shown together with results obtained by CT-selective pulses (“1pls”). The data were collected at MAS rates of (a, b) 24.0 kHz and (c, d) 14.0 kHz, by using 3.2 mm and 4.0 mm triple-resonance MAS probeheads, respectively. The two NMR spectra in each of (a, b) are shown on the same absolute intensity scale, whereas those in (c, d) are scaled to display equal peak-maxima of the ¹⁷O^[2] resonances.

Each MAS NMR spectrum of Fig. 1 was deconvoluted into signal contributions from ¹⁷O^[2], Si–¹⁷O^[1], and Al–¹⁷O^[1] moieties (see the ESI†). The fractional populations are presented in Table 1, together with MD-derived O speciations (see ref. 9e,h). ¹⁷O NMR reveals fractional populations x^[1]_Al ≈ 0.015–0.04 of Al–O^[1] species. An overall good agreement is observed between experiments and simulations for the total NBO population (x^[1] = x^[1]_Si + x^[1]_Al), the main discrepancy being clearly over-estimated Al–NBO contacts in the glass models. The latter also reveal non-negligible populations of oxygen triclusters (x^[3]) and “free O²⁻ ions” (x^[0]), as discussed further in ref. 9e,f,h. The attribution of the weak NMR signal to Al–¹⁷O^[1] motifs is consistent with the following trends/observations:

(i) As expected from Al–NBO fragments, there is a concomitant increase of x^[1]_Al with the total NBO content in the glass structure (compare the results for Lu²⁸₅₅(2.21) with Lu²⁵₅₁(2.45) in Table 1), as well as with the Al content [compare Lu²¹₆₅(2.21) and Lu²⁸₅₅(2.21)].

(ii) The isotropic chemical shifts associated with the various ¹⁷O^[1] sites vary significantly with the nature of the RE³⁺ cation, as is also evident from the NMR spectra of Fig. 1. Yet, the Al–¹⁷O^[1] isotropic shifts remain consistently ∼60–100 ppm higher than their Si–¹⁷O^[1] counterparts, in qualitative accordance with reported trends of Ca-based aluminate and Si-poor/Ca-rich AS glasses (we stress, however, that no Al–NBO signals were observed for Ca AS glasses exhibiting >20 mol% SiO₂).^4a,b

(iii) The low (average) quadrupolar products C_Qη = C_Q(1 + η²/3)^1/2 ≈ 2.6 MHz observed for the Si–¹⁷O^[1] sites are consistent with previous reports from AS glasses,^1b whereas the Al–¹⁷O^[1] species reveals C_Qη ≈ 1.7 MHz (obtained by spectra deconvolution; see the ESI†). This is to our knowledge the first estimate of quadrupolar products for Al–¹⁷O^[1] sites. We note that its lower value relative to Si–¹⁷O^[1] is expected from the higher ionic character of the Al–¹⁷O^[1] bond and consistent with the well-established trend observed for ¹⁷O^[2] sites: C_Qη(Al–O–Al) < C_Qη(Al–O–Si) < C_Qη(Si–O–Si).^1b The NMR parameters of the various ¹⁷O species will be discussed in detail elsewhere.

To conclude, we have provided the first direct experimental evidence for Al–NBO contacts in highly polymerized RE₂O₃–Al₂O₃–SiO₂ glasses with variable RE/Al/Si contents. The results are corroborated by MD simulations. Given the following MD-derived propensity trends of AlO_p groups to associate with NBO species, AlO₄ > AlO₅ > AlO₆,^9h we attribute most of the Al-associated NBO species to be located at AlO₄ tetrahedra. Notwithstanding a strong preference for SiO₄ groups to accommodate the NBO ions, the presence of Al–NBO moieties of ≲4% out of the total O speciation (7–11% of all NBO) appears to be a general feature of AS glasses that incorporate trivalent cations with high field strength: apparently they stabilize otherwise energetically disfavoured structural motifs.

While the relative Al–NBO populations grow concurrently with the Al content of the glass, they persist in SiO₂-rich networks (at least up to ≈65 mol% SiO₂), despite that their net NBO population remain relatively low (x^[1] ≈ 0.22). This is in stark contrast to AS glasses based on low-CFS mono/divalent cations, where non-negligible Al–NBO contacts have hitherto only been observed directly for fragmented networks rich in modifiers (≳50 mol% M₍₂₎O) and simultaneously featuring low SiO₂ (≲30 mol%) contents and high molar ratios n(Al₂O₃)/n(SiO₂) > 2.^4a

This work was supported by the Carl Trygger Foundation, the Magn. Bergvall Foundation, and the Swedish Research Council (contract VR-NT 2010-4943). We gratefully acknowledge NMR equipment grants from the Swedish Research Council and the Knut and Alice Wallenberg Foundation.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental and numerical procedures; examples of best-fit ¹⁷O NMR spectra; additional NMR spectra obtained by strong rf pulses. See DOI: 10.1039/c5cp02985f

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