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

By using solid-state 17 O 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 B 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.

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-Al 2 O 3 -SiO 2 glasses normally involve a monovalent alkali (M + ) or divalent alkaline-earth (M 2+ ) metal cation.The glass networks comprise SiO 4 and AlO 4 tetrahedra that are cornershared by bridging oxygen (BO) atoms.The additional negative charge of each AlO À 4 moiety (relative to SiO 4 ) 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. 1,2he 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 AlO 4 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 AlO 5 and AlO 6 polyhedra form whenever zn M o n Al or n Si o 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 chargeaccumulation 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 SiO 4 in silica-rich AS glasses. 1,4owever, 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 AlO 5 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 AlO 5 /AlO 6 populations 6 ).Moreover, while Loewenstein's rule holds strictly for crystalline AS phases featuring n Al r n Si (such as zeolites and minerals 1a ), minor deviations thereof are reported for M (2) O-Al 2 O 3 -SiO 2 glasses. 7Yet, 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. 2Nevertheless, property (iii) is nowadays assumed to apply universally for any SiO 2 -dominated M (2) O-Al 2 O 3 -SiO 2 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 Al 2 O 3 , while SiO 2 is a minor component (o40 mol%). 4owever, the presence of trivalent rare-earth (RE 3+ ) cations in RE 2 O 3 -Al 2 O 3 -SiO 2 glasses 8 leads to markedly higher configurational and chemical disorder, as mirrored in the following structural features: (1) significant AlO 5 /AlO 6 populations prevail throughout the entire range of RE AS compositions, i.e., not only for those featuring n Al 4 n Si and/or insufficient modifier contents. 9The relative amounts of higher-coordination polyhedra were demonstrated to grow for decreasing SiO 2 content, 9d-f and particularly when the RE 3+ cation field-strength, CFS = z/R 2 , 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 La 3+ o Y 3+ o Lu 3+ o Sc 3+ .9e,10 (2) The high positive charge of the RE 3+ cations implies clear violations of the Loewenstein rule, reflected in a pronounced Al/Si atomic disorder for RE AS glass networks, as demonstrated by 29 Si and 27 Al NMR, 9f,11 as well as by molecular dynamics (MD) simulations.][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 29 Si NMR of a significant BO/NBO intermixing among SiO 4 /AlO 4 groups in La 2 O 3 -Al 2 O 3 -SiO 2 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 SiO 2 -rich RE 2 O 3 -Al 2 O 3 -SiO  O [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 SiO 4 groups; Si-17 O [1] (B137-158 ppm).While the 17 O [2] NMR signal dominates, that from Si-17 O [1] grows concurrently with r, i.e., when the glass-network polymerization decreases.Moreover, a weak but significant 17 O resonance appears in the high-ppm region (B175-250 ppm) of all NMR spectra in Fig. 1: it is assigned to Al-17 O [1] motifs.Incidentally, such a signal was previously reported by Schaller and Stebbins in the 17 O MAS NMR spectrum from one Y 2 O 3 -Al 2 O 3 -SiO 2 glass.9b However, despite noting that the ''NBO peak may include oxygens bonded to AlO 4 or SiO 4 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 17 O [1] resonance.This 17 O NMR peak appears to be a general feature of high-CFS RE-based AS glasses, but we did not detect it from La 2 O 3 -Al 2 O 3 -SiO 2 glasses (data not shown), in accordance with observations made in ref. 9b.
Note that neither the 17 O MAS NMR spectra (Fig. 1) nor their 27 Al counterparts may directly inform about the presence of x [1] x [2] x [3] x [1] x [1] Si x a Nominal aRE 2 O 3 -bAl 2 O 3 -cSiO 2 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 17 O MAS NMR, shown together with the contributions from Si-NBO (x [1] Si ) and Al-NBO (x [1] Al ) species, where Values within parentheses are the corresponding MD-derived data.The uncertainties are AE0.015 and AE0.010 for the populations derived from NMR and MD simulations, respectively.Al-NBO contacts.The 27 Al NMR spectra were recently reported for Y, 9e Lu, 9e and Sc 9g AS glasses, all revealing coexisting AlO 4 , AlO 5 , and AlO 6 groups.Yet, unambiguous evidence for the assignment of the high-shift 17 O resonance to Al-17 O [1] species is provided by the 17 O{ 27 Al} TRAPDOR NMR 12 data presented in Fig. 2.Here the 17 O-27 Al dipolar interaction is recoupled by applying a strong radio-frequency (rf) pulse for t rec = 2.5 ms.For all 17 O sites in close spatial proximity to 27 Al, an attenuated integrated 17 O NMR signal intensity [S(t rec )] results relative to that observed in the absence of 17 O-27 Al recoupling by using a spin-echo [S 0 (t rec )].Indeed, due to the presence of Si-O-Al and Al-O-Al structural motifs, the BO-deriving 17 O NMR signals manifest a significant signal dephasing.This also applies to the weak 17 O resonance B175-250 ppm in Fig. 2 (assigned to Al-17 O [1] bonds), as is evidenced by its high dephasing ratio DS/S 0 = [S 0 (t rec ) -S(t rec )]/S 0 (t rec ) obtained by deconvoluting the net 17 O NMR peakshapes, as exemplified for the S 0 (t rec ) spectra in Fig. 2(b, d and f).
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) 17 O [2] resonances.This strongly suggests that the main 17 O [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 17 O NMR spectra recorded by the 27 Al-17 O RAPT-CP technique. 13In this experiment, solely 17 O species in close proximity to 27 Al are detected.Indeed, while the 17 O resonancerange 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-17 O [1] moieties (note that the weak 27 Al-17 O polarization-transfer efficiency coupled with the low abundance of Al-17 O [1] groups precludes their observation).
Each MAS NMR spectrum of Fig. 1 was deconvoluted into signal contributions from 17 O [2] , Si-17 O [1] , and Al-17 O [1] moieties (see the ESI †).The fractional populations are presented in Table 1, together with MD-derived O speciations (see ref. 9e,h).17 O NMR reveals fractional populations x [1] Al E 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 nonnegligible populations of oxygen triclusters (x [3] ) and ''free O 2À ions'' (x [0] ), as discussed further in ref. 9e,f,h.The attribution of the weak NMR signal to Al-17 O [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 Lu (ii) The isotropic chemical shifts associated with the various 17 O [1] sites vary significantly with the nature of the RE 3+ cation, as is also evident from the NMR spectra of Fig. 1.Yet, the Al-17 O [1] isotropic shifts remain consistently B60-100 ppm higher than their Si-17 O [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 420 mol% SiO 2 ).4a,b (iii) The low (average) quadrupolar products MHz observed for the Si-17 O [1] sites are consistent with previous reports from AS glasses, 1b whereas the Al-17 O [1] species reveals À C QZ E 1.7 MHz (obtained by spectra deconvolution; see the ESI †).This is to our knowledge the first estimate of quadrupolar products for Al-17 O [1] sites.We note that its lower value relative to Si-17 O [1] is expected from the higher ionic character of the Al-17 O [1] bond and consistent with the well-established trend observed for 17 O [2] sites: The NMR parameters of the various 17 O species will be discussed in detail elsewhere.
To conclude, we have provided the first direct experimental evidence for Al-NBO contacts in highly polymerized RE 2 O 3 -Al 2 O 3 -SiO 2 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, Fig. 2 (a, c, e) 17 O NMR spectra recorded at 9.4 T and 14.0 kHz MAS from the as-indicated Y and Lu bearing AS glasses by employing 17 O{ 27 Al} TRAPDOR NMR. 12 The spectra labelled by S(t rec ) and S 0 (t rec ) were obtained in the presence and absence of dipolar dephasing, respectively (t rec = 2.5 ms).The difference between the black and red traces reflects the degree of 17 O-27 Al contacts among the BO, Al-NBO and Si-NBO species, with the number on top of each signal representing the dephasing degree, DS/S 0 (uncertainty AE0.03).(b, d, f) Experimental S 0 (t 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.
This journal is © the Owner Societies 2015 AlO 4 4 AlO 5 4 AlO 6 , 9h we attribute most of the Al-associated NBO species to be located at AlO 4 tetrahedra.Notwithstanding a strong preference for SiO 4 groups to accommodate the NBO ions, the presence of Al-NBO moieties of t4% 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 SiO 2 -rich networks (at least up to E65 mol% SiO 2 ), despite that their net NBO population remain relatively low (x [1] E 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 SiO 2 (t30 mol%) contents and high molar ratios n(Al 2 O 3 )/ n(SiO 2 ) 4 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.

Fig. 1
Fig. 1 17 O NMR spectra recorded from the as-indicated RE 2 O 3 -Al 2 O 3 -SiO 2 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.

Fig. 3
Fig. 3 17 O NMR spectra recorded at 14.1 T from the as-indicated RE AS glasses by employing (a, b) 17 O{ 27 Al} TRAPDOR 12 and (c, d) 27 Al -17 O RAPT-CP 13 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 peakmaxima of the 17 O [2] resonances.
2 glasses with RE = {Y, Lu, Sc}, by utilizing magic-angle spinning (MAS) 17 O NMR.Each specimen was prepared with E20% 17 O-enrichment and is denoted RE a b (r), where a and b represent the nominal Al 2 O 3 and SiO 2 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% SiO 2 and n(RE 2 O 3 ) o n(Al 2 O 3 ); see Table 1.The ESI † describes all sample preparation and basic characterization procedures, as well as the NMR experimentation discussed below.Fig. 1 displays 17 O 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 17