Extent of Disorder in Magnesium Aluminosilicate Glasses: Insights

Dec 9, 2015 - Consequently, the extent of framework disorder and the nature of polymerization in the Mg-aluminosilicate glasses, remains an unsolved ...
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Extent of Disorder in Magnesium Aluminosilicate Glasses: Insights from 27Al and 17O NMR Sung Keun Lee,* Hyo-Im Kim, Eun Jeong Kim, Kwan Young Mun, and Saebom Ryu Laboratory of Physics and Chemistry of Earth Materials, School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, Korea ABSTRACT: The quantification of the intrinsic disorder in archetypal noncrystalline magnesium aluminosilicates remains unsolved. This lack of knowledge is because of the increased structural perturbation caused by Mg2+, a high field strength cation, resulting in substantial broadening in both spectral and scattering responses. Most progress regarding amorphous aluminosilicate has thus been made with relatively low field strength cations (e.g., Na+ and Ca2+). Here, we quantified the nature of structural disorder in Mg-aluminosilicate glasses in the enstatite (MgSiO3)-pyrope (Mg3Al2Si3O12) join using 17 O and 27Al NMR. While Mg-aluminosilicate glasses show a much larger topological and configurational disorder around Al than those of Na- and Ca-analogues, the fraction of [5,6]Al (∼8−10%) and the magnitude of topological disorder do not vary significantly with composition. This implies spatial proximity between Mg2+ and the under-bonded bridging oxygens, such as Al-O-Al and Si-O-Al, while Mg2+ preferentially forms Mg-O-Si over Mg-O-Al. The estimated degree of Al avoidance (Q) of ∼0.65 for Mgaluminosilicates based on 17O NMR is close to a random distribution of Si/Al (Q = 0) and is thus much smaller than those estimated for Na- and Ca-aluminosilicate glasses (from ∼0.95 to ∼0.85) that often show evidence for Si/Al ordering (Q = 1, complete Al avoidance). The results also revealed that degree of Al avoidance decreases linearly with increasing cation field strength of non-network-forming cations, highlighting the first simple predictive relationship between the nature of chemical disorder and the types of non-network forming cation. This established correlation can be utilized to explain and predict the diverse properties of the Mg-bearing multicomponent glasses and melts with complex composition-dependence.

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prominent with increasing field strength (defined by charge/ square of ionic radius) of non-network-forming cations such as Mg2+ (e.g., refs 21 and 29−37). This is in contrast to other aluminosilicate glasses with relatively low field strength cations, such Ca2+ and Na+, where the detailed coordination environments around the framework cations (e.g., [4,5]Al and [4]Si) and the degree of polymerization among the cations have recently been quantified.38−46 Furthermore, because of their relatively high liquidus temperature, model systems with much lower melting temperatures, such as Pb analogues, have been explored (e.g., refs 15, 47, and 48). Consequently, the extent of framework disorder and the nature of polymerization in the Mg-aluminosilicate glasses, remains an unsolved problem in geochemistry, physical chemistry, and glass sciences. The extent of Si/Al mixing in aluminosilicate glasses can be directly estimated by exploring the atomic environment around bridging oxygen (BO)-linking framework cations, such as [4]SiO-[4]Si, [4]Si-O-[4,5]Al, and [4]Al-O-[4,5]Al. For example, extensive mixing of [4]Si and [4]Al toward chemical order results in an increase in the fraction of [4]Si-O-[4]Al, whereas a tendency

INTRODUCTION Magnesium aluminosilicate glasses are among the prototypical amorphous oxides with fundamental implications for physical chemistry, geochemistry, and glass and materials sciences. Multicomponent Mg-bearing aluminosilicate glasses also have a wide range of applications in the glass and ceramics industry because the addition of Mg to aluminosilicate glasses enhances their chemical durability and mechanical properties (e.g.,1−8). They can also be potentially useful host materials for highpower laser generation.9 Aluminosilicate glasses in the MgSiO3 (enstatite)-Mg3Al2Si3O12 (pyrope) pseudobinary join are important model systems for iron-free, mafic (Mg- and Ferich) mantle melts with atomistic insights into dynamics, magma transport, and thus the thermal evolution of the Earth’s lower mantle (ranging from 660 km to 2880 km in depth) (e.g., refs 10−14). The key to understanding the unique macroscopic properties of oxide glasses and their corresponding melts is the determination of detailed atomic structures and the extent of disorder among constituent cations and anions (e.g., refs 15−28). However, structural evolution of Mg-aluminosilicate glasses with composition is not well-understood. This is because the overlap among spectral features, peaks, and structure factors typical in oxide glasses becomes even more © XXXX American Chemical Society

Received: November 4, 2015 Revised: December 8, 2015

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DOI: 10.1021/acs.jpcc.5b10799 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Al (i.e., [5,6]Al) also increases with increasing cation field strength of non-network-forming cations for ternary aluminosilicate glasses34,53,69,70 and/or average cation field strength for multicomponent aluminosilicate glasses with more than two types of non-network-forming cations.37,51 The significant fractions of [5,6]Al in partially depolymerized REE-bearing aluminosilicate glasses also imply a distinct role of large ionic radii (and the charge) of those high z-elements on the Al coordination environments.56,69−72 An increase in pressure leads to an increase in Al coordination number in diverse aluminosilicate glasses including Mg-bearing multicomponent glasses under static or dynamic compression.20,23,33,41,57,73−77 Most of the aforementioned progress in Al coordination environments in the glasses has been made using 27Al NMR spectroscopy, offering an excellent opportunity to yield quantitative fractions of distinct Al sites and local network distortion around Al via exploration of the magnitude of the quadrupolar interactions (between the electric field gradient and nuclear quadrupole moment) and thus the quadrupolar coupling constant (Cq).34,43,78−81 The Cq of [n]Al also tends to increase with increasing field strength of non-network forming cations.34,37,43,51,56,58,69−72 Despite the progress with these newly revealed structural insights, the details of proximity between Al and Mg2+, as well as the nature of topological disorder around Al and its composition dependence needs to be explored. The potential results will allow us to quantify the extent of disorder in Mg-aluminosilicate glasses. Because of the aforementioned challenges in accessing the atomic structure of Mg-aluminosilicate glasses, a systematic and predictive model of structure−composition−property relationships has not been fully established. Here, we explore the structure of Mg-aluminosilicate glasses with varying degrees of polymerization in the enstatite-pyrope join using 17O and 27Al NMR spectroscopy to determine the nature of the intrinsic degree of disorder and to propose a link between the extent of disorder and their properties. On the basis of our spectroscopic results, we propose structural schemes to account for the current experimental observation regarding the roles of Mg2+ and its proximity toward diverse BOs and the extent of polymerization in the Mg-aluminosilicate glasses. Together with our previous results on the degree of Al avoidance of other silicate glasses, we report the first quantitative relationship between cation field strength of nonframework cations and the degree of Al avoidance (Q) in Mg-aluminosilciate glasses. We then discuss how the observed link between Q and composition can explain composition-induced changes in the properties of glasses and their corresponding liquids.

toward disorder leads to an increase in the fractions of both [4] Al-O-[4]Al and [4]Si-O-[4]Si.42,49 To quantify the degree of intermixing, the order parameter (Q, the degree of Al avoidance) was proposed, spanning from complete chemical order (Q = 1) to a random distribution of Si and Al (Q = 0).42,49 While the nature of Si/Al disorder has been explored for diverse aluminosilicate glasses with relatively low field strength cations, the Q for Mg-aluminosilicate glasses is not currently available. A relatively small, negative configurational enthalpy estimated from the liquidus surface implies that the Q of Mg-aluminosilicate glasses is distinct from those estimated for other aluminosilicate glasses that often show evidence for Si/Al ordering.24 Spectroscopic evidence to account for the observed macroscopic configurational properties is anticipated. As an increase in the degree of polymerization is accompanied by a decrease in the fraction of nonbridging oxygen (NBO, e.g., Mg-O-[4]Si) (e.g.,21,50) and change in detailed local atomic configuration around NBO, estimation of the NBO fraction and its local environment in the Mgaluminosilicate glasses is necessary.21,30 Additionally, the detailed cation distributions around NBO, such as preferential partitioning of Ca2+ and Mg2+ into NBOs and the proximity of Na+ to BOs37,51−53 and bonding preference to form Si-NBO (i.e., Mg-O-Si) over Al-NBO (Mg-O-Al) are expected to control the corresponding melt properties. While such a bonding preference has been confirmed in Ca- and Naaluminosilicate glasses,54,55 the cation field strength of Mg2+ is much larger than those of other nonframework cations, the increased local disorder due to Mg2+ may lead to a decrease in the degree to which such preferences are obeyed. Indeed, Sirich, rare earth element (REE) bearing aluminosilicate glasses are reported to consist of a noticeable fraction of Al-NBOs.56 While Q-species distribution [QnSi refers to a Si tetrahedron with nBOs and (4 − n)NBOs as the nearest neighbors] from Raman and 29Si MAS NMR studies can often be used to indirectly estimate the NBO and BO content in glasses,21 overlap among each Q-species in these spectra make it difficult to unequivocally determine the Q-species distribution and thus the degree of network polymerization. In contrast, oxygen-17 NMR spectroscopy [particularly, 17O triple-quantum magic angle spinning (3QMAS) NMR] is able to directly resolve various BO and NBO sites in glasses with varying compositions and temperatures at both 1 atm and high pressure.17,18,20,31,41,44,56−62 Therefore, this provides a unique opportunity to determine both the degree of Al avoidance and the extent of polymerization, unveiling the cation ordering,63−66 and/or demixing around NBOs in multicomponent glasses.37,51 17O NMR also revealed a universal decrease in the NBO fraction in oxide glasses with pressure20 and increase in the chemical disorder with increasing temperature.17,67 Although earlier pioneering studies showed the structural changes in endmember glasses,53,63,65 considerable overlap among peaks poses a challenge for unambiguously estimating the oxygen site fractions in Mg-aluminosilicate glasses with varying composition, temperature, and pressure. To solve the illusive puzzle regarding intrinsic disorder among framework cations, information on their quantitative coordination environments are necessary. While Si is predominantly four-coordinated, regardless of composition, in general the average Al coordination number tends to increase with increasing Al content from peralkaline (e.g., binary Mgsilicate) to fully polymerized aluminosilicate glasses (e.g., MgO/Al2O3 = 1).8,34,53,68 The fraction of highly coordinated

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EXPERIMENTAL SECTION

Sample Preparation. Mg-aluminosilicate glasses in the MgSiO3 (enstatite)-Mg3Al2Si3O12 (pyrope) join [(MgSiO3)1−x (Al2O3)x with XAl2O3 = Al2O3/(MgSiO3 + Al2O3) of 0 (enstatite composition), 0.05, 0.1, 0.15, 0.2, and 0.25 (pyrope composition)] were synthesized from mixtures of MgO, Al2O3, and 20% 17O-enriched SiO2. The latter was prepared via hydrolysis of SiCl4 with 17O-enriched H2O, which was then heated at 1100 °C for 1 h under an Ar environment. The MgO powder was dried at 1300 °C for 2 h, which resulted in oxide that is much less prone to hydration. Approximately ∼0.2 wt % cobalt oxide (CoO) was added to reduce the spin-relaxation time and thus the total NMR collection time. The powdermixtures were fused at 1625 °C in an Ar environment for 1 h in B

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and 3QMAS NMR. Figure 1 shows the 27Al MAS NMR spectra of the Mg-aluminosilicate glasses. The dominant peak

a tube furnace. The melts were quenched into glasses by lowering the Pt crucible into water. Table 1 shows the nominal Table 1. Nominal Compositions of Mg-Aluminosilicate Glasses in the MgSiO3 (Enstatite)-Mg3Al2Si3O12 (Pyrope) Join [(MgSiO3)1−x(Al2O3)x with Varying XAl2O3 [=Al2O3/ (MgSiO3 + Al2O3)] composition

nominal composition (mol %)

MgSiO3

Al2O3

XAl2O3

Xpyrope

MgO

Al2O3

SiO2

1 0.95 0.9 0.85 0.8 0.75

0 0.05 0.1 0.15 0.2 0.25

0 0.05 0.1 0.15 0.2 0.25

0 0.2 0.4 0.6 0.8 1

50.0 48.7 47.4 45.9 44.4 42.9

0 2.6 5.3 8.1 11.1 14.3

50.0 48.7 47.4 45.9 44.4 42.9

compositions of the glasses. Negligible weight loss (∼1−2 wt %) was reported during synthesis and thus the compositions of the resulting glasses were expected to be close to their nominal compositions. The resulting glasses showed no evidence of crystalline phases under polarizing microscopy. NMR Spectroscopy. The 27Al MAS and 3QMAS NMR spectra of Mg-aluminosilicate glasses in the MgSiO 3− Mg3Al2Si3O12 join were collected using a Varian solid-state NMR 400 spectrometer (9.4 T) at 104.23 MHz with a 3.2 mm Varian double-resonance probe (Seoul National University, Korea). 27Al MAS NMR spectra were collected using singlepulse acquisition with an rf pulse length of 0.3 μs (approximately 30° for solids) and a recycle delay of 1 s. 27Al 3QMAS NMR spectra were collected using a fast amplitude modulation (FAM)-based shifted-echo pulse sequence (consisting of two hard pulses with durations of 3.0 and 0.7 μs, followed by an echo delay of ∼500 μs, and a subsequent soft pulse with a duration of 15 μs) with a relaxation delay of 1 s.78,82,83 The estimated rf field strength and spinning speed were ∼125 and 18 kHz, respectively. 3648 (collected for 56 h)−6912 (collected for 117 h) scans were averaged to achieve the current signal-to-noise ratio (with a minimum contour line of 2.5%) in the 27Al 3QMAS NMR spectra. As a background signal from the empty rotor was observed in the spectra for MgSiO3:Al2O3 = 0.95:0.05 glass, the final spectrum was obtained following subtraction of the background signals. The 27 Al NMR spectra were referenced to an external 0.1 M AlCl3 solution. 17 O 3QMAS NMR spectra were collected on Varian 400 MHz spectrometers (9.4 T) with a Doty Scientific MAS probe (with a 4 mm Si3N4 rotor) using a FAM-based shifted-echo pulse sequence [4.5 μs−τ (delay)−1.1 μs−echo-delay (approximately 0.36 ms)−19.5 μs], with recycle delays of 0.95−1 s. The rf fields used for these experiments were approximately 72 kHz. A phase table with 96 cycles was used to select a full echo with a recycle delay of 1 s. 3360 (collected for 44 h)−6720 (collected for 90 h) scans were averaged to achieve the signalto-noise ratio shown in the 17O 3QMAS NMR spectra. A magic angle spinning speed of 14 kHz was used. The 17O NMR spectra were measured using tap water as the external reference.

Figure 1. 27Al MAS NMR spectra of Mg-aluminosilicate glasses in the MgSiO3 (enstatite)-Mg3Al2Si3O12 (pyrope) join [(MgSiO3)1−x(Al2O3)x with XAl2O3 [=Al2O3/(MgSiO3 + Al2O3)] of 0.05, 0.1, 0.15, 0.2, and 0.25 (pyrope composition)] as labeled. The rotor backgrounds were subtracted from the spectra for low Al2O3 glasses. The blue spectrum shows an 27Al MAS NMR spectrum for CaNa-aluminosilicate [(CaO)0.75·(Na2O)0.5]1.5·(Al2O3)0.5· 2(SiO2)] glass. ∗ represents the spinning sideband.

with a maximum at ∼50 ppm corresponds to four-coordinated Al ([4]Al). The shape of the peak is also characterized by long tails extending toward lower frequencies, indicating a larger degree of distribution of Cq and the isotropic chemical shift (δiso) of Al environments.78,79 The peak widths in the 27Al MAS NMR spectra of the Mg-aluminosilicate glasses [with a full width at half-maximum (fwhm) of ∼60 ppm] are much broader than those of complex quaternary Ca-Na aluminosilicate glasses (with an estimated fwhm of ∼22 ppm, blue spectrum).20 This suggests that the degree of network distortion around the Al and/or configurational disorder is much larger for Mgaluminosilicate glasses. Note that the Al background signal was subtracted from the spectra, as the 27Al MAS NMR spectrum of the empty rotor showed a signal because of the Al background (mostly [6]Al, Figure 1 bottom).82 Despite the peak broadening because of Mg-induced structural disorder, features from [5]Al (∼30 ppm) are also observed, which can be betterresolved using 2D 27Al 3QMAS NMR spectroscopy. Figure 2 shows the 27Al 3QMAS NMR spectra for Mgaluminosilicate glasses in the enstatite−pyrope join where fully resolved [4]Al is dominant and a noticeable fraction of [5]Al peaks are observed in all the studied glasses (∼−22 ppm in the isotropic dimension). A minor fraction of [6]Al is also observed for glasses with low Al concentration, primarily due to the rotor background. Although it has previously been suggested that the

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RESULTS AND DISCUSSION Effect of Composition on Al Coordination Environments in the Glasses in the MgSiO3 (Enstatite)Mg3Al2Si3O12 (Pyrope) Join: Insights from 27Al MAS C

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MHz.34,43,58 Rather, these estimated Cq values are comparable to those of pure Al2O3 glass, where the estimated Cq for [4]Al varies from 7−8 MHz,82,84 indicating larger topological disorder because of bond length and angle distribution and thus network distortion around Al in Mg-aluminosilicate glasses. Those values are also comparable to those of [4]Al (7−8 MHz) in the Mg-aluminosilicate glasses with similar Mg/ Si ratio reported previously based on simulation of 27Al MAS NMR spectra (at 17.5 T) using a Gaussian isotropic model.34 These Cq values for Mg-aluminosilicate glasses are smaller than those in REE-bearing aluminosilicate glasses (9−11 MHz), indicating that the REE tend to perturb glass networks more significantly.69,71 Despite the pronounced topological disorder around Al in the Mg-aluminosilicate glasses, the extent of topological disorder may not change noticeably upon addition of Al to the enstatite glass. This invariance implies that the structural arrangement around Al in Mg-aluminosilicate glasses may show a moderate degree of regularity (deviated from random Mg2+ distribution), characterized by the proximity between Mg2+ and [4,5]Al (see below for further discussion). Figure 3 shows the total isotropic projections of the 27Al 3QMAS NMR spectra for Mg-aluminosilicate glasses in the

Figure 2. 27Al 3QMAS NMR spectra of Mg-aluminosilicate glasses in the MgSiO3 (enstatite)-Mg3Al2Si3O12 (pyrope) join with varying XAl2O3, as labeled. Contour lines are drawn from 8 to 98% relative intensity with a 6% increment, with four additional lines at the 2.5, 4, 6, and 11% levels to better represent low-intensity features. The contour lines at 3.0 and 4.5% are drawn for the spectra of MgSiO3:Al2O3 = 0.95:0.05 glass. The red square in the spectrum of MgSiO3:Al2O3 = 0.95:0.05 glass shows the signal due to the rotor background.

fraction of [5,6]Al varies with increasing degree of polymerization from depolymerized binary silicate to charge-balanced join,29,34,35,43,53,68 no apparent changes in the peak intensity for each Al species with varying pyrope content are observed. Furthermore, the measured peak widths (fwhm) of [4]Al in the MAS dimension of the 2D spectra (∼43 ppm) are invariant with composition. As the peak width in the MAS dimension is directly proportional to the magnitude of the quadrupolar interactions, the degree of network distortion around the [4,5]Al does not vary significantly with composition (Xpyrope). As expected from the similarity in peak width of each Al species in the MAS dimension, the estimated Cq for [4,5]Al does not change significantly with Xpyrope. Note that we estimated the quadrupolar coupling product (Pq) from the center of gravity of the [4,5]Al peak in the 27Al 3QMAS NMR spectra, while the asymmetry parameter (η) was assumed to be 0.5 to calculate Cq [= Pq/(1 + η2/3)1/2]. The average Pq value of Als in the glasses are ∼7.2 ± 0.2 MHz ([4]Al) and ∼5.2 ± 0.2 MHz ([5]Al), respectively. The corrected average 27Al Cq values are ∼7 ± 0.2 MHz ([4]Al) and ∼5.0 ± 0.2 MHz ([5]Al), respectively. These Cq values are much larger than those of Ca- and Naaluminosilicate glasses with typical Cq ranges for [4]Al of 4−6

Figure 3. Total isotropic projection of the 27Al 3QMAS NMR spectra of Mg-aluminosilicate glasses with varying composition in the MgSiO3 (enstatite)-Mg3Al2Si3O12 (pyrope) join with varying XAl2O3, as labeled. The red arrow shows the background signal due to empty ZrO2 rotor. Dotted lines crossing the peak maxima for [4]Al with varying composition are shown.

enstatite-pyrope join, which provides additional details of Al coordination environments. First, the peak maximum of [4]Al in the glass shifted slightly from −43 ± 1.5 ppm (Xpyrope = 0.2) to −45 ± 1.5 ppm (Xpyrope = 1) in the isotropic dimension, indicating a change in Q4Al(nSi) species ([4]Al species with n number of Si as next nearest neighbors without any NBO): as will be discussed in the 17O NMR results below, absence of AlNBO (Mg-O-Al) indicates that Al is fully polymerized and thus forms only the Q4Al species55). On the basis of the previously established trend between the peak position of the Q4Al(nSi) species and n,37,58 while the majority of Al in the 0.95:0.05 glass should be Q4Al(4Si), the observed changes in peak position D

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field strength. In contrast to the pronounced coupled configurational and topological disorder due to addition of Mg2+, relatively negligible changes in Cq and [5]Al fraction with varying composition indicate potential spatial proximity between Mg2+ and highly coordinated Al species. Specifically, as will be shown in the 17O NMR results below, Al is fully polymerized without forming Mg-O-Al. Therefore, the observed trend indicates proximity between Mg2+ (in Mg-OSi) and Al-O-Al or other Al-bearing under-bonded bridging oxygen species (i.e., Si-O-[4,5]Al). Extent of Polymerization and Chemical Disorder around Oxygen in Mg-Aluminosilicate Glasses in the Enstatite−Pyrope Join. 17O 3QMAS NMR Results. The NBO and BO fractions as well as the atomic configurations around NBOs have been suggested to account for the anomalous changes in the transport properties of silicate glasses and melts over diverse composition and pressure ranges.20,57 While the 17 O MAS NMR spectra of the Mg-aluminosilicate glasses show a single broad peak because of overlapping BOs and NBOs (not shown), Figure 4 shows the 2D 17O 3QMAS NMR spectra

indicate that the fractions of Q4Al(2Si) and Q4Al(1Si) increase with increasing Xpyrope. This change is also manifested in the changes in estimated isotropic chemical shift for [4]Al, which increases with increasing pyrope content from 65.7 ppm (at Xpyrope = 0.2) to 68.1 ppm (at Xpyrope = 1). Second, the estimated peak width (fwhm) for [4]Al in the isotropic projection slightly increases from ∼14 ppm (Xpyrope = 0.2) to ∼15.5 ppm (Xpyrope = 1). This suggests a somewhat wider variation in Q4Al(nSi) species for the pyrope glasses than that of low-Al glasses toward the MgSiO3 endmember, thus enhancing the complexity of Q4Al(nSi) distribution. We also note the peak width of [4]Al is much wider than that of the other Ca- and CaMg-aluminosilicate glasses; while it depends on the other compositional variations (in addition to the cation field strength of non-network formers) (e.g., Si/Al), those of [4]Al in the CaMgSi2O6-CaAl2SiO6 range from 9.5 to 11.4 ppm.51 These results indicate that the presence of Mg2+ leads to an increase in both the chemical disorder (mainly shown by the peak width in the isotropic dimension) and topological disorder in the glasses (manifested in the larger Cq). The chemical and topological disorders in the aluminosilicate glasses are positively correlated and thus are not completely decoupled. Finally, the calibrated total fraction of [5,6]Al is ∼7−10% and does not change with increasing Xpyrope: as the 3QMAS NMR signal intensity depends mainly on the magnitude of quadrupolar interactions of Al sites (and thus Cq), the [n]Al fractions are calibrated by taking into consideration the uneven multiple-quantum transition process for each [n]Al.78,85,86 While the presence is not obvious in the 2D NMR spectra, a minor fraction (∼1%) of [6]Al species was observed in the isotropic projection. Note that the fraction of [5]Al in the glasses studied is somewhat smaller than those reported for the 1D 27Al MAS NMR spectra of the glasses collected at high field (e.g., 18.8 T);34,53 the relevant issues regarding quantification of Al and O sites fractions in the silicate glasses and their NMR parameters, have been discussed in the previous studies.51,84 Briefly, these studies showed the calibration method used in the current study provides relatively robust (though not perfect) estimation of NMR parameters and relative site fractions. As for the 27Al NMR spectra at relatively low magnetic field (9.4 T), unless there are significant fractions of the highly coordinated Als, the fractions of [5.6]Al may not be rigorously quantified using 1D MAS NMR alone. 2D NMR results at relatively low magnetic field tend to underestimated the Cq values of [n]Al sites as has been extensively discussed. 51,71,84 The current method (calibration of 2D NMR intensity) at relatively low magnetic field (9.4 T) may also lead to somewhat underestimated fractions of highly coordinated Al, whereas the 1D NMR study combined with spectral deconvolution using a Gaussian isotropic model distribution (and/or two Gaussian functions) can lead to slightly overestimated fractions of highly coordinated Al (see refs 51, 71, and 84 for a detailed discussion). Despite the uncertainty in the absolute fraction of each Al site, it is clear that the relative fraction does not change with composition. The current results demonstrate that the presence of Mg2+ leads to the formation of [5]Al, increasing the degree of network distortion and thus Al-O bond length and angle distribution around [5]Al, as evidenced by larger Cq values than those of aluminosilicate glasses with lower field cation strengths (Na+ and Ca2+). It also enhances the total configurational disorder, as evidenced by the large peak width in the isotropic dimension compared with other aluminosilicate glasses with low cation

Figure 4. 17O 3QMAS NMR spectra of Mg-aluminosilicate glasses in the MgSiO3 (enstatite)-Mg3Al2Si3O12 (pyrope) join with varying XAl2O3 [= Al2O3/(MgSiO3 + Al2O3)] from 0 (enstatite composition) to 0.25 (pyrope composition). Contour lines are drawn from 8 to 98% relative intensity with a 6% increment, with four additional lines at the 2.5, 4, 5.5, and 11% levels to better represent low-intensity peaks.

of the glasses where oxygen peaks are resolved: the peaks stemming from distinct BOs and NBOs ([4]Si-O-[4]Si, Al-O-Si + Mg-O-Si, and [4]Al-O-[4]Al) are partially resolved for the pyrope composition glass as labeled, consistent with the 17O NMR spectra of the glasses collected at higher magnetic field.53 Note that the resolution among oxygen sites was improved when collected at 9.4 T, as the peaks with distinct Cq values were much better resolved at lower magnetic field.53 The peaks E

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The Journal of Physical Chemistry C corresponding to Mg-O-[4]Si (∼-32 ppm in the isotropic dimension) and [4]Si-O-[4]Si (∼-48 ppm in the isotropic dimension) in the MgSiO3 glass were also fully resolved. The spectra of the glasses with an intermediate composition showed multiple BO and NBO peaks; however, while [4]Si-O-[4]Si is partially resolved from other peaks, the [4]Si-O-[4]Al peak overlaps with the Mg-O-[4]Si peak. On the basis of the correlation between peak positions of Ca-O-Si and Ca-O-Al in Ca-aluminosilicate glasses near Ca-aluminate join,55 expected peak positions of Mg-O-[4]Al, if it would exist, are ∼−50 ppm (in the isotropic dimension) and ∼80 ppm (in the MAS dimension), respectively. The presence of any feature near the expected position in the 2D NMR spectra for Mgaluminosilicate glasses is not evident, indicating that the fraction of Mg-O-[4]Al is likely to be negligible. Finally, the spectra also show a non-negligible fraction of Al-O-Al species (∼-21 ppm in the isotropic dimension) toward endmember pyrope glass, indicating the manifested stability of Al-O-Al in the depolymerized silicate glasses. The Al-O-Al peak intensity apparently increases with increasing pyrope content. In contrast, the Si-O-Si fraction decreases with increasing Al content. The current peak assignments for oxygen sites are based on previous 17O NMR studies of crystalline and noncrystalline aluminosilicates and are straightforward;17,41,54,59−61,66,87−92 however, we note that the Si-O-Al and Al-O-Al peaks in the current spectra may also stem from [4]Si-O-[4,5]Al and [4]AlO-[4,5]Al, as expected from the noticeable presence of [5]Al in the studied glasses (Figure 2). Previous 17O NMR studies of Na-aluminosilicate glasses quenched from melts at high pressure confirmed the formation of [4]Si-O-[5,6]Al, with a peak position of ∼−40 ppm in the isotropic dimension.41 Therefore, while the increase in peak intensity (∼-35 ppm in the isotropic dimension) with increasing pyrope content is mostly due to the formation of [4]Si-O-[4]Al, it could also be partially because of the formation of [4]Si-O-[5]Al. The presence of [4]Al-O-[5]Al is not currently clear. While the following discussion is speculative, we note that previous study of Mgaluminoborate glasses showed strong spatial proximity between [4] Al (∼65%) and [5]Al (∼26%) through dipolar coupling.54 Although spatial proximity between [4]Al and [5]Al may not necessarily warrant the formation of [4]Al-O-[5]Al, it certainly supports the possibility. Furthermore, the peak position of AlO-Al in Mg-aluminoborate glasses (∼−20 ppm)54 is also similar to that estimated in the current study and thus the presence of the peak cannot be completely disregarded. Figure 5 presents the isotropic projections for the 17O 3QMAS NMR spectra of glasses in the join. Again, Mg-O-Si and Si-O-Si peaks are clearly resolved in the MgSiO3 glass, whereas the Mg-O-Si peak overlaps with the Si-O-Al peaks in the intermediate composition glass. The position of the combined peak (Mg-O-Si + Si-O-Al) gradually shifts to a lower frequency from ∼−31 ppm (for MgSiO3 glass) to −34 ppm (for the pyrope glasses). These results indicate that the fraction of Mg-O-Si decreases, while that of Si-O-Al (mostly [4] Si-O-[4]Al and a small fraction of [4]Si-O-[5]Al) increases with increasing pyrope content. The NMR parameters were also estimated from the center of gravity (for resolved peaks, Mg-OSi and Si-O-Si) and peak maxima; 17O δiso and Pq of Mg-O-Si are ∼49 ppm and ∼2.8 MHz, respectively, while those of Si-OSi peaks are ∼58 ppm and ∼4.6 MHz, respectively. The Cq of the mixed peak Si-O-Al + Mg-O-Si is ∼2.9 MHz. These are used to calibrate the NMR peak intensity and thus to yield

Figure 5. Total isotropic projection of the 17O 3QMAS NMR spectra of Mg-aluminosilicate glasses in the MgSiO3 (enstatite)-Mg3Al2Si3O12 (pyrope) join with varying XAl2O3 [=Al2O3/(MgSiO3 + Al2O3)] from 0 (enstatite composition) to 0.25 (pyrope composition), as labeled. Dotted lines crossing the peak maxima for Si-O-Al + Mg-O-Si peak with varying composition are shown.

quantitative fractions of fractions of Si-O-Si and Al-O-Al (see below). Degree of Polymerization in the Glasses in the Enstatite− Pyrope Join. Here, we summarizes the oxygen site specific structural mechanisms that are consistent with the current experimental observations from 17O and 27Al NMR spectra. We note that similar schemes have been proposed and/or confirmed in other aluminosilicate glasses with varying composition and pressure. The first scheme determines the degree of polymerization, where the addition of MgO leads to a decrease in the degree of polymerization by depolymerizing three distinctive types of BOs: [4]

Si‐O‐[4]Si + Mg‐O‐Mg → 2(Mg‐O‐[4]Si)

(1.1)

[4]

Si‐O‐[4]Al + Mg‐O‐Mg → Mg‐O‐[4]Si + Mg‐O‐[4]Al (1.2)

[4]

Al‐O‐[4]Al + Mg‐O‐Mg → 2(Mg‐O‐[4]Al)

(1.3)

The scheme starting with eq 1.1 has been used to account for the degree of polymerization in binary silicate glasses (e.g., refs 21, 50, 93, and 94). As are the cases for Ca- and Naaluminosilicate glasses,55 there is no clear evidence for the MgO-[4]Al peak in the 17O NMR spectra. Therefore, the formation of NBO in Mg-aluminosilicate glasses occurs at the silicate network, forming Mg-O-[4]Si over Mg-O-[4]Al. The following schemes thus describe such a bonding preference: [4]

Si‐O‐[4]Al + Mg‐O‐[4]Al → [4]Al‐O‐[4]Al + Mg‐O‐[4]Si (2.1)

[4]

Si‐O‐[4]Si + Mg‐O‐[4]Al →

[4]

Si‐O‐[4]Al + Mg‐O‐[4]Si (2.2)

[4]

Note that formation of Mg-O- Al presented in eqs 1.2 and 1.3 is compensated by those shown in eqs 2.1 and 2.2. Mg-induced F

DOI: 10.1021/acs.jpcc.5b10799 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C increase in the coordination number of Al can be described using the following quasi-chemical equations: [4]

Si‐O‐[4]Al + Mg‐O‐[4]Si →

[4]

Si‐O‐[5]Al‐O‐[4]Si + Mg* (3.1)

[4]

Al‐O‐[4]Al + Mg‐O‐[4]Si →

[4]

Si‐O‐[5]Al‐O‐[4]Al + Mg* (3.2)

2+

where network-modifying Mg may play a dual role as a charge-balancing cation (Mg*) near the under-bonded BOs ([4]Si-O-[4]Al, [4]Al-O-[4]Al) and BOs with [5]Al ([4]Si-O-[5]Al, [4] Al-O-[5]Al). This mechanism involves the formation of [4]SiO-[5]Al and [4]Al-O-[5]Al and is thus similar to the pressureinduced Al coordination transformation in the aluminosilicate glasses.23,41,61 The mechanisms shown in the scheme with eqs 3.1 and 3.2 result in a reduction in NBO fraction, leading to an increase in the network polymerization. As for the extent of mixing between framework cations (e.g., Si and [4,5,6]Al), the following quasi-chemical equations account for the relative population of BOs observed in 17O NMR spectra: [4]

Si‐O‐[4]Si + [4]Al‐O‐[4]Al → 2([4]Si‐O‐[4]Al)

[4]

Si‐O‐[4]Si + [5]Al‐O‐[5]Al → 2([4]Si‐O‐[5]Al)

[4]

Si‐O‐[4]Si + [6]Al‐O‐[6]Al → 2([4]Si‐O‐[6]Al)

(4.1) (4.2) (4.3)

Equation 4.1 accounts for the BO fractions in aluminosilicate glasses at 1 atm with negligible and/or relatively minor fractions of highly coordinated Al and Si. Equations 4.2 and 4.3 can play an important role, if the fractions of [5,6]Al are dominant (e.g., aluminosilicate glasses at high pressure33,41,73 and REE-bearing aluminosilicate glasses at 1 atm70). The energy difference among these BOs affects most of the configurational thermodynamic properties, such as the configurational entropy and the activity coefficient in the pseudobinary join (see implication section below for further details). In the current study, the fraction of [5]Al is not significant and the peaks due to oxygens linking [4,5]Al and [4]Si (e.g., [4]Si-O-[5]Al and [4]AlO-[5]Al) are not clearly resolved and thus their presence is not firmly confirmed. Therefore, the following scheme instead of three distinct eqs 4.1−4.3) may be used to describe the observed changes in oxygen site populations in Figure 6: Si‐O‐[4]Si + ZAl‐O‐ZAl → 2([4]Si‐O‐ ZAl)

[4]

Figure 6. Schematic local structure of Mg-aluminosilicate glasses based on current NMR measurements.

degree of intermixing between Si and [4.5.6]Al (as shown in eqs 4.1 to 4.3).39,95 Taking into consideration the NMR results and aforementioned structural mechanisms, Figure 6 shows the schematic local structure of Mg-aluminosilicate glasses. Note that MgO-[4]Si is preferentially formed (as shown in Figures 4 and 5, based on the schemes in eqs 1 and 2). However, Mg2+ is also expected to play a charge-balancing role near under-bonded oxygens (Si-O-Al and Al-O-Al, described in the scheme in eq 3). The high field strength of Mg2+ may allow for the formation of [5]Al bonded either to [4]Si and/or [4]Al. These BOs ([4]SiO-[4,5]Al and [4]Al-O-[4,5]Al, in Figure 6A−C) thus have spatial proximity to Mg2+. While this aspect has been known, the strong proximity allows us to account for a compositionindependent nature of the Al site fractions. The current results also explicitly indicate that Mg-O-Si (NBO) has a proximity toward [4]Si-O-[4,5]Al and/or [4]Al-O-[4,5]Al (BO). In the following section, we quantify the effects of cation field strength on the degree of Al avoidance (Q) in Mgaluminosilicate glasses. Quantification of the Degree of Al Avoidance in MgAluminosilicate Glasses. Fitting Protocols of 17O NMR Spectra. To better quantify the extent of Al/Si disorder, estimation of the fraction of Si-O-Si is necessary. Here, the

(4.4)

where Z is the “average” coordination number of Al. In the current study, Z is ∼4.1 (=0.91 × 4 + 0.08 × 5 + 0.01 × 6). Therefore, instead of utilizing three unknown energy differences among oxygens in the eqs 4.1−4.3 [e.g., 2E([4]Si-O-[4]Al) − E([4]Si-O-[4]Si) − E([4]Al-O-[4]Al), 2E([4]Si-O-[5]Al) − E([4]Si-O-[4]Si) − E([5]Al-O-[5]Al)], the single energy difference among BOs in eq 4.4 (2W = 2E([4]Si-O-ZAl) − E([4]Si-O-[4]Si) − E(ZAl-O-ZAl)) allows us to estimate the degree of Al avoidance (Q), as defined below:15,22 Q = 1 − exp(2W /RTf )

(5)

where R and Tf are the gas constant and fictive temperature, below which the supercooled liquid is kinetically frozen, respectively. On the basis of variation in 2W, Q is expected to vary from 0 (random distribution where 2W approaches 0) to 1 (complete Al avoidance with large negative 2W value). The multiple order parameters are necessary to fully describe the G

DOI: 10.1021/acs.jpcc.5b10799 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C fraction of each BO and NBO was obtained from the isotropic projections of the 17O 3QMAS NMR spectra (Figure 5). The spectra were fitted using three Gaussian functions representing Al-O-Al, [Si-O-Al + Mg-O-Si], and Si-O-Si. Here, the Al-O-Al peak represents [n]Al-O-[n]Al (mostly [4]Al-O-[4]Al with a small fraction of [4]Al-O-[5]Al). Therefore, in the current calculation, the average coordination number of Al is set to 4.1 instead of distinguishing all potential peaks. The following protocols were employed to obtain robust fitting results. The position and width of the Mg-O-Si peak were relatively well-constrained from the spectrum of MgSiO3 glass (∼-32 ppm with fwhm of ∼12 ppm). Those of the main, intense peaks due to Si-O-Al + Mg-O-Si were well-constrained (∼-32 − -34 ppm with fwhm of ∼11−12 ppm). The Si-O-Si peak positions and widths were also relatively well-constrained from the 2D 17O 3QMAS spectrum of MgSiO3 and were constrained to ∼−48 and ∼10.3 ppm, respectively, for the ternary compositions. The information for the Al-O-Al peak position was not wellconstrained. Nevertheless, previous 17O NMR studies of Naaluminosilicate glasses with varying degrees of polymerization showed that the difference in peak position between Na-O-Si (Na-NBO with Na as the network-modifying cation) and Al-OAl (BO with Na+ as the charge-balancing cation) was ∼8 ppm.40 Taking into consideration the similarity in peak positions for Na-O-Si and Mg-O-Si,65 the difference in peak position between Mg-O-Si and Al-O-Al (with Mg2+) is likely to be similar to that between Na-O-Si and Al-O-Al (∼8 ppm). Furthermore, the Al-O-Al peak in Mg-aluminoborate glasses is estimated to be ∼−20 ppm.54 On the basis of these constraints, together with the fact that the peak for Al-O-Al is much broader than those of other BOs,89 the estimated peak position for the Al-O-Al peak was ∼−21 ppm (with a fwhm of ∼17 ppm). With the exception of Si-O-Al + Mg-O-Si peaks (directly estimated from the isotropic projection), the widths and positions can vary by ±2 ppm (peak position) and ±3 ppm (width). Figure 7 shows the fitting results, demonstrating the structural evolution of oxygen with composition, where the changes in fraction of each BO with increasing pyrope content, particularly Si-O-Si and Al-O-Al, are quantitatively estimated. Effect of Composition on the Degree of Al Avoidance. Figure 8 shows the calibrated populations of BOs and NBO for glasses in the enstatite-pyrope join with varying pyrope content estimated from the fitting results (Figure 7): the signal intensity of each peak in the 2D NMR spectra was calibrated considering all the experimental details (power and duration of pulses, and Cq of BOs and NBO). The estimated fraction of BO for the MgSiO3 glass is 31.9%, which is consistent with that expected from the composition (33.3%), indicating the robustness of the current fitting protocol. Calibrated Si-O-Si fractions along the enstatite-pyrope join decrease with increasing pyrope content from approximately 31.9% (MgSiO3 glass) to 18.1% (pyrope glass). Figure 8 also presents the predicted fractions of BO [SiO-Al (violet), Si-O-Si (black), and Al-O-Al (blue)] with varying degrees of Al-avoidance (Q) from 1 (complete Al avoidance, long-dashed line) to 0 (random distribution of Si and Al, shortdashed line). The predicted fraction of each BO with Q = 0.8 (thick, pale line), Q = 0.7 (middle, solid line), and Q = 0.6 (thin line) are also shown. These were calculated based on eq 4.4 along with the statistical mechanical models developed in our earlier study.49 The detailed derivation of the oxygen populations with varying Q and composition can be found elsewhere.15,22,42,49 Briefly, depending on the 2W value, the mole fractions of bridging oxygens [e.g., X([4]Si-O-[4]Si)] can

Figure 7. Results of fitting Gaussian functions (thin lines) to the isotropic projection of 17 O 3QMAS NMR spectra of Mgaluminosilicate glasses in the MgSiO3-Mg3Al2Si3O12 join. The thick lines represent the experimental spectra.

Figure 8. Variation of the fractions of bridging and nonbridging oxygens in Mg-aluminosilicate glasses in the MgSiO3-Mg3Al2Si3O12 join. Closed circles and open squares denote the estimated fractions of Si-O-Si and Al-O-Al from the 17O isotropic projections of the spectra, respectively (see text). The red curve shows the calculated Mg-O-Si fraction in the join. The black, blue, and violet curves show the expected (calculated) fractions of Si-O-Si, Al-O-Al, and Si-O-Al, respectively, with varying degree of Al avoidance (Q) from 0 (random distribution of Si and Al, thin, dashed line) to 1 (complete obedience of Al avoidance, thick, long-dashed line). The calculated fraction of each BO with Q = 0.8 (thick, pale line), Q = 0.7 (middle, solid line), and Q = 0.6 (thin line) are also shown.

H

DOI: 10.1021/acs.jpcc.5b10799 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C be energetically constrained, i.e., X2([4]Si-O-ZAl)/[X([4]SiO-[4]Si) × X(ZAl-O-ZAl)] = exp(−2W/RTf) = 1/(1 − Q). Then, X([4]Si-O-[4]Si) and X(ZAl-O-ZAl) can be expressed with Si and Al contents in the glasses and [4]Si-O-ZAl. This allows to predict the fraction of each BO with composition and Q as shown in Figure 8. At constant Xpyrope (and thus constant Si/Al ratio), the fractions of [4]Si-O-[4]Si and Al-O-Al increase with decreasing Q (i.e., less negative 2W value in eq 4.4), while the fraction of SiO-Al increases with increasing Q (thus more negative 2W value in eq 4.4). Regardless of Q, the fraction of Si-O-Si tends to decrease with increasing Xpyrope. Based mainly on the Si-O-Si fraction in the join, the estimated degree of Al avoidance of Mgaluminosilicate glasses is 0.65 ± 0.08. While Q can also vary with Si/Al ratio and/or the degree of polymerization, the Q value in the series does not significantly change with varying pyrope composition. These results confirm that the Alavoidance rule is violated in peralkaline Mg-aluminosilicate glasses. While there is an intrinsic uncertainty in the fitting results because of peak overlap, the experimental results are well-constrained in the estimated values within relatively narrow Q values; in particular, the Si-O-Si fraction of pyrope composition glass varies from 4% (Q = 1) to 22% (Q = 0). Thus, a robust Q value can be obtained from the estimated SiO-Si fraction: uncertainty of the estimated Q value results mostly from the uncertainties in 3QMAS peak intensity of each BO sites (calibration and the peak overlap). The estimated uncertainty in Si-O-Si peak intensity in the pyrope glass is ±2− 3%, which leads to an uncertainty in Q value of ±0.08. The current estimation of the degree of Al avoidance was compared with previous estimations of the Q values for other aluminosilicate glasses and melts. Figure 9 shows the effect of

the degree of chemical order linearly decreases with increasing cation field strength of non-network-forming cations and can be simply described using the following relationship: Q ≈ −0.1 × CFS + 1

(6)

This highlights the first simple predictive relationship between the nature of chemical disorder and the types of non-network forming cation in the glasses. With a further increase in the cation field strength and/or addition of REE, the formation of highly coordinated [5,6]Al and/or Al-NBO is prevalent. The Q for glasses with higher-field strength cations and/or with REE thus may deviate from the observed linear trend. Implications for Macroscopic Properties. The estimated Q and relevant structural information can account for the composition-dependence in melt viscosity. While we did not directly estimate the fraction of XNBO (i.e., Mg-O-Si) because of its overlap with Si-O-Al, the Mg-O-Si fraction was indirectly obtained through estimation of the Si-O-Al fraction based on the estimated fractions of Si-O-Si and Al-O-Al. The estimated Mg-O-Si fraction decreases with increasing Xpyrope from ∼68% (MgSiO3 glass) to 33% (Mg3Al2Si3O12 composition). Although the experimental melt viscosity data for the entire pseudobinary join are not available, the increase in the degree of polymerization due to reduction of the Mg-O-Si fraction implies that the melt viscosity (μ) at constant temperature is expected to increase [as μ ∝ 1/exp(A + XNBO), where A is material dependent](e.g.,97,98). However, though Q does not apparently change with varying composition in the series, it is expected that the contribution from chemical disorder (Q) to total configurational entropy (Sconfig) increases with increasing pyrope content from Al/Si = 0 (where the effect of Q on Sconfig is zero as there is no Al) to Al/Si = 0.67 (where Si/Al disorder strongly contributes to Sconfig). A decrease in the melt viscosity is thus expected because η is also proportional to exp(1/ Sconfig).99−101 Taking into consideration these competing mechanisms, the actual melt viscosity may show a nonlinear variation with composition in the join. The estimated degree of Al avoidance can be used to calculate thermodynamic properties, including the activity coefficient of silica (γSiO2) and configurational enthalpy and stemming from framework Si/Al disorder.15,49 Taking into consideration the extent of disorder estimated from eq 4.4 only, the Si-O-Si fraction (XSi-O-Si) has a 1:1 correspondence to the activity (and thus the activity coefficient) of SiO2 in glasses:20 melt XSi−O−Si = XSi 2 γSiO 2

(7)

where coordination number normalized XSi is the mole fraction of Si [e.g., 4Si/(4Si + 4.1Al)]. Note that the fraction of Si-O-Si is the mole fraction of Si-O-Si among BOs [i.e., Si-O-Si/(Si-OSi + Al-O-Al + Si-O-Al)]. At constant XSi, because the fraction of [4]Si-O-[4]Si increases with decreasing Q, γSiO2 is also expected to increase with a decrease in the degree of Al avoidance. In order to extend its application to modeling the depolymerized full ternary silicates with NBOs, additional terms including NBO-BO and/or Mg-Al mixing would be necessary. Furthermore, modeling the thermodynamic properties of depolymerized silicates often requires a fraction of metalbridging oxygen (MBO, e.g., Mg-O-Mg). Recent studies of the glasses near orthosilicate compositions (e.g., Mg/Si = 2) provided useful discussion regarding this issue (e.g., refs 15 and 102). Nevertheless, the fractions of MBO in Mg-aluminosilicate glasses are expected to be rather small.32,103 Finally, Q also has

Figure 9. Effect of the field strength of nonframework cations (i.e., charge/square of ionic radius, r) on the degree of Al avoidance (Q).

field strength of nonframework cations (CFS, i.e., charge/ square of ionic radius, r) on the degree of Al avoidance. The ionic radii of Ca2+, Na+, Li+, and Mg2+ used in the current study are, 1, 1.02, 0.76, and 0.72 Å, respectively.96 The estimated Q value for Na-, Li-, and Ca-aluminosilicate glasses are approximately 0.94, 0.90, and 0.85, respectively.22,42,89 The current estimation using 17O NMR results shows that the Q for Mg-aluminosilicate glasses (0.65) is much closer to a random distribution (Q = 0) than other aluminosilicate glasses with low field strength cations. These results show that the Q and thus I

DOI: 10.1021/acs.jpcc.5b10799 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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direct implications for the configurational enthalpy (and thus the excess Gibbs free energy of mixing). On the basis of our previous studies,15,49 the configurational enthalpy of alumiconfig nosilicate glasses due to Si-Al mixing HSi-Al in the chargebalanced join can be expressed as follows: HSiconfig ‐Al

⎡ ′⎢ = XSi′ XAl ⎢⎣ 1 +

⎤ ⎥ ′ Q ⎥⎦ 1 − 4XSi′ XAl

Article

AUTHOR INFORMATION

Corresponding Author

*(S.K.L) E-mail: [email protected]. Telephone: 822-8806729. Notes

The authors declare no competing financial interest.

■

Ztot 2W

ACKNOWLEDGMENTS This work was supported by a research grant (2014-053-046) to S.K.L. from the National Research Foundation of Korea. We deeply appreciate careful and constructive suggestions and helps by two anonymous reviewers, which greatly improve the clarity and quality of the manuscript.

(8)

where Ztot = 4XSi + 4.1XAl, XSi′ = 4XSi/Ztot, and XAl ′ = 4.1XAl/Ztot refer to the normalized mole fractions of A and B, and XSi and XAl are the mole fractions of Si and Al involving the formation of BOs. With increasing Q from 0 to 1, negative deviation in Hconfig is expected to increase.15,104 The current experimental estimation of Q (0.65) thus indicates that the configurational enthalpy in Mg-aluminosilicate glasses and melts is expected to show less negative deviation from ideal mixing than those of other aluminosilicate glasses (with Q ranging from 0.8 to 0.95). This is consistent with the indirect estimation from the liquidus surface for MgO-SiO2−Al2O3 ternary melts.24

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REFERENCES

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CONCLUSION Despite their importance and implications, detailed atomic structure, the nature of disorder of Mg-aluminosilicate glasses, and their systematic structure−property relationships are among the fundamental puzzles in physical chemistry. The current NMR results allow us to unveil the degree of structural disorder in Mg-aluminosilicate glasses. First, spatial proximity among Mg2+-O-Si (NBO) and the under-bonded BOs is prevalent, confirming a dual role of Mg2+ as both network modifier and charge balancing cation, as has been proposed and confirmed for other aluminosilicate glasses.(e.g.,21,50) An increased chemical and topological disorder in the Mgaluminosilicate glasses (compared with Ca-, Na-, and Lialuminosilicate glasses) leads to a formation of a larger fraction of [5]Al and Al-O-Al, which in turn results in a stronger spatial proximity among Mg2+ and these BOs. This strong spatial correlation between Mg2+ and [5]Al may be responsible for the constant topological disorder and [5]Al fraction with varying composition from depolymerized enstatite to more polymerized pyrope glasses. The 17O NMR spectra have allowed us to quantify the degree of Al avoidance (Q) in the Mgaluminosilicate glasses that varies between complete Al avoidance (Q = 1) and random distribution (Q = 0). The estimated Q of 0.65 is much smaller than those estimated for other alkali and alkaline earth aluminosilicate glasses with relatively low field strength cations (ranging from 0.8 to 0.95). The current results highlight the fact that Q linearly decreases with cation field strength. The strong linear correlation between the cation field strength of nonframework cations and the degree of Al avoidance can be utilized to explain the diverse configurational and thermodynamic properties of the glasses and melts in the join. The quantitative link to the degree of Alavoidance in other aluminosilicate glasses consisting of cations with much larger ionic radii (e.g., La3+, Y3+) remains to be established. As the mafic mantle melts show a wide range of compositional evolution with varying pressure and temperature, the current results with high Mg2+ content can provide unique constraints on the atomistic origins of the properties of Mgbearing multicomponent melts. J

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