Effect of Network Polymerization on the Pressure-Induced Structural

ARTICLE pubs.acs.org/JPCC

Effect of Network Polymerization on the Pressure-Induced Structural Changes in Sodium Aluminosilicate Glasses and Melts: 27Al and 17O Solid-State NMR Study Sung Keun Lee,*,† Yoo Soo Yi,† George D. Cody,‡ Kenji Mibe,§ Yingwei Fei,‡ and Bjorn O. Mysen‡ †

Laboratory of Physics and Chemistry of Earth Materials School of Earth and Environmental ciences, Seoul National University, Seoul, 151-742, Korea ‡ Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington D.C. 20015, United States § Earthquake Research Institute, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan ABSTRACT: Probing the pressure-induced structural changes and the extent of disorder in aluminosilicate glasses and melts at high pressure remains a challenge in modern physical and chemical sciences. With an aim of establishing a systematic relationship between pressure, composition, and glass structures, we report 27Al and 17 O 3QMAS NMR spectra for sodium aluminosilicate glasses [Na2O:Al2O3:SiO2 = 1.5:0.5:2n with n = 1 (NAS150520, XSiO2 = 0.5), 2 (NAS150540, XSiO2 = 0.67), and 3 (NAS150560, XSiO2 = 0.75)], quenched from melts at pressures up to 8 GPa. We also explore the stability of the [4]Al
O
[4]Al cluster in the highly depolymerized, NAS150520, glass at high pressure. For given glass composition, the [5,6]Al peak intensity increases with increasing pressure. The population of [5,6]Al increases linearly with XSiO2 from NAS150520 (XSiO2 = 0.5) to NAS150560 glass (XSiO2 = 0.75) at both 6 and 8 GPa. The [5,6]Al/XSiO2 ratio also tends to increase with pressure, indicating a possible relationship between [5,6]Al fraction and XSiO2 that depends on pressure. The effect of pressure on the network connectivity in the sodium aluminosilicate glasses is manifested in the increase in [4] Si
O
[5,6]Al peak intensity and the decrease in the nonbridging oxygen (NBO) fraction with increasing pressure. The fraction of [4] Si
O
[5,6]Al in NAS150520 is smaller than in NAS150560. Taking into consideration the pressure-induced Al coordination transformation in the fully polymerized glass (albite, Na2O:Al2O3:SiO2= 1:1:6, NBO/T = 0), the fraction of [5,6]Al at a given pressure varies nonlinearly with variations of NBO/T. [5,6]Al fraction at 8 GPa increases with decreasing degree of melt polymerization from ∼8% for fully polymerized albite melt (NBO/T = 0) to ∼37% for partially depolymerized melt (NAS150560, at NBO/T = 0.29). Then it gradually decreases to ∼15% for NAS150520 with further increase in NBO/T of 0.67. This observed trend in the densification behavior at a given pressure indicates competing densification mechanisms involving steric hindrance vs changes of NBO fraction in the silicate melts. The NMR results also suggest that both NBO and BO, particularly [4] Si
O
[4]Si, interact with Na+, and thus the Na+ distribution is likely to be homogeneous around both NBO and BO at high pressure without spatial segregation of silica-rich and alkali-rich domains for the glass compositions studied here. The presence of the [4]Al
O
[4]Al cluster is distinct in the NMR spectrum for NAS150520 glass at both 6 and 8 GPa. A new scheme of pressureinduced structural transitions in silicate melts involving [4]Al
O
[4]Al includes the formation of [4]Al
O
[5]Al.

’ INTRODUCTION The atomic structure (e.g., coordination number and the network polymerization) of aluminosilicate liquids at high pressure is essential to understand the complex pressure-induced changes in transport (viscosity and diffusivity) and thermodynamic properties [configurational entropy, heat capacity, activity coefficient of silica, element partitioning coefficient, solubility of elements including noble gases into melt, etc.] of dense silicate magmas in the Earth’s interiors.1
25 The potential structural origins of the pressure-induced changes in macroscopic properties allow us to account for distribution of elements and evolution of chemical composition of the Earth’s primary mantle and atmosphere.26
29 Despite the importance of these fundamental properties and the structure of dense silicate melts, determining a detailed atomic and nanometer scale structure of aluminosilicate r 2011 American Chemical Society

glasses and melts at high pressure has remained difficult due to a lack of suitable experimental probes: conventional vibrational spectroscopies and X-ray scattering methods have yielded structural insights into the pressure-induced structural changes in the aluminosilicate glasses and melts, inherent topological and chemical disorder prevalent for the oxide glasses, and severe overlaps among peaks and/or structure factors for the glasses using these techniques, however, make it difficult to resolve detailed Special Issue: Chemistry and Materials Science at High Pressures Symposium Received: July 15, 2011 Revised: August 24, 2011 Published: September 08, 2011 2183

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The Journal of Physical Chemistry C atomic structures of noncrystalline oxides at elevated pressure conditions (see refs 18, 22, and 30 and references therein). Recent progress in high-resolution solid-state NMR (e.g., triple quantum magic angle spinning, 3QMAS) for oxide glasses synthesized at high pressure has enabled us to unveil pressureinduced changes in atomic configurations around quadrupolar nuclides (e.g., 11B, 23Na, and 27Al) and oxygen (17O) in pure borate,31,32 sodium borosilicates,33 binary and ternary mixed cation silicates,15,34
37 and ternary and multicomponent aluminosilicate glasses.2,4,12,15,35,38
40 These studies have reported the pressure-induced formation of highly coordinated framework cations, such as [4]B, [5.6]Si, and [5,6]Al, in these oxide glasses (refs 4, 12, 15, 31, 32, and 37
41). Topologically, the bond length between the framework cation and oxygen in oxide glasses gradually increases with pressure, and that between the nonframework cation and oxygen may increase or decrease depending on the composition of the glasses. The distribution of bond angle ([4]Si
O
[4]Si) and lengths in silicate glasses increases with increasing pressure, and the fraction of the boroxol ring (planar B3O63
ring) in pure borate glasses tends to decrease with pressure (refs 4, 12, 15, 31, 32, and 37
41). A detailed review of the progress can be found elsewhere (see ref 42 and references therein). In the case of aluminosilicate glasses, Al become highly coordinated with increased pressure at lower pressure than that where coordination changes of Si take place (ref 4, 12, 15, 32, and 37
41), consistent with earlier suggestions from 1D NMR.24,25,43
46 The previous NMR results show the strong link between structural transitions in glasses at high pressure and composition: [5,6]Al fraction in the melts at high pressure increases with increasing cation field strength (i.e., charge/ ionic radius) of non-network forming cations (e.g., Na+, Ca2+, K+, etc.). The fractions of [5,6]Al at high pressure are larger for the glasses with high alkali contents in the melts.4,12,39,40 The average Al and Si coordination numbers at high pressure can be linked to the overall density increase in these melts.4,12 By probing directly local environments around bridging (BO, e.g., [4]Si
O
[4]Si, [4]Si
O
[4]Al) and nonbridging oxygen (NBO, e.g., Na
O
[4]Si), 17O 3QMAS NMR studies of silicate glasses formed at high pressure reveal pressure-induced changes in the network connectivity.2,15,34,41 The results of studies have shown that the degree of melt polymerization (i.e., total BO fraction) increases with increasing pressure (refs 2, 15, 34, and 41). The NBO fraction in the melts at high pressure also decreases with increasing alkali content,37 consistent with earlier results from the O-17 1D MAS NMR study.44 Two different types of NBO environments (i.e., Na
O
Si and mixed {Ca
Na}
O
Si) in mixed cation silicate glasses show distinct pressure dependence.36 Rapid decompression of melts results in an increase in NBO fractions.34,38 Topological disorder due to distortion of framework units and bond length distribution tends to increase with increasing pressure. These trends have strong composition dependence (e.g., effect of polymerization of melts at 1 atm, Si/Al ratio).35,37 In spite of this complex composition
pressure dependence in structural transitions in silicate glasses at high pressure, the pressure-induced changes in glass structure can be shown to follow a relatively simple scaling relationship, where experimentally determined NBO fractions for a diverse suite of silicate and aluminosilicate glasses investigated over a range of high pressures can be shown to converge into a single decaying function.2 Further extensive experimental studies of glasses and melts with varying melt polymerization (i.e., degree of NBO/T values, the

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number of nonbridging oxygens per tetrahedral cation) and pressure are required to confirm the observed convergence of NBO fraction. Previous studies have focused on highly polymerized silica-rich silicate glasses.4,12,15,32,37
41 The effect of composition [i.e., NBO/T and/or and mole fraction of SiO2 = SiO2/(Na2O + Al2O3 + SiO2)] on network polymerization in aluminosilicate melts at high pressure, thus, remains to be explored. Another intriguing yet difficult question to address regarding structural transitions in silicate melts at high pressure is whether the stability of high-energy clusters, such as [4]Al
O
[4]Al, decreases or increases with increasing pressure. In contrast to its well-known effect on the degree of Si
Al mixing in networks and the corresponding changes in configurational thermodynamic properties at 1 atm,14,47 the stability of [4]Al
O
[4]Al at high pressure has not been investigated so far. Here, we explore the effect of composition on the pressureinduced structural changes in aluminosilicate glasses. We report 27 Al and 17O 3QMAS NMR spectra for peralkaline sodium aluminosilicate glasses (Na2O:Al2O3:SiO2 = 1.5:0.5:2n, where n = 1, 2, 3) quenched from melts with varying composition and, therefore, degree of polymerization, NBO/T (NBO/T = 0.67, 0.4, and 0.29), and pressure up to 8 GPa with an aim to establish a systematic relationship between pressure, composition, and the glass structures. We also explore the stability of the [4]Al
O
[4]Al cluster in the highly depolymerized Al-rich aluminosilicate melts (Na2O:Al2O3:SiO2 = 1.5:0.5:2) at high pressure, giving previously unknown insights into Al
Si disorder at high pressure.

’ EXPERIMENTS Materials and Synthesis. Sodium aluminosilicate glasses with varying composition [Na2O:Al2O3:nSiO2 = 1.5:0.5:2n with n = 1 (NAS150520), 2 (NAS150540), and 3 (NAS150560)] were synthesized from mixtures of Na2CO3, Al2O3, and 40% 17Oenriched SiO2 (Figure 1). Approximately 0.2 wt % of cobalt oxide (CoO) was added to reduce the spin
lattice relaxation time, which allows us to collect a spectrum with improved signal-tonoise ratio in the same amount of time. These samples have a varying degree of network polymerization with distinct NBO/T values of 0.67 (for NAS150520), 0.40 (for NAS150540), and 0.29 (for NAS150560), respectively, under the assumption that all Al3+ is in tetrahedral coordination at 1 atm pressure. Mole fractions of SiO2 [XSiO2 = SiO2/(Na2O + Al2O3 + SiO2)] of the glasses are 0.5 (for NAS150520), 0.67 (for NAS150540), and 0.75 (for NAS150560). The glass starting materials were synthesized at ambient pressure by fusing the sample mixtures for ∼1 h at 1474
1873 K in a tube furnace at an Ar environment. Taking into consideration negligible weight loss during synthesis, the composition of the resulting glass is expected to be close to its nominal composition. These glass samples (approximately 15 mg) were sealed in the Pt tube and then arc welded. Upon welding, the Pt tube was surrounded by a wet paper towel to minimize potential heating of the sample. Pt tubes with glass samples were inserted in an Al2O3 tube in graphite furnaces. About 12
15 mg of recovered sample was used for the NMR experiment. The NAS150560 and NAS150520 glass sample assemblies were loaded into the 1500 ton multianvil press with a 18/11 (i.e., octahedron edge length/truncated edge length of the anvil) assembly at Geophysical Laboratory, Carnegie Institution of Washington. The NAS150540 glass at the 6 GPa sample was synthesized in a 500 ton multianvil press at Earthquake 2184

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The Journal of Physical Chemistry C Research Institute, University of Tokyo, with an 18/12 assembly. The glasses were fused at 6 and 8 GPa at 1923
2073 K for 10 min and then quenched to glasses while at a given pressure.48 The resulting glasses were optically transparent and show no evidence

Figure 1. Chemical compositions of sodium aluminosilicate glasses (Na2O:Al2O3:SiO2 = 15:5:60, 15:5:40, and 15:5:20) in the Na2O
Al2O3
SiO2 ternary diagram.

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of crystalline phases under polarizing microscopy and Raman spectroscopy. The quench rate was g500 K/s. Previous studies reported that there can be minor but detectable structural changes only with a rapid-pressure quench (5.7 GPa/s) (refs 34 and 38). The current decompression rate of ∼2.78  10
4 GPa/s (or ∼1 GPa/h) was thus too slow to affect any additional structural relaxation once the glass formed from the melts after quenching (temperature) glasses at high pressure. The structure of glasses studied here represents that of supercooled liquids frozen at the glass transition temperature at high pressure. NMR Spectroscopy. NMR spectra were collected with a Varian 400 solid-state NMR spectrometer at 9.4 T (at Seoul National University) with a 3.2 mm thick wall ZrO2 rotor in a Varian double-resonance MAS probe at the Larmor frequency of 54.2 MHz for 17O and 104.3 MHz for 27Al. The 3QMAS NMR spectra were collected using a fast amplitude modulation (FAM)based shifted-echo pulse sequence, with hard pulses with durations of 3.0 μs and 0.6
1 μs and a soft pulse with a duration of 15 μs. A relaxation delay of 1 s and an echo delay of approximately 0.5 ms were used with a rotor spinning speed of 15 kHz and with a phase table of 96 cycles, designed to select full echo.49
51 The spectra were referenced to 0.1 M AlCl3 (aq) (27Al) and tap water (17O). We also collected 17O 3QMAS NMR spectra for NAS150520 glass using a shifted-echo pulse sequence [3.3 us-τ (t1 delay)
1.3 us-τ(delay)
20 us] on a CMX Infinity 300 (7.1 T) spectrometer at the Larmor frequency of 40.7 MHz for 17O (at Geophysical Laboratory). A 2.5 mm ZrO2 rotor in a Chemagnetic double resonance MAS probe was used with the

Figure 2. 27Al 3QMAS spectra for sodium aluminosilicate glasses (Na2O:Al2O3:SiO2 = 15:5:60, 15:5:40, and 15:5:20) quenched from melts at 1 atm (left), 6 GPa (center), and 8 GPa (right) as labeled. Contour lines are drawn at 4.7% intervals from 4% to 98% (total 20 contour lines). 2185

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Figure 3. Population of [4]Al, [5]Al, and [6]Al species for sodium aluminosilicate glasses quenched from melts at ambient and high pressure (black solid line, Na2O:Al2O3:SiO2 = 15:5:60; red solid line, Na2O:Al2O3:SiO2 = 15:5:20, NAS150520) as labeled. Closed square, diamond, and triangle refer to [4]Al, [5]Al, and [6]Al, respectively. Trend lines connecting experimental data are drawn for visual clarity only.

relaxation delay of 1 s and a spinning speed of 19 kHz. The spectra for NAS150560 glass collected at 7.1 T were reported previously35 and were recollected at 9.4 T in the current study for comparison with spectra for other more depolymerized glasses (NAS150540 and NAS150520) quenched from melts at ambient and high pressure.

’ RESULTS AND DISCUSSION Effect of Composition on Pressure-Induced Changes in Al Coordination Environments: A View From 27Al NMR. Figure 2

shows the 27Al 3QMAS NMR spectra for depolymerized peralkaline (Na/Al > 1) sodium aluminosilicate glasses (Na2O:Al2O3: SiO2 = 1.5:0.5:2n, n = 1,2,3) quenched from melts at high pressure up to 8 GPa (ref 35). The Al environments ([4,5,6]Al) are well-resolved. For each glass composition, only the [4]Al peak observed in spectra of the glasses formed at 1 atm and the [5,6]Al peak intensity increases with increasing pressure. The fraction of [5,6] Al at a given pressure appears to increase with increasing degree of network 1 atm polymerization from NAS150520 to NAS150560 glass. This conclusion is consistent with existing data where higher alkali cation content in the silicate glasses at 1 atm enhances the formation of highly coordinated Si and Al at high pressure.15,25,35,39 Figure 3 shows the population of [4]Al, [5]Al, and [6]Al species of the sodium aluminosilicate glasses quenched from melts with varying pressure and composition. The NMR signal intensity in the 3QMAS NMR experiment was calibrated by taking into account the relationship between 3QMAS efficiency and quadrupolar coupling constant (Cq, at a fixed quadrupolar asymmetry parameter η of 0.5): 3QMAS efficiency is determined by excitation of the triple quantum coherence and subsequent reconversion into single quantum coherence, and it nonlinearly varies

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with the magnitude of interaction between the nuclear quadrupole moment and electric field gradient, which can be parametrized by Cq (refs 52 and 53). The [4]Al fraction decreases with pressure from 100% to ∼65% for NAS150560 and ∼83% for NAS150520, and the fractions of [5,6]Al increase with pressure, regardless of composition. The [5,6]Al fractions for NAS150520 glass (NBO/T = 0.67) at high pressure are smaller than that for NAS150560 glass (NBO/T = 0.29), indicating that for the glass composition studied here, with increasing degree of melt polymerization (and thus a smaller NBO/T) at 1 atm, the fraction of highly coordinated Al at high pressure is more significant. The population of [5,6]Al (Y-[5,6]Al) in these compositions increases linearly with XSiO2 at both 6 and 8 GPa [i.e., Y
[5,6]Al = aXSiO2 + b] (Figure 4, see figure caption for the error bars in the fits). The slopes (a, [5,6]Al/XSiO2) are approximately 92 ( 20 (at 8 GPa) and 43 ( 7 (at 6 GPa), respectively, indicating that the relationship between the fraction of [5,6]Al and XSiO2 has pressure dependence: for the glass compositions studied here, the [5,6]Al/XSiO2 ratio increases with increasing pressure. Figure 5 presents the 2D contour plot of an expected population of [4,5,6]Al species in the sodium aluminosilicate glasses with varying composition (XSiO2 from 0.5 to 0.75) and pressure (from 1 atm to 8 GPa): because only a limited number of experimental data points are available (Figures 1
3 above), the data in between these data points were interpolated. The figure thus shows how the Al coordination environments change with pressure and XSiO2: with decreasing degree of 1 atm melt polymerization (e.g., from NAS150560, XSiO2 = 0.75 to NAS150520, XSiO2 = 0.5), the formation of [5,6]Al at high pressure is suppressed perhaps because of steric hindrance by nearby network modifying cations (see below for further discussion). Effect of Composition on the Pressure-Induced Changes in the Degree of Melt Polymerization: A View from 17O 3QMAS NMR. The 17O 3QMAS NMR spectra of ternary sodium aluminosilicate glasses quenched from melts with varying pressure and compositions show four types of bridging oxygen clusters (i.e., [4]Si
O
[4]Si, [4]Al
O
[4]Al, [4]Si
O
[4]Al, and [4]Si
O
[5,6]Al) and nonbridging oxygen clusters (i.e., Na
O
[4]Si) (Figure 6). These BO and NBO clusters are well resolved in the spectra for the glasses at 1 atm. In detail, the [4]Si
O
[4]Si peak intensity apparently decreases with increasing 1 atm NBO/T from NAS150560 to NAS150520 glass, indicating a decrease in the network polymerization of the aluminosilicate melts with composition. A small but non-negligible fraction of [4]Al
O
[4]Al in the NAS152020 glass at 1 atm is also observed. This suggests that the distribution of [4]Al and [4]Si in the network deviates from a complete chemical order that tends to minimize the fraction of oxygen linking two [4]Al, [4]Al
O
[4]Al (i.e., Al
Al avoidance): with increasing degree of disorder in [4]Al and [4]Si in the framework, the fraction of [4]Al
O
[4]Al increases23 (see below for further discussion). Although [4]Al may be linked to BO and NBO, we note that all [4]Al in the glasses studied here are only linked to BO ([4]Si
O
[4]Al and [4]Al
O
[4]Al) as evidenced by the absence of a nonbridging Na
O
[4]Al peak,54,55 while [4]Si can form NBO (i.e., Na
O
[4]Si) at 1 atm. The effect of pressure on the network connectivity in the sodium aluminosilicate glasses is manifested in the increase of [4] Si
O
[5,6]Al peak intensity (∼
35 to
40 ppm in the isotropic dimension) with pressure (Figure 6). The fraction of [4] Si
O
[5,6]Al in NAS150520 is smaller than in NAS150560, which is consistent with a decrease in the [5,6]Al fraction with increasing XSiO2. The total fraction of NBO appears to decrease 2186

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Figure 4. Variation of the population of [5,6]Al (%) [i.e., ([5]Al + [6]Al)/ ([4]Al + [5]Al + [6]Al)] in sodium aluminosilicate glasses with varying XSiO2 [= SiO2/(Na2O + Al2O3 + SiO2)] and pressure. Triangle, square, and diamond symbols refer to glasses quenched from melts at 1 atm, 6 GPa, and 8 GPa, respectively. The linear trend line can be described by the following relations: [5,6]Al at 8 GPa = 91.9 ((20) XSiO2
31.4 ((12); [5,6]Al (%) at 6 GPa = 42.7 ((7) XSiO2
19.1 ((4).

with increasing pressure, leading to an increase in the overall polymerization of melts. The proportion of high-energy [4]Al
O
[4]Al clusters, though minor, is observable in the 2D NMR spectrum for NAS150520 at both 6 and 8 GPa, suggesting moderate stability of the cluster in the highly depolymerized peralkaline glasses at high pressure (see below for further discussion). 17O 3QMAS NMR spectra for the glasses at high pressure also do not show a detectable fraction of oxygen linking highly coordinated framework units, such as [5]Al
O
[5]Al, [6]Al
O
[6]Al, and [5] Si
O
[5]Si clusters, indicating that a clustering of highly coordinated framework cations is prohibited for the glass compositions studied here at higher pressure. There is a pressure-induced increase in the [4]Si
O
[5,6]Al fraction at the expense of Na
O
[4]Si and [4]Si
O
[4]Al as seen in the 17O 3QMAS NMR spectra collected at 7.1 T for NAS150560 glass with varying pressure (Figure 7). The results also show the presence of [4]Al
O
[4]Al at 6 and 8 GPa. Whereas the overall information in the spectra at 7.1 T is identical to those collected at 9.4 T, we note that the Na
O
[4]Si, [4] Si
O
[4]Al, and [4]Al
O
[4]Al peaks are better resolved because the atomic clusters with a larger Cq difference are better resolved at lower magnetic field. A decrease in [4]Si
O
[4]Si peak intensity (and an increase in Na
O
[4]Si) with decreasing XSiO2 from 0.75 (for NAS150560) to 0.5 (for NAS150520) can be seen in the total isotropic projection of 17O 3QMAS NMR spectra for the glasses quenched from melts with varying composition at 1 atm (Figure 8 top). Additionally, the peak positions of oxygen clusters in the isotropic dimension vary with composition. The peak position of [4]Si
O
[4]Si (as well as [4]Si
O
[4]Al and Na
O
[4]Si clusters) in the isotropic dimension varies with XSiO2. This trend indicates that both NBOs and BOs, particularly [4]Si
O
[4]Si, interact with Na+ without strong preferential proximity between Na+ and NBOs over BOs: a preferential spatial proximity between Na+ and NBOs would lead to changes in NMR peak positions for NBO peaks only. Therefore, the Na+ distribution is likely to be

Figure 5. Interpolated proportion of Al species ([n]Al) in sodium aluminosilicate glasses with varying XSiO2 [= SiO2/(Na2O + Al2O3 + SiO2)] and pressure where the populations of Al species with glass composition between the experimental data points were calculated from the trend lines.

homogeneous around both BO and NBO in the glass network for the glass compositions studied here at 1 atm.30,54,56 A pressure-induced increase in the [4]Si
O
[5,6]Al fraction is at the expense of NBO and [4]Si
O
[4]Al clusters, while the fraction of the [4]Si
O
[4]Si peak is apparently invariant with pressure as shown in the isotropic projection of 17O 3QMAS NMR spectra for the glasses with varying pressure and composition (Figure 8 middle). The peak positions of oxygen clusters (both NBOs and BOs, including [4]Si
O
[4]Si) move slightly to more negative chemical shifts in the isotropic dimension (corresponding to an increase in 17O isotropic chemical shift) with increasing pressure from 1 atm to 8 GPa because of an increase in Si
O bond length.35,37 This trend is suggesting that there is no clear preferential NBO
Na+ interaction and is thus consistent with a homogeneous Na+ distribution in the glass networks at both ambient and high pressure without spatial segregation of silica-rich and alkali-rich domains predicted from modified random network models.54,55 2187

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Figure 6. 17O 3QMAS spectra for sodium aluminosilicate glasses (Na2O:Al2O3:SiO2 = 15:5:60, 15:5:40, and 15:5:20) quenched from melts at 1 atm, 6 GPa, and 8 GPa as labeled. Contour lines are drawn at 4.7% intervals from 4% to 98% (total of 20 contour lines), with additional lines at 5% and 6.5% levels.

Effect of Composition on the Structural Transitions and the Extent of Disorder in Sodium Aluminosilicate Melts at High Pressure. The current experimental data reveal that the

1 atm NBO/T can affect the degree of melt polymerization at high pressure with increasing XSiO2 from 0.5 (NAS150520) to 0.75 (NAS150560) (Figure 4). Taken together with previous results for the fully polymerized albite composition glass (Na2O:Al2O3: SiO2= 1:1:6, NBO/T = 0),35 these new results demonstrate that the fraction of [5,6]Al and thus Si
O
[5,6]Al at a given pressure increases with increasing NBO/T and then decreases with a further increase in NBO/T (i.e., nonlinear trend hereafter). For instance of constant pressure, the [5,6]Al fraction at 8 GPa increases with decreasing degree of melt polymerization from ∼8% for fully polymerized melt35 (albite composition with NBO/T = 0) to ∼37% for partially depolymerized melt (NAS150560, at NBO/T = 0.29) and then decreases with a further decrease in the degree of polymerization to ∼15% for NAS150520 with NBO/T = 0.67. While more extensive experimental studies are necessary to quantitatively constrain the pronounced nonlinearity, this trend in the pressure-induced structural changes in the melts at a given pressure indicates competing densification mechanisms involving steric hindrance due to network modifying cations and NBO fraction in the silicate melts. The latter (i.e., increase in NBO fraction) facilitates pressure-induced coordination transformation in the melts at high pressure because the highly coordinated Al is formed at the

expense of NBO. In contrast, the addition of network modifying cations with increasing 1 atm NBO/T leads to an increase in steric hindrance and thus prohibits coordination transformation. The effect of steric hindrance becomes prevalent with increasing 1 atm NBO/T at high pressure: from albite glass (NBO/T = 0) to NAS150560 (NBO/T = 0.29) glass, although Na content increases, the fraction of [5,6]Al at high pressure increases, indicating that the formation of NBO plays an important role (instead of steric hindrance). From NAS150560 to NAS150520 (NBO/T = 0.67) glass, the fractions of non-network forming cations around NBO increase. This prohibits NBO from being connected to other nearby framework cations (i.e., [5,6]Al), and this steric hindrance becomes dominant with further increase in NBO/T. The pressure-induced changes in oxygen cluster fractions in the aluminosilicate glasses and melts at high pressure have been described using the following quasi-chemical mechanisms for pressure-induced structural transtions15,22,34,40,44,45 ½4

Si
O
½4 Al þ Na
O
½4 Si ¼

½4

S
O
½5 Al
O
½4 Si þ Na

ð1-1Þ

Si
O
½4 Si þ Na
O
½4 Si ¼

2188

½4

½4

Si
O
½5 Si
O
½4 Si þ Na

ð1-2Þ

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Figure 7. 17O 3QMAS spectra for sodium aluminosilicate glasses (Na2O:Al2O3:SiO2 = 15:5:20) quenched from melts at 1 atm, 6 GPa, and 8 GPa as labeled. Contour lines are drawn at 5% intervals from 3% to 88%, with additional lines at 4.5% and 6.5% levels.

where the role of Na in the melts changes from network modifying to charge balancing (Na*) the [5]Si
O
[4]Si and [5] Al
O
[4]Si at high pressure. The pressure-induced stability of the [4]Al
O
[4]Al cluster is shown in the isotropic projection of 2D 17O 3QMAS NMR spectra for NAS150520 glass (Figure 8). Although it is difficult to quantify its fraction at 6 and 8 GPa due to relatively small peak intensity, its fraction apparently decreases with increasing pressure [Figure 8 (bottom)]. The results allow us to propose a new scheme for structural transitions at high pressure involving [4]Al
O
[4]Al clusters in aluminosilicate melts, which can be described as follows ½4

Al
O
½4 Al þ Na
O
¼

½4

Al
O


½5

½4

Si

Al
O
½4 Si þ Na

ð1-3Þ

The pressure-induced decrease in the [4]Al
O
[4]Al fraction is thus accompanied by the formation of [4]Al
O
[5]Al. While the NMR parameters for the [4]Al
O
[5]Al peak will be determined by future ab inito chemical calculations, our previous

Figure 8. (Top) Total isotropic projection in 17O 3QMAS spectra for sodium aluminosilicate glasses (Na2O:Al2O3:SiO2 = 15:5:60, 15:5:40, and 15:5:20) quenched from melts at 1 atm. (Middle) Total isotropic projection in 17O 3QMAS spectra for sodium aluminosilicate glasses quenched from melts at 1 atm, 6 GPa, and 8 GPa as labeled. (Bottom) Total isotropic projection in 17O 3QMAS spectra for NAS150520 glass quenched from melts at 1 atm, 6 GPa, and 8 GPa as labeled with an enlarged view for the [4]Al
O
[4]Al cluster.

calculations for [4]Si
O
[5,6](Al, Si) showed that 17O δiso for [n] (Si,Al)
O
[4]Si increases with increasing coordination number (n) (ref 15). Assuming a similar relationship between 17O δiso of [n]Al
O
[4]Al and the coordination number of Al, the 17 O δiso of [4]Al
O
[5]Al is likely to be larger than that of [4] Al
O
[4]Al (more negative peak position in the isotropic dimension). The increase in the peak intensity ∼
18 to
20 ppm, thus, may indicate the formation of [4]Al
O
[5]Al. Note that stability of oxygen clusters at high pressure can be predicted from relative cluster populations of BO and NBO peaks in the spectrum. On the basis of almost constant [4] Si
O
[4]Si peak intensity with increasing pressure, it can be concluded that stability of the [4]Si
O
[4]Si cluster is much higher than those for [4]Al
O
[4]Al and [4]Si
O
[4]Al. On the basis of the presence of [4]Al
O
[4]Al in the NMR spectra at 2189

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The Journal of Physical Chemistry C

ARTICLE

high pressure and a similar trend in the pressure-induced decrease in [4]Si
O
[4]Al [Figure 8 (bottom)], qualitatively, the energy penalty for the formation of the [4]Al
O
[4]Al cluster in aluminosilicate glasses at high pressure (free energies for the reactions in eqs 1-1 and 1-3) is comparable to that of [4] Si
O
[4]Al. Otherwise, one would expect to observe a more significant decrease in the peak intensity with increasing pressure. Whereas [4]Al
O
[4]Al is a higher energy cluster at 1 atm, the relative stability of [4]Al
O
[4]Al at high pressure stems from the fact that its annihilation is prohibited by the formation of another higher-energy cluster [5]Al
O
[4]Al at high pressure. As explicitly shown in eq 1, the decrease in NBO in the aluminosilicate melts is mainly accompanied by the pressureinduced Al coordination transformation (and mostly formation of Si
O
[5,6]Al and additionally [4]Al
O
[5,6]Al) in silicate melt composition studied here
ð∂XNBO =∂PÞT ¼ ð∂½5;6 Al=∂PÞT ¼ ½∂ð½4 Si
O
½5;6 Al þ

½4

Al
O
½5 AlÞ=∂PT

ð2Þ While eq 2 describes qualitative relations between the pressure dependence of Al coordination number and oxygen cluster fractions (Figures 4 and 6), a quantitative fraction of [5,6]Al with pressure can be used to confirm the composition (i.e., NBO/T) dependence of (∂XNBO/∂P)T [and
(∂ [5,6]Al/∂P)T]. By introducing the transition pressure in which the NBO fraction is expected to be 50% of the value at 1 atm (PXNBO=0.5), the normalized NBO fraction X0 NBO(P0 ) [= X0 NBO(P0 )/X0 NBO (1 atm)] tends to decrease from 1 to 0.5 with increasing normalized pressure P0 (=P/PXNBO=0.5, 0 e P0 e 1) and can be described as follows2 X 0NBO ðP0 Þ ¼ 1
exp½ðP0
1Þ=α

ð3Þ

where α is a constant relevant to the degree of network rigidity upon pressurization (see ref 57 for definition of network rigidity). The α value of ∼0.4 accounts for the pressure-induced changes in NBO fraction in NAS150560 and NAS105020 glass.2 While the fictitious pressure where the NBO fraction in melts is expected to be 0 (PXNBO=0) was used in the previous study (ref 2), PXNBO=0.5 was used in the current study to describe the effect of composition on (∂XNBO/∂P)T. This is because a trend (∂XNBO/∂P)T in melts may vary at higher pressure where the NBO fraction is less than 50% of the value at 1 atm. The PXNBO=0.5 increases with decreasing degree of polymerization of melts at 1 atm from NAS150560 to NAS150520 glass.2 Additionally, on the basis of the relationship between the [5,6] Al fraction and NBO/T (from fully polymerized melts, partially depolymerized, to highly depolymerized melts), we also note that the PXNBO=0.5 of the glass may increase with increasing degree of polymerization from NAS150560 to the other fully polymerized melts (NBO/T = 0). The structure of sodium aluminosilicate glasses at high pressure studied in the current study represents that of supercooled liquids frozen at the glass transition temperature at high pressure. Much less is known about the structure of aluminosilicate silicate liquids above the melting temperature at highpressure ranges (∼8 GPa) studied here (see ref 22 and references therein). To have a better understanding of the structure of corresponding liquids above their melting temperature at high pressure, the effect of temperature on structure of aluminosilicate

glasses under compression at elevated pressure is necessary. The effect of the fictive temperature on the degree of polymerization and coordination number for diverse aluminosilicate melts at 1 atm was reported to be rather minor.58 Similar studies at high pressure, however, remain to be explored. While in situ solid-state NMR of oxide glasses at high pressure is not currently available, X-ray Raman scattering—an element specific an in situ highpressure probe—has also been effective in providing structural transitions in low-z elements (e.g., O, Li, B) in oxide glasses at much higher pressure than the NMR study can provide.1,57,59,60 For example, boron K-edge X-ray Raman scattering studies of pure B2O3 and alkali borate glasses revealed the pressure-induced boron coordination transformation from [3]B to [4]B up to 25 GPa.1,57,59,60 Oxygen K-edge X-ray Raman scattering study of MgSiO3 glass at pressures above 20 GPa suggests a pressureinduced oxygen coordination transformation from [2]O to triply coordinated oxygen ([3]O) (ref 1). Quantum mechanical and molecular dynamics simulations have provided insight into the pressure-induced structural transitions in aluminosilicate melts at much higher pressure conditions (>100 GPa) relevant to Earth’s lower mantle.61
64 The current results show that the relationship among atomic structure (i.e., coordination number and connectivity)
pressure
composition can be established in the ternary aluminosilicate glasses quenched from melts (e.g., Figures 2 and 7). The current structural information may help elucidate connections between the microscopic signatures of anomalous and nonlinear changes in the macroscopic properties of the corresponding liquids.2,14 For instance, recent thermodynamic modeling showed that the pressure-induced NBO fractions and coordination number can account for the anomalous pressure dependence of configuration entropy, viscosity, and oxygen diffusivity in silicate melts.2 A decrease in NBO fraction with pressure is likely to lead to a decrease in activity coefficient of silica, while the solubility of elements into silicates may be dependent on relative chemical affinity between NBO and elements of interest.2,14 The current and potential results help to develop a more quantitative relationship between macroscopic properties and structures of oxide glasses at high pressure relevant to earth’s interiors.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: 822-880-6729. Fax: 822871-3269.

’ ACKNOWLEDGMENT This study was supported by a grant from the National Research Foundation, Korea (2007-000-20120), to SKL. We thank P. Grandinetti for providing us with the RMN software for 2D NMR data processing and two anonymous reviewers for constructive suggestions. ’ REFERENCES (1) Lee, S. K.; Lin, J. F.; Cai, Y. Q.; Hiraoka, N.; Eng, P. J.; Okuchi, T.; Mao, H. K.; Meng, Y.; Hu, M. Y.; Chow, P.; Shu, J. F.; Li, B. S.; Fukui, H.; Lee, B. H.; Kim, H. N.; Yoo, C. S. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 7925. (2) Lee, S. K. Proc. Natl. Acad. Sci. 2011, 108, 6847. (3) Mibe, K.; Yoshino, T.; Ono, S.; Yasuda, A.; Fujii, T. J. Geophys. Res.-Solid Earth 2003, 108. 2190

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