# The effect of iodine on the local environment of network-forming elements in aluminoborosilicate glasses: An NMR study

Sami Soudani, Michael Paris, Yann Morizet

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## To cite this version:

Sami Soudani, Michael Paris, Yann Morizet. The effect of iodine on the local environment of network-forming elements in aluminoborosilicate glasses: An NMR study. Journal of the American Ceramic Society, In press, 107 (7), pp.4401-5138. 10.1111/jace.19764. hal-04495097v2

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# The effect of iodine on the local environment of network-forming elements in aluminoborosilicate glasses: An NMR study

Sami Soudani<fnref n="1" />,<fnref n="2" /> ID |

<figure><img src="image_5.png" /></figure>

Michael Paris<fnref n="2" /> |

Yann Morizet<fnref n="1" />,<fnref n="2" /> ID

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## Correspondence

**Sami Soudani, Laboratoire de Planétologie et Géosciences (LPG), UMR 6112, Nantes Université, 2 rue de la Houssinière, 44322 Nantes Cedex, France.
Email: sami.soudani@univ-nantes.fr

## Funding information

Conseil Régional des Pays de la Loire; Agence Nationale de la Recherche, Grant/Award Number: ANR-20-CE08-0018

## Abstract

Knowledge of the structural organization of nuclear waste glasses at the atomic scale is the basis for further studies of macroscopic properties (e.g., fracture toughness, chemical durability, and leaching rate). Recent studies have focused on the iodine dissolution mechanism in oxide-based glasses synthesized under high-pressure conditions, but there is still a lack of key information on the extent of the modifications caused by iodine entering the glass network. We therefore decided to conduct an in-depth study by nuclear magnetic resonance on two series of aluminoborosilicate glasses synthesized under high pressure conditions and doped with either I₂ or I₂O₅. The obtained ¹¹B, ²³Na, and ²⁷Al nuclear magnetic resonance (NMR) spectra were fitted and correlated with the iodine content in order to find possible relationships with the local environment change for the probed cations. These data showed that iodine has no effect on the coordination of aluminum and very little effects on that of boron. In contrast, we found an expected relationship of the I content with the ²³Na NMR parameters. In comparison with previous studies on the non-bridging oxygen (NBO) distribution in aluminoborosilicate, we suggest that Na⁺ is mainly scavenged from silica tetrahedra with one or two NBO (Q³ or Q<fnref n="2" />) as no boron planar triangles with one NBO are expected for such compositions. From those information, we establish a model for iodine dissolution in nuclear waste glass that is consistent with previous studies.

## KEYWORDS

borosilicate glass, nuclear magnetic resonance, nuclear waste

# 1 | INTRODUCTION

The ¹²⁹I radioisotope represents a major issue in the immobilization of nuclear wastes produced by human nuclear plant activities. Although this radioisotope is not produced under large quantity, its long half-life and high volatility

makes ¹²⁹I a troublesome radioactive element that requires specific matrix for immobilization.<fnref n="1" />⁻⁸ Currently, there are landmark works that have developed various matrices able to incorporate iodine in their structure: from glasses with various chemical compositions⁹⁻¹⁴ to crystalline phases¹⁴,¹⁵⁻¹⁹ and even glass composites¹⁴,²⁰⁻²³; however,

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

© 2024 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society.

<footnote n="1">¹Nantes Université, Université Angers, Le Mans Université, CNRS, UMR 6112, Laboratoire de Planétologie et Géosciences, Nantes, France</footnote>

<footnote n="2">²Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN, Nantes, France</footnote>
each matrix has advantages and drawbacks, and there is
currently no ideal solution.

Recent works<sup>24-26</sup> proposed the use of high-pressure synthesis conditions for producing I-bearing aluminoborosilicate glasses. Using relevant chemical glass compositions such as typical low-activity waste glass compositions,<sup>10,27,28</sup> they demonstrated that using high-pressure (up to 2 GPa) and high-temperature (1250°C) conditions is beneficial for dissolving a large quantity of iodine into the glass structure (up to 2.5 mol.%) corresponding to iodine concentrations several orders of magnitude higher than what is currently achieved for ambient pressure equivalent glasses.<sup>10-14,29,30</sup> Nonetheless, there is still a lot to achieve in order to validate such approach and to establish an industrial-ready protocol. For instance, the influence of I dissolution on the glass structure is currently poorly known, and the essential knowledge comes from the previous studies conducted at ambient pressure.<sup>10-13</sup> Recent study by Vénague et al.<sup>31</sup> on alkali-bearing borosilicate glasses suggested I dissolution induces an increase in the glass degree of polymerization. Although it appears to be consistent with the early established I dissolution mechanisms,<sup>10,11,13</sup> it remains to be demonstrated for I-bearing aluminoborosilicate glass equilibrated under high-pressure conditions. Along with the effect of pressure, the I dissolution could induce a major change in the network structure of the aluminoborosilicate glass that is currently not accounted for. In order to propose a specific immobilization matrix, the main difficulty arises from the fact that both the I dissolution and the pressure conditions induce an effect on the glass structure.

Applying high-pressure conditions is known to increase the average coordination number of network-forming cations<sup>32-43</sup> and to modify the concentration of non-bridging oxygen (NBO).<sup>42,44-46</sup> In detail, for aluminoborosilicate glass compositions, cations with higher field strength (i.e., alkaline-earth) favor the formation of highly coordinated Al species AlO<sub>5</sub> and AlO<sub>6</sub><sup>47-50</sup> owing to their poor charge compensation capacity for AlO<sub>4</sub> and BO<sub>4</sub> species.<sup>51,52</sup> Notwithstanding, the conversion from AlO<sub>4</sub> into higher coordination AlO<sub>5</sub> and AlO<sub>6</sub> upon pressure increase involves a modification in the charge distribution as the charge compensation cation is released from the AlO<sub>5</sub> and AlO<sub>6</sub> units. Conjointly, the conversion from BO<sub>3</sub> into BO<sub>4</sub> requires the presence of a compensating cation.<sup>53-57</sup> Previous studies<sup>58-63</sup> showed that pressure induces an increase in the proportion of BO<sub>4</sub> increasing the N<sub>4</sub> value defined as the ratio [BO<sub>4</sub>]/([BO<sub>4</sub>] + [BO<sub>3</sub>]).

Recent studies by Morizet et al.<sup>24,64,65</sup> showed that charge compensation by alkali or alkaline-earth cations is required for I species: either I<sup>-</sup> or IO<sub>3</sub><sup>-</sup>. The most probable mechanism is the scavenging of cation-positive charges initially ascribed to a network-modifying role and associ-

ated with NBO atoms. The alternative solution involves I
species charge compensation by cations already implied
in a charge-compensating role for AlO₄ and BO₄ species.
In such case, it would force AlO₄ species to convert into
AlO₅ and AlO₆, and BO₄ to convert into BO₃. In the first
case, the I dissolution would result in an increase in the
degree of polymerization due to the decrease in the NBO
concentration. In the second case, the I dissolution would
result in the decrease in the degree of polymerization con-
sidering that the BO₃ species have lower connectivity than
BO₄ species and that AlO₅ and AlO₆ species are considered
mix network-forming and network-modifying species.<sup>66,67</sup>
As the degree of polymerization is related to the chemi-
cal and mechanical durability of glass,<sup>68-73</sup> it is currently
required to determine how exactly the glass structure is
modified by either the pressure or iodine incorporation.
If I dissolution induces an increase in the glass polymer-
ization, as suggested recently in Morizet et al.<sup>74</sup> based on
X-ray photoelectron spectroscopy (XPS), it implies that the
glass chemical composition range that can be used for the
immobilization of <sup>129</sup>I radioisotopes can be extended to
initially less polymerized ones, the parameters of which
would be improved by I-doping.

In the present work, we investigated the change in struc-
ture for I-bearing and I-free sodium aluminoborosilicate
glasses synthesized under high-pressure condition (1 GPa).
I solubility and speciation have been determined. The
network structure (<sup>11</sup>B and <sup>27</sup>Al) is scrutinized with a solid-
state nuclear magnetic resonance (NMR) approach as well
as the local environment for Na atoms, which is the only
available charge compensation cation. A discussion on the
iodine dissolution and its impact on the glass structure is
proposed.

2 | MATERIALS AND METHODS

## 2.1 | Starting material and high-pressure synthesis

We prepared two series of aluminoborosilicate glasses with different B₂O₃ contents: B15N20 with 15 mol.% B₂O₃ and B5N20 with 5 mol.% B₂O₃ (the glass compositions are reported in Table 1) and with a constant Na₂O content of 20 mol.%. The starting materials were synthetized in the SiO₂–Al₂O₃–B₂O₃–Na₂O system from spec-pure oxides (SiO₂, Al₂O₃), carbonates (Na₂CO₃), and boric acid (H₃BO₃). We synthetized the starting glasses using a two-step method: (1) melting of the SiO₂–Na₂O part in a Pt crucible at 1100°C for 1 h under air, (2) melting of the recovered SiO₂–Na₂O glass mixed with Al₂O₃ and H₃BO₃ at 1100°C for 2 h under constant argon flux (20 L/h) in a Pt crucible. The obtained glasses are optically free of crystal but present
<table><caption>TABLE 1 Experimental conditions, major element concentrations, and spectroscopic results.</caption><thead><tr><th rowspan="2">Sample</th><th rowspan="2">Pressure (GPa)</th><th colspan="4">Major elements in mol.% (± 0.2)<sup>a</sup></th><th colspan="6">X-ray photoelectron spectroscopy (XPS)<sup>d</sup></th></tr><tr><th>SiO<sub>2</sub></th><th>B<sub>2</sub>O<sub>3</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>Na<sub>2</sub>O</th><th>I</th><th>R<sup>tb</sup></th><th>K<sup>tc</sup></th><th>(%)</th><th>Γ</th><th>(%)</th><th>I<sup>0</sup></th><th>(%)</th><th>I<sup>5+</sup></th><th>I overall redox state (ORS)<sup>e</sup></th><th>Calculated BO<sub>4</sub> mol.%<sup>g</sup></th><th>Calculated AlO<sub>4</sub> mol.%<sup>g</sup></th><th>Proportion of network modifying Na<sup>+</sup>g</th></tr></thead><tbody><tr><td>B15N20-p</td><td>0.0001</td><td>61.3</td><td>13.1</td><td>5.2</td><td>20.4</td><td>1.14</td><td>3.42</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.64</td><td>16.7</td><td>10.1</td><td>34</td></tr><tr><td>B15N20-1free</td><td>1</td><td>61.3</td><td>13.1</td><td>5.2</td><td>20.4</td><td>1.14</td><td>3.42</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.66</td><td>17.3</td><td>10.1</td><td>33</td></tr><tr><td>B15N20-12</td><td>1</td><td>61.5</td><td>13.1</td><td>5.3</td><td>19.1</td><td>1.0</td><td>1.05</td><td>3.39</td><td>67.8</td><td>32.2</td><td></td><td></td><td></td><td>-0.68</td><td>0.64</td><td>16.8</td><td>10.3</td><td>29</td></tr><tr><td>B15N20-12O5</td><td>1</td><td>62.2</td><td>13.1</td><td>5.3</td><td>18.5</td><td>0.9</td><td>1.03</td><td>3.45</td><td>54</td><td>31.8</td><td>14.2</td><td></td><td></td><td></td><td>0.65</td><td>17.1</td><td>10.2</td><td>26</td></tr><tr><td>B5N20-1free</td><td>1</td><td>65.4</td><td>4.2</td><td>6.1</td><td>24.3</td><td></td><td></td><td>2.34</td><td>6.28</td><td></td><td></td><td></td><td></td><td></td><td>0.76</td><td>6.4</td><td>11.4</td><td>63</td></tr><tr><td>B5N20-12</td><td>1</td><td>66.1</td><td>4.2</td><td>6.0</td><td>22.9</td><td>0.8</td><td>2.22</td><td>6.41</td><td>77.5</td><td>22.5</td><td></td><td></td><td></td><td>-0.78</td><td>0.79</td><td>6.6</td><td>11.4</td><td>61</td></tr><tr><td>B5N20-12O5</td><td>1</td><td>66.6</td><td>4.2</td><td>6.1</td><td>22.0</td><td>1.1</td><td>2.12</td><td>6.4</td><td>84</td><td>16</td><td></td><td></td><td></td><td>-0.84</td><td>0.86</td><td>7.2</td><td>11.6</td><td>57</td></tr></tbody></table>

<sup>a</sup>The glass major element composition has been determined using SEM-EDS except for B<sub>2</sub>O<sub>3</sub>, which has been measured using LA-ICP-MS.

<sup>b</sup>R' = [Na<sub>2</sub>O]/[Al<sub>2</sub>O<sub>3</sub> + B<sub>2</sub>O<sub>3</sub>].

<sup>c</sup>K' = [SiO<sub>2</sub>]/[Al<sub>2</sub>O<sub>3</sub> + B<sub>2</sub>O<sub>3</sub>].

<sup>d</sup>The details of the XPS fitting of the 1 3d peaks are provided in the Supporting Information section.

<sup>e</sup>ORS = -1*|Γ| + 0*|Γ| + 5*|Γ|.

<sup>f</sup>N<sub>4</sub> = [BO<sub>4</sub>]/[BO<sub>3</sub> + BO<sub>4</sub>].

<sup>g</sup>To compute the proportion of network-modifying Na<sup>+</sup>, we used the molar percentage of BO<sub>4</sub> and AlO<sub>4</sub> calculated from the N<sub>4</sub> and the proportion of AlO<sub>4</sub> ([MO<sub>4</sub>] = [M<sub>2</sub>O<sub>3</sub>] × 2 × X<sub>M</sub>, where X<sub>M</sub> is the proportion of tetrahedral cation obtained from NMR fit).

a grayish color, which is believed to originate from remaining CO<sub>2</sub> reduced under argon into nanometric graphite.

The recovered glasses have been crushed into powder and then loaded into Pt capsules (4 mm outer diameter and 13 mm of length) along with iodine for the high-pressure experiments. We used either I<sub>2</sub> or I<sub>2</sub>O<sub>5</sub> as a source for iodine at 4–6 mol.% initial content so as to insure the saturation of iodine with respect to I solubility. The excess I also insures to obtain homogeneous I-bearing glasses after the high-pressure experiments. The use of I<sub>2</sub> and I<sub>2</sub>O<sub>5</sub> is believed to have an important effect on the I solubility and speciation.<sup>25,65</sup> Reference I-free samples have also been prepared for each composition to discriminate the effect of pressure to the effect of iodine incorporation on the glass structure.

High-pressure experiments were carried out using an end-loaded Oxford-type piston-cylinder apparatus at 1.0 GPa and 1250°C. The Pt capsule is loaded into MgO Magnorite ceramics loaded in 1/4 in. talc-Pyrex high-pressure assemblies. Resistive heating is performed with a graphite furnace surrounding the base ceramics. The entire assembly is loaded into a ¼ in. pressure plate. The detailed protocol for conducting the high-pressure experiments is described elsewhere,<sup>75</sup> but it consists of three main steps: (1) pressurizing to 1.0 GPa; (2) heating to 550°C, allowing high-pressure assembly relaxation; and (3) heating up to the final temperature. During the experiment, the temperature is monitored by a Eurotherm with a type S (PtRh<sub>10</sub>/Pt) thermocouple located at the top of the capsule. The pressure is automatically controlled during the experiment using a Stigma needle pump. The accuracy is ±1°C and ±10% for temperature and pressure, respectively. A fast isobaric quench is performed 2 h after the beginning of the experiment by turning the power off, with an estimated quench rate of 100°C/s. The recovered glass chips are coated with dark residues turning brown-yellow to red when washed with ethanol, corresponding to iodine residues. This indicates that the I saturation was attained, and the I content measured in the glass corresponds to the iodine solubility for each composition at given intensive conditions. Under an optical microscope, the glasses are colored yellow and free of bubbles and crystals.

## 2.2 | Scanning electron microscopy and energy-dispersive spectrometry (SEM-EDS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)

Chemical analysis and imaging have been performed using a Jeol JSM 5800LV scanning electron microscope (SEM) coupled to an SSD SAMx energy-dispersive spectrometer
(EDS). Prior to analysis, glass chips have been mounted in epoxy plots and polished down to ¼ µm before carbon coating (15 nm). The analytical conditions were 15 kV for voltage and 0.5 nA for current (regularly controlled). The spot size is set to approximatively 20 µm to avoid Na loss under the electron beam. Each sample has been analyzed 5–10 times on different glass chips with a measurement time of 1 min to check for any heterogeneity. Internal standards have been used for the element quantification: corundum for Al₂O₃, wollastonite for SiO₂, NaCl for Na₂O, and RbI for I. The uncertainty on element measurement is estimated to be about 0.2 mol.%. However, we measured an average standard deviation of about 0.13 mol.% from the replicated measurements, which is considered representing the homogeneity in the element distribution within the glass. As boron is a very light element (and because part of the signal is absorbed by the beryllium windows), the B₂O₃ quantification by EDS shows important uncertainty even with adapted internal standards and is not quantified with this technique in our study.

Laser ablation–inductively coupled plasma-mass spectroscopy (LA-ICP-MS) is performed for B₂O₃ quantification, on a quadrupole ICP-MS (Varian 820-MS) with ablation by an ArF excimer laser (193 nm, Analyte G2, Photon Machines) in an HelEx II 2-volume cell with He as carrier gas. The laser energy density is 4.54 J cm⁻² with a repetition rate of 10 Hz. The spot size is set to 110 µm, and the acquisitions are performed on 30 s with points collected every 0.25 s (120 pts per analysis), preceded, and followed by a 30 s blank acquisition. In these conditions, the pit size is estimated to be about 20 µm in depth. We realized five analyses on each glass sample. The data were calibrated using internal standards to quantify any possible drift.

Boron and aluminum have been recorded, and the latter one is used as an internal standard to adjust B₂O₃ quantification (i.e., Al₂O₃ content by LA-ICP-MS is referenced against measurement from SEM–EDS that is believed robust). We used the aluminoborosilicate glasses investigated by Jolivet et al.⁷⁶ that have been characterized using multiple techniques (SEM–EDS, ¹¹B magic angle spinning [MAS] NMR, and ICP optical emission spectrometry) for the quantification of B₂O₃. The LJ5 and ISG glasses investigated in Jolivet et al.⁷⁶ with 25.2 and 16.5 mol.% B₂O₃, respectively, were used for the LA-ICP-MS calibration of the B₂O₃. The standard deviation on the measured B₂O₃ is better than 0.6 mol.% for most of the glasses.

## 2.3 | Solid-state nuclear magnetic resonance (NMR)

¹¹B, ²³Na, and ²⁷Al NMR data were acquired on a Bruker Avance III 500 MHz (11.7 T) spectrometer with a 2.5 mm

MAS probe at the Larmor frequency of 160.46, 132.29, and 130.32 MHz, respectively. The glass powder was loaded into the ZrO₂ rotor with Teflon caps. ¹¹B and ²⁷Al MAS spectra were acquired at an MAS frequency of 20 kHz with a single π/12 excitation for a radio frequency field strength of 20 kHz. For ²³Na MAS spectra, the MAS frequency was set to 30 kHz, and we used a single π/8 excitation (r.f. field strength of 20 kHz). The recycle delays between consecutive scans were 5 s for ¹¹B and 1 s for both ²⁷Al and ²³Na. ¹¹B, ²⁷Al, and ²³Na spectra are referenced against H₃BO₃ solution at 19.6 ppm, Al(NO₃)₃ solution at 0 ppm, and NaNO₃ solution at 0 ppm, respectively.

The ¹¹B 3Q-MAS spectra were acquired at an MAS frequency of 20 kHz and a recycle delay of 1 s. We used a four-pulse sequence, including Z filter. The 3Q excitation pulse length was 3.9 µs for an r.f. field of 140 kHz. For 3Q reconversion, we used a 12.5 µs (quarter of the MAS period) divergent DFS (double frequency sweep) pulse from 100 kHz to 1.5 MHz using an r.f. field strength of 130 kHz.⁷⁷

The ¹¹B SQ–DQ (single quantum–double quantum) homonuclear correlation spectra were acquired using the BR2₁ pulse scheme for dipolar recoupling.⁷⁸ We used π-pulse lengths of 50 µs and short excitation/reconversion times of 400 µs. The MAS frequency was 20 kHz, and the recycle delay was 3 s.

## 2.4 | X-ray photoelectron spectroscopy (XPS)

We performed XPS on a Kratos Axis Ultra with an X-ray monochromatic Al Kα source at 1486.6 eV and 300 W. The glass chips were scrubbed with ethanol and dried (air) before analysis to remove possible remaining iodine from the surface and loaded into the sample chamber operating under high vacuum (<8.10⁻⁸ mbar). The beam size on the sample is 300 × 700 µm². We acquired first survey spectra at a pass energy of 160 eV with an instrument resolution of 1.95 eV. The high-resolution spectra were acquired with a 40 eV pass energy and a step of 0.1 eV, with a total acquisition time of 32 min per spectra for I 3d (608–642 eV) to investigate the iodine speciation. Under those conditions, the instrumental resolution is 0.54 eV. The obtained spectra were treated with CasaXPS software. The spectra were referenced against the adventitious C 1s transition at 284.8 eV, and the background was subtracted using a Tougaard function.⁷⁹ The I 3d spectra were fitted with asymmetric Gaussian–Lorentzian lines, and only the I 3d₅/₂ peak was considered for fitting as it shows the higher intensity and resolution compared to the I 3d₃/₂ peak.
3 | RESULTS AND DISCUSSION

**3.1 | Chemical analysis from SEM-EDS and LA-ICP-MS**

The iodine and major oxide contents in mol.% from SEM-EDS are given in Table 1. The data are corrected from the B₂O₃ content obtained with LA-ICP-MS. The I content in all samples is somehow comparable given the ±0.2 mol.% estimated error on I analysis, with 1.0 and 0.9 mol.% I for B15N20-I₂ and B15N20-I₂O₅, respectively, and 0.8 and 1.1 mol.% I for B5N20-I₂ and B5N20-I₂O₅, respectively. Strangely, both glass series exhibit comparable I content at ~1 mol.%. We expected a higher I solubility for the B15N20 glass series compared to the B5N20 one, considering that B₂O₃ is thought to have a positive impact on iodine solubility by losing the glass network.³⁰ One possible explanation is that for “low” boron content, I solubility is not controlled by the concentration of B₂O₃ itself but by the Na₂O concentration. As a matter of fact, iodine, whether it is dissolved as I⁻ or I⁵⁺ is systematically surrounded by Na⁺ cations for charge compensation.²⁵,²⁶,⁶⁴,⁷⁴ In the investigated glass compositions, the Na₂O content is roughly the same (see Table 1), which could lead to comparable I solubility. Nonetheless, we were expecting a higher I solubility for glasses loaded with I₂O₅ (which decomposes into I₂ and O₂ above 350°C) as compared to glasses loaded with I₂. Previous studies²⁵,⁶⁵ showed that I solubility in nuclear waste glasses is enhanced when an oxidized I source (I₂O₅) is used in the experiment. However, the I 3d XPS measurements (Table 1 and Figure S1) showed that I speciation is almost identical. For instance, we obtained 68% I⁻, 32% I⁰ and 54% I⁻, 32% I⁰, and 14% I⁵⁺ for B15N20-I₂ and B15N20-I₂O₅ glasses, respectively. In a recent work,⁷¹ for comparable glass compositions, the use of I₂ or I₂O₅ does not induce a dramatic change in I solubility for identical pressure conditions. It should be pointed out that the measured I solubility in our B15N20 glasses is comparable to the I solubility reported in Morizet et al.⁷⁴ for similar glass composition and intensive conditions. Within error, we can consider that I solubility is almost constant for the investigated glass series and for the different I sources. Another point to address is the control of the red/ox state in the capsule during the experiment. As the final iodine speciation is roughly the same, we conclude for these samples that adding I₂O₅ has little effect as compared to other samples from previous studies.²⁵,⁶⁴

**3.2 | Network cation local environment characterization using NMR spectroscopy**

The NMR spectra for ²⁷Al are shown in Figure 1 and presented as normalized to the area. The ²⁷Al NMR spectra

are shown along with the experimental conditions (i.e., pressure and I source), I solubility, and derived Al coordination number (CN<sub>Al</sub>). An example of fit is presented at the bottom of the figure, and all fits are presented in Figure S2. For both compositions, the ²⁷Al spectra are representative of aluminoborosilicate glasses with low Al₂O₃

<figure><img src="image_7.png" /><figcaption>**FIGURE 1** ²⁷Al nuclear magnetic resonance (NMR) spectra of the two series: (A) B5N20 and (B) B15N20. The black dashed line corresponds to the pristine sample, the full black line corresponds to the high-pressure iodine-free sample, the full green and purple lines correspond to the high-pressure I-doped glasses, with I₂ and I₂O₅, respectively. The average coordination number is calculated from the fit, and the I content is resulting from scanning electron microscope/energy-dispersive spectrometer (SEM/EDS) measurements.</figcaption></figure>
and high Na₂O contents.⁵¹,⁸⁰ The spectra are constituted by a very intense IVAl component at 61 ppm, a small shoulder located around 35 ppm attributed to VAl, and no evidence for the presence of VIAl signal at ~0 ppm. (Details of the NMR fit in the Supporting Information section.)⁴⁸,⁵⁴,⁵⁵,⁶³,⁸⁰⁻⁸³

In Figure 1, we observe no evolution of the ²⁷Al NMR line shape as a function of I content or I source. This indicates that the Al local environment is not perturbed by the I dissolution in the glass network. We simulated the B15N20-p spectrum with two Czjzek lines⁸⁴,⁸⁵ corresponding to IVAl and VAl species in the DMfit software.⁸⁶ The obtained NMR fit parameters were used to fit the ²⁷Al NMR spectra for I-bearing sample. Within slight minor adjustments, we obtained the same NMR parameters (i.e., isotropic chemical shift, $\delta_{iso}$, quadrupolar coupling constant, $C_q$, and the width of the Gaussian distribution of isotropic chemical shift, FHWM CS; see Supporting Information section) for these samples, suggesting that AlO₄ and AlO₅ local environments are not affected by the I dissolution. We followed the same protocol for the B5N20 series (Figure 1) and also did not observe any change in the derived NMR parameters between the I-free and I-bearing glasses, suggesting an absence of I effect on Al local environment.

In the acquisition of the ¹¹B MAS NMR, the glass signal is hindered by the boron signal coming from the NMR probe and the first satellite transition. Hence, both need to be removed prior fitting. The protocol followed to clean up the signal is detailed in previous works,⁷⁶,⁸⁷,⁸⁸ and consists of two main steps: (1) subtraction of the probe signal using a reference spectrum of an empty rotor acquired in the same conditions; (2) subtraction of the signal from the first satellite transition by subtracting the same spectra shifted by the MAS rotation frequency (20 kHz). The treated ¹¹B MAS NMR spectra are shown in Figure 2 for the two glass series and normalized to the area.

The ¹¹B spectra are composed of a quasi-symmetric peak located around 0 ppm related to boron in tetrahedral coordination (BO₄) and a broad asymmetric band centered around 12 ppm related to boron trigonal plan coordination (BO₃). For the B5N20 series, Figure 2A, we observe a very intense BO₄ signal and a very weak and poorly resolved BO₃ signal. This result was expected given the low B₂O₃ content (~5 mol.% B₂O₃) and relatively high Na₂O content (ranging from 18.6 to 20.5 mol.% Na₂O), as previous models⁵³,⁵⁶ showed that Na₂O favors the formation of BO₄ units for composition with such R' and K' values (1.1 and 3.4, respectively). For the B15N20 series shown in Figure 2B, on the other hand, we observe a much more intense BO₃ component but with still a dominant BO₄ peak as expected from the composition (about 13 mol.% B₂O₃ and 22.0–24.3 mol.% Na₂O).

<figure><img src="image_8.png" /><figcaption>FIGURE 2 ¹¹B nuclear magnetic resonance (NMR) spectra of the two series: (A) B5N20 and (B) B15N20. The N₄ is obtained from the fit (example shown at the bottom of part B), and the I content is obtained from scanning electron microscope/energy-dispersive spectrometer (SEM/EDS) measurements.</figcaption></figure>

Those bands are composed of multiple signals arising from different environments of the given BO₃ or BO₄. It is common to use different Gaussian lines to fit the peak corresponding to tetrahedral boron, based on the number of surrounding Si atoms. In glasses with high K' (K' = [SiO₂]/[Al₂O₃ + B₂O₃]), the proportion of B₂O₃ is much less than the proportion of SiO₂, which favors the formation of tetrahedral boron with four surrounding
silicon atoms. On the other hand, when the ratio of SiO₂ over B₂O₃ is lower, the number of tetrahedral boron linked to four SiO₄ tetrahedra will decrease as more B is available. From the NMR point of view, a tetrahedral boron linked to four silicon atoms [BO₄(0B,4Si)] will stand on lower frequencies in the spectrum than a tetrahedral boron linked to one or several BO₃ [e.g., BO₄(1B,3Si)].⁸⁹⁻⁹¹

One approach to determine if boron species are present around BO₄ is the SQ-DQ method, which allows to perform dipolar homonuclear correlation.⁹²⁻⁹⁵ This method applied to boron gives information on the presence of BO₃ in close vicinity to either BO₃ or BO₄, and then on the possibility to have boron in the direct surrounding of a tetrahedral boron. From the SQ-DQ, the projection of the spectra (see Figure S3) on the direct dimension allows us to identify some BO₄ species present in the sample.⁹⁰,⁹⁶ Indeed, on this projection, the peak centered near 0 ppm corresponds to the tetrahedral boron and can be fitted with Gaussian lines, which will reveal the position of each BO₄ environment (i.e., BO₄[1B,3Si], BO₄[2B,2Si]) but not the BO₄(0B,4Si) as no boron-to-boron interaction is taking place in this case. The SQ-DQ spectra for our samples and their projections used to prepare the following fit are available in Figures S3 and S4.

There is a common agreement for the boron in planar triangle configuration that multiple environments can be found in the region around 12 ppm.⁹⁰,⁹⁷,⁹⁸ We decided to fit this part of the spectra with two components corresponding to BO₃ in boroxol ring formation (at higher frequency) and BO₃ that are not in a ring formation (at lower frequency), as this is one of the most common fitting methods.⁸⁸,⁹⁹ From the NMR point of view, it is complex to attribute a line function that would authentically reveal the complexity of these species. The most recent studies are using functions based on the shape of the crystalline line with a Gaussian distribution of the associated parameters¹⁰⁰ (isotropic chemical shift: δᵢₚₐ, quadrupolar coupling constant: Cq, and asymmetry parameter: ηq). With these information, it is possible to establish a fit of the ¹¹B MAS NMR spectra with the DMfit software.⁸⁶ We used Gaussian line shapes to fit the low-frequency part of our spectra as BO₄ exhibits very low quadrupolar coupling constant.⁸⁷,⁸⁹,¹⁰⁰ We first placed the line corresponding to BO₄[1B,3Si] with fixed position and width based on the results we obtained from fitting the projection of the SQ-DQ spectra. Then we added a free Gaussian line at lower chemical shift values, accounting for BO₄[0B,4Si]. We initialized the high-frequency part (BO₃ lines) with two bands with δᵢₚₐ, Cq, and ηq (and the associated distributions) coherent with previous studies on aluminoborosilicate glasses.⁸⁷⁻⁹¹ The BO₃ frequency region is difficult to investigate, so we fitted in parallel the MQMAS spectrum corresponding to each glass in order to maintain coherence

with 1D spectra (only for the B15N20 series as there is too few BO₃ in the B5N20 one and its 1D spectra is fitted with a unique component). With this method to fit the BO₃ part of the spectra, and the fixed BO₄[1B,3Si], we observe a missing intensity in the BO₄ region around 1 ppm in chemical shift.

Several boron units can be at the origin of this missing signal. First, the BO₄[2B,2Si] which would appear at 0.8-1.5 ppm higher than the BO₄[1B,3Si]. However, the SQ-DQ indicates that those species are not present, which is consistent with the low B/Si ratio suggesting that enough Si is present to surround all the BO₄ tetrahedra. Similarly, the energetically unfavorable AlO₄-BO₄ bonding is not expected in our glasses given the very low Al₂O₃ content (≤6 mol.%) and the usually good Si-Al mixing in glasses with similar compositions. Finally, it is known that NBO can make the NMR signal of the network former unit shift to higher chemical shift values (as it is the case for ²⁹Si¹⁰¹⁻¹⁰³). Currently, there is no evidence of BO₄-NBO in aluminoborosilicate glasses, although those species are predicted to take place in glasses rich in both BO₄ and NBO species.⁹⁵,¹⁰⁴ Thus, as the origin of the missing signal is unclear and deserves in-depth studies beyond the scope of the paper, we have added one or two Gaussian lines in the BO₄ region in order to correctly fit the spectra and obtain a reliable N₄ value (fit results are available in Table S2).

A typical fit is shown at the bottom of Figure 2B, and the final N₄ value is displayed in Table 1. For the B15N20 series, we can observe that the proportion of BO₃ ring is decreasing between the pristine sample and the high-pressure I-free one of about 3%, where the BO₃ non-rings are constant. Alongside, for the tetrahedral boron, the most noteworthy feature is the increase in the proportion of BO₄(1B,3Si) of about 3%. It seems at first sight that planar borons in ring configurations are more prone to form BO₄ in the B15N20 series when pressure increases. When iodine is introduced into the glass, we observe no change in the proportion of ring and non-ring BO₃, but there is a near 1:1 exchange of BO₄(1B,3Si) in favor of BO₄(0B,4Si) (the last Gaussian line remaining almost constant). This result seems to indicate that the mixing between the BO₄ and SiO₄ units increases when iodine incorporates the glass network. On the other hand, for the B5N20 series, it is complicated to evaluate the change in the proportions of the different boron species given the very low B₂O₃ content. As a matter of fact, as the measured N₄ is about 0.8, we expect that less than 2 mol.% BO₃ is present in the glass as calculated from absolute B₂O₃ concentration ([BO₃] = 2·[B₂O₃]·(1 - N₄)); therefore, a reliable quantitative simulation of the proportion of ring and non-ring BO₃ is difficult. Moreover, the SQ-DQ analysis is not sensitive enough to qualify the presence of BO₃-BO₃ links
<figure><img src="image_9.png" /><figcaption>FIGURE 3 The fraction of four-coordinated boron (N₄) plotted against I content. Black symbols are standing for the measured data, red symbols are N₄ calculated with the Du and Stebbins⁵⁷ model based on the chemical composition only.</figcaption></figure>

with these few BO₃ contents. However, the position of the BO₄(1B,3Si) line found with the SQ-DQ analysis is noteworthy lower than expected. Indeed, usually, BO₄(1B,3Si) are found near 1 ppm and BO₄(0B,4Si) at about -1.2 ppm. In the B5N20 glass, we found BO₄(1B,3Si) at -1.5 ppm (see Supporting Information section). This suggests that the whole BO₄ region may be shifted to lower frequency, as seen in Jolivet et al.⁷⁶

The N₄ is plotted against I solubility in Figure 3. The calculated error bars obtained from the fit are within the data point and on the order of ±0.025. We compare our result from the N₄ calculated using the model of Du and Stebbins⁵⁷ for each sample, considering the slight changes in glass composition (see Table 1). The first noticeable feature is the non-negligible difference between the calculated and the measured N₄ values; nevertheless, the N₄ shift between the two glass series seems to be equivalent, indicating that the changes in composition affect in the same way the calculated and the measured N₄. Hence, the difference between the calculated and measured N₄ values could be due to a possible Na configuration that is not accounted for in the model for N₄ calculation that induces a decrease in the N₄ value (i.e., clustering of planar boron with NBO).

We observe that the N₄ values for the B15N20 glass series seem not to be much affected by I content or pressure: N₄ ranging from 0.64 to 0.66. The effect of pressure was expected to be very low here as the pressure range stud-

ied is rather small, and changes in the network forming species environment are often found to appear above 1 GPa depending on the composition.³⁷,¹⁰⁴,¹⁰⁵ For the B5N20 glass series, we observe almost no change in the N₄ values between the high-pressure I-free sample and the I-bearing loaded with I₂ sample: N₄ changing from 0.76 to 0.79. On the contrary, there is a clear difference with the sample loaded with I₂O₅ with N₄ increasing to 0.86. At first glance, we could attribute this change to the slight variation in the Na₂O content (2 mol.% between B5N20-I₂ and B5N20-I₂O₅) that would decrease the NBO concentration and then promote the formation of BO₄ (BO₄ are converted into BO₃ at high alkali content to promote NBO formation⁵⁶). As this change in the Na₂O content produces a change in the R' value (R' = [Na₂O]/[Al₂O₃ + B₂O₃]), which is known to greatly affect the glass structure,⁵³,⁵⁶,⁵⁷ this would affect in the same way the calculated data. However, the observed increase in the calculated N₄ barely represents half the increase observed for the measured N₄.

## 3.3 | Charge-compensating/Network-modifying cation local environment characterization using NMR spectroscopy

We used ²³Na NMR as a proxy for the evolution of the BO₄ species proportion, considering that Na⁺ in our glasses is the only cation that compensates the excess negative charge on BO₄ and AlO₄ units. The ²³Na NMR spectra are displayed in Figure 4 and consist in a simple asymmetric line with a peak maximum between -10 and -25 ppm. For both B5N20 (Figure 4A) and B15N20 (Figure 4B), there is no visible change in the line shape, suggesting a negligible effect of either pressure conditions or I content, regardless of its initial shape.

We fitted the ²³Na NMR spectra with a simple Czjzek line.⁸⁴,⁸⁵ The obtained NMR parameters are provided in the Supporting Information section. It has been demonstrated¹⁰⁵⁻¹¹⁰ that the derived parameters for ²³Na (i.e., δᵢₐ and Cq) could be related to the local configuration of the Na atoms in the glass structure: The isotropic chemical shift δᵢₐ is linked to the ⟨Na-O⟩ bond length, and the quadrupolar coupling constant Cq reflects the change in the symmetry of the electric field gradient, linked to change in the Na⁺ coordination number. We plotted in Figure 5 the obtained ²³Na Cq versus δᵢₐ in order to differentiate whether the Na⁺ cations are mainly network modifiers (higher Cq and δᵢₐ) or Na⁺ cations are mainly charge compensators (lower Cq and δᵢₐ). In this figure, we can observe two distinct behaviors for the investigated glass series: the data points for B5N20 fall in the network modifier area, whereas the data points for B15N20 fall in the charge compensator area. This indicates that
<figure><img src="image_10.png" /><figcaption>FIGURE 4 $^{23}$Na nuclear magnetic resonance (NMR) spectra of the two series: (A) B5N20 and (B) B15N20. The NMR parameters ($\delta_{iso}$ and $C_q$) are displayed in Table S1 and are used to plot Figure 5.</figcaption></figure>

$Na^+$ cations are used mainly to create NBOs for glasses in the B5N20 series, whereas $Na^+$ cations are mostly used as a charge compensator role for $BO_4$ and $AlO_4$ for glasses in the B15N20 series. For the B5N20 series, data points are overlapping, which indicates that the average role of sodium remains constant upon iodine dissolution. On the other hand, for the B15N20 series, we observe an important shift toward lower $C_q$ and $\delta_{iso}$ values. Respectively, the $\delta_{iso}$ is shifting from -5.6 in the high-pressure I-free B15N20 sample, to -6.1 when $I_2$ is loaded, and -7.1 ppm when $I_2O_5$ is loaded. The $C_q$ is simultaneously shifting from 2.9

<figure><img src="image_11.png" /><figcaption>FIGURE 5 $^{23}$Na nuclear magnetic resonance (NMR) quadrupolar coupling constant ($C_q$) plotted against isotropic chemical shift ($\delta_{iso}$). $Na^+$ in a network modifying role have higher $C_q$ and higher $\delta_{iso}$. $Na^+$ in a charge-compensating role have lower $C_q$ and lower $\delta_{iso}$. Some relevant glasses from the literature have been added for comparison.</figcaption></figure>

to 2.7 MHz. This indicates that the proportion of sodium that acts as a charge compensator is increasing. With the proportion of $AlO_4$ and $BO_4$ for these series, we can calculate the proportion of Na used for charge compensation ($[AlO_4 + BO_4]/(2.[Na_2O])$). We observe that the proportion of $Na^+$ in a network-modifying role is very high for B5N20-I-free (63%, Table 1) and decreases for B5N20-I$_2$ and B5N20-I$_2O_5$ to 61% and 57%, respectively. For the B15N20 series, the proportion of network modifying sodium is lower as more $Na^+$ are used to compensate tetrahedral boron. However, we observe the same variations, with a higher network-modifying $Na^+$ content for B15N20-I-free (33%) and lower content for B15N20-I$_2$ and B15N20-I$_2O_5$ with 29% and 26%, respectively. As the proportion of $BO_4$ and $AlO_4$ is almost constant along a series, we can attribute the major part of this change to the decrease in the $Na_2O$ content and, to certain extent, to iodine (that is present as I- or $IO_3^-$) that is expected to scavenge $Na^+$ on NBO for its compensation. It is tough to discern both effects, but as 1 mol of I$^-$ (or $IO_3^-$) requires 1 mol of $Na^+$,⁶⁴ a non-negligible part of the observed shift can be attributed to iodine. The changes in the proportion of network modifying $Na^+$ for both series are about the same order, but here again, the larger proportion of initial network modifying $Na^+$ for the B5N20 series dominates the signal, masking any effect of changes in composition or iodine content. Finally, as the measured I content for B15N20-I$_2$
and B15N20-I₂O₅ is about the same (1 mol.%), the portion of the observed shift on Figure 5 attributed to iodine must be the same for both samples. It is important to note that the iodine content in these samples is relatively small compared to the sodium content. Therefore, it is challenging to draw conclusions based on the small variations observed by magnetic resonance.

## 3.4 | Predicted non-bridging oxygen regime and implication for iodine solubility—changes in the glass polymerization

According to the model proposed in previous studies,⁵⁶,⁵⁷,¹¹¹ the presence of NBOs and their relationship to Si and B networks can be inferred on the basis of the calculated R' value. For aluminoborosilicate glasses, we can distinguish three main regimes in which the nature and proportion of NBOs vary: (1) No NBOs are expected in glasses with low R' (R' < 0.5 + K'/16), (2) only NBOs on SiO₄ species are expected in glasses with R' ranging from 0.5 + K'/16 to 0.5 + K'/4, and (3) NBOs are expected on both SiO₄ and BO₃ species in glasses with R' is above 0.5 + K'/4 and below K' + 2. Those three regimes correspond to the three colored regions in Figure 6 plotted with our data points. The B15N20 glass series lies on the NBO-Si region, with 100% of NBO present linked to SiO₄ forming Q³ species (only one NBO per tetrahedron). The B5N20 glass series lies on the NBO-Si/NBO-B region, with a majority of Q³, few Q² (two NBO per tetrahedron), and almost no NBO-B, which is consistent with the low BO₃ content.

With that in mind, we tried to establish simple chemical reactions for the iodine dissolution in the glass that could describe the change in the glass structure. For B15N20, as only NBO-Si species are available, iodine will use available Na⁺ cations associated with Q³ units, such as:

$$2(\text{Na}^+ \cdots \text{O} - \text{Si}) + \text{I}_2 \rightarrow \text{Si} - \text{O} - \text{Si} + 2\text{NaI} + 1/2\text{O}_2 \quad (1)$$

$$2(\text{Na}^+ \cdots \text{O} - \text{Si}) + \text{I}_2\text{O}_5 \rightarrow \text{Si} - \text{O} - \text{Si} + 2\text{NaIO}_3 \quad (2)$$

Equations (1) and (2) are valid for I₂ and I₂O₅ incorporations, respectively. Owing to the very large presence of NBO-Si in B5N20 glass series, the I dissolution mainly scavenges Na⁺ cations from the Q³ units as it is the case for the B15N20 series. However, there is actually no information on the preferences for iodine to scavenge certain types of NBO, and equation of dissolution considering both the Q² and the boron NBO-B units can be established, such as:

$$2(\text{Na}^+ \cdots \text{O} - \text{B}) + \text{I}_2 \rightarrow \text{B} - \text{O} - \text{B} + 2\text{NaI} + 1/2\text{O}_2 \quad (3)$$

<figure><img src="image_12.png" /><figcaption>FIGURE 6 The fraction of four-coordinated boron (N₄) plotted against the R' value (R' = [Na₂O]/[Al₂O₃ + B₂O₃]). Black symbols are standing for the measured data, red symbols are N₄ calculated with the Du and Stebbins⁵⁷ model based on the chemical composition only. Lines corresponding to the K' value (K' = [SiO₂]/[Al₂O₃ + B₂O₃]) are plotted for information purposes. The colored regions correspond to the three different non-bridging oxygen (NBO) regimes (inexistent/only on SiO₄/on SiO₄ and BO₃).⁵⁶,⁵⁷,¹¹¹</figcaption></figure>

$$2(2\text{Na}^+ \cdots 2\text{O} - \text{Si}) + \text{I}_2 \rightarrow 2(\text{Na}^+ \cdots \text{O} - \text{Si}) + 2\text{NaI} + \text{O}_2 \quad (4)$$

$$2(\text{Na}^+ \cdots \text{O} - \text{B}) + \text{I}_2\text{O}_5 \rightarrow \text{B} - \text{O} - \text{B} + 2\text{NaIO}_3 \quad (5)$$

$$2(2\text{Na}^+ \cdots 2\text{O} - \text{Si}) + \text{I}_2\text{O}_5 \rightarrow 2(\text{Na}^+ \cdots \text{O} - \text{Si}) + 2\text{NaIO}_3 + 1/2\text{O}_2 \quad (6)$$

Such dissolution mechanisms are consistent with the proposed dissolution mechanisms by Morizet et al.⁶⁴ From recent studies,⁶⁴,⁶⁵ we know that iodine (I⁻ or I⁵⁺ in IO₃⁻ clusters) is not compensated by a unique Na⁺ cation in aluminoborosilicate glasses but either by three to four Na⁺ sharing partial charges. This contributes to the integration of iodine in the glass network but implies that I⁻ or IO₃⁻ units will be surrounded by either NBO, tetrahedral species requiring compensation (AlO₄/BO₄) or a mix of both, which is a key information to understand the mechanism of dissolution that can favor iodine leaking from the glass.
We infer that the Na⁺ consumption upon I dissolution is taken from NBO local environments. It implies that I dissolution induces an increase in the degree of glass polymerization in agreement with recent results.⁷⁴ This mechanism has been also suggested in previous studies²⁵,³¹; however, we currently have few evidences for this mechanism, and further, more advanced work is required (i.e., advanced ¹⁷O and ²⁹Si NMR investigations).

In the case of nuclear waste immobilization using vitrification, an increase in the glass polymerization is sought as there will be an increase in the glass fracture toughness (i.e., increase in the mechanical resistance) and an increase in the chemical durability as NBO species are more likely to be altered in humid environment.¹¹²⁻¹¹⁴

From a more general viewpoint, the I dissolution is very similar to the dissolution of other volatile species extensively studied in Earth sciences. For instance, it has been demonstrated that the CO₂ dissolution as CO₃²⁻ and S dissolution as SO₄²⁻ in high-pressure silicate glasses require the charge compensation by available cations (e.g., Na⁺ and Ca²⁺¹¹⁵⁻¹²⁰) and are accompanied by an increase in the glass degree of polymerization (i.e., decreasing NBO/T¹²¹).

An increase in the glass transition temperature (T_g) should be associated with the increase in the degree of polymerization¹²²⁻¹²⁴; however, for CO₂, T_g is not affected or slightly decreases,¹²⁵ which is explained by a two-liquid viscosity mixing. Actually, we suggest that the same process takes place for I dissolution. Jolivet et al.¹²⁶ showed that there is a slight decrease in T_g with increasing I content for depolymerized glass composition that appears to be counterintuitive providing an increase in degree of polymerization occurs with I dissolution.

Previous works²⁹ showed that the presence of B₂O₃ is beneficial for I dissolution; however, our study suggests a more complex picture. There is no doubt that the B₂O₃ is required to promote the I dissolution as it decreases the melting point and softens the melt structure, making it more flexible. However, more predominantly, the quantity and the nature of the charge-compensating cations for I dissolution are at stake, as suggested in previous studies.²⁴⁻²⁵,⁶⁴ Clearly, more work is required in order to investigate the complex interplay between the boron network and the silicon network. For instance, direct evidence for Q³, Q², and NBO-B units decreasing upon I dissolution remains to be observed.

## 4 | CONCLUSIONS

We have examined two series of aluminoborosilicate glasses with different boron contents synthesized under

high-pressure conditions with and without iodine, using ¹¹B, ²³Na, and ²⁷Al solid-state NMR and I 3d XPS. These analyses revealed that the boron and aluminum environments are similar to previously reported NMR results for Na-rich compositions. Pressure has no effect on the CN<sub>Al</sub> and little effect on the N<sub>4</sub>, as expected for such compositions.

For all I-doped samples, the I content was measured to about 1 mol.%, mainly I⁻ (54%-84%) and I⁰ (16%-32%). However, one sample doped with I₂O₅ shows an I⁵⁺ content of about 14%. For all samples, we have shown that the I incorporation has little or no effect on the Al³⁺ and B³⁺ environments but has a slight influence on the Na⁺ cation local environment. This effect is only noticeable for the high-B₂O₃ series, where the Na mainly plays a charge-compensating role to form BO₄ and AlO₄. We attribute this feature to the compensation requirement for the I⁻ and IO₃⁻ molecular units, scavenging Na⁺ from NBO sites either Q³ for the B15N20 series, or Q² and NBO-B for the B5N20 series. Further studies on the behavior of compensating cations in such glasses would be useful to understand to what extent the environment of the network forming cations can be modified by iodine. Finally, as iodine appears to use Na⁺ from NBO sites, we expect an increase in glass polymerization, which can be demonstrated by ¹⁷O NMR on labeled samples.

## ACKNOWLEDGMENTS

The authors are grateful to the Agence Nationale de la Recherche, which funded the current work through the ANR project "Iodine-CLEAN-UP" (ANR-20-CE08-0018). The authors also thank the Région Pays de la Loire for financing the present research through the "Trajectoire nationale de la recherche ligérienne" program. The authors thank the Laboratoire de Planétologie et Géosciences, Nantes Université and the CNRS for providing access to the analytical facilities, and Carole La for support for the LA-ICP-MS analysis. Solid-state NMR, XPS, and SEM/EDS measurements were performed on IMN's equipment platform, PLASSMAT, Nantes, France. The authors would like to thank J. Hamon (XPS) and N. Stephant (SEM/EDS) for the measurements.

## ORCID

Sami Soudani ID https://orcid.org/0009-0001-9301-2566

Yann Morizet ID https://orcid.org/0000-0001-9599-245X

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**How to cite this article:** Soudani S, Paris M, Morizet Y. The effect of iodine on the local environment of network-forming elements in aluminoborosilicate glasses: An NMR study. J Am Ceram Soc. 2024;107:4557–71.

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