# Deciphering vanadium redox and structure in aluminosilicate glass: A spectroscopic study

Adrien Donatini, Peggy Georges, Tiphaine Fevre, Laurent Cormier, Daniel R Neuville

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Adrien Donatini, Peggy Georges, Tiphaine Fevre, Laurent Cormier, Daniel R Neuville. Deciphering vanadium redox and structure in aluminosilicate glass: A spectroscopic study. Journal of Non-Crystalline Solids, 2025, 666, pp.123689. 10.1016/j.jnoncrysol.2025.123689. hal-05355947

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# Deciphering vanadium redox and structure in aluminosilicate glass: A spectroscopic study

Adrien Donatini<sup>a,b,c</sup>, Peggy Georges<sup>a</sup>, Tiphaine Fevre<sup>a</sup>, Laurent Cormier<sup>b</sup>, Daniel R. Neuville<sup>*</sup>,<sup>c</sup>

## ARTICLE INFO

### Keywords:

Vanadium

Aluminosilicate

Raman spectroscopy

Optical absorption spectroscopy

XANES spectroscopy

redox

## ABSTRACT

Vanadium oxide can be used to color silicate glasses and to influence their crystallization behavior. However, vanadium's role in glass is strongly dependent on its redox state and local structure, both of which are poorly understood. In this study, XANES, Raman and optical absorption spectroscopy were used to determine vanadium redox state in an aluminosilicate glass. Raman spectroscopy of oxidized and reduced samples allowed for the attribution of the Raman signatures of vanadium ions. The species present in the glass synthetized under air are $[V^{5+}O_5]^{3-}$, $[V^{5+}O_4]^{3-}$ and $[V^{4+}O_5]^{6-}$. When the glass is synthesized under reducing conditions, the redox state is a mixture of $V^{3+}$ and $[V^{4+}O_5]^{6-}$. Our results show that optical absorption spectroscopy is well-suited to monitor changes in $V^{4+}$ concentration, while Raman spectroscopy allows the monitoring of $V^{5+}$. Vanadium reduction mechanism was investigated by performing Raman and optical absorption spectroscopy measurements along a reduction profile. Our results show that $V^{3+}$, $V^{4+}$ and $V^{5+}$ coexist over a range of oxygen fugacity. This leads to sample color not being linear with oxidation state as the coexistence of $V^{3+}$ and $V^{5+}$ might be responsible for the rise of another optical absorption that darkens the glass.

## 1. Introduction

The addition of transition elements in oxide glasses can be used to modify numerous properties of the glass, including color [1,2], lumi- nescence behavior [3], viscosity [4] or bioactivity [5,6]. Most of these transition elements can be present in different oxidation states in oxide glasses. Each of these oxidation states will contribute to the aforemen- tioned properties differently. For instance, it has been shown that a reduction of iron leads to a decrease of melt and glass viscosity in alkali silicate glasses [4,7]. The control of the redox state of transition ele- ments is therefore paramount to obtain the desired properties. The redox state is dependent of numerous parameters such as temperature, oxygen fugacity, glass composition and the presence of other multivalent ele- ments [8–12]. Controlling all those parameters is challenging and a major hurdle in the fabrication of glasses containing transition elements.

Vanadium is a multivalent transition element that can be found in four different oxidation states in silicate glasses ($V^{2+}/V^{3+}/V^{4+}$ and $V^{5+}$) [13,14]. This range of stable oxidation states results in vanadium

bearing oxide glasses exhibiting colors from purple via green to yellow [1,15]. Moreover, molecular dynamic simulations show that vanadium ions can be found with different coordination numbers such as $[V^{5+}O_4]^{3-}$, $[V^{5+}O_5]^{5-}$ and $[V^{5+}O_6]$ for pentavalent vanadium alone [16]. This diversity of coordination number have been somewhat verified experimentally with XANES [14] and Raman spectroscopy [15]. Pentavalent $V_2O_5$ is often considered as a network former due to its similarity with $P_2O_5$ [17–19]. Therefore, the incorporation of vanadium within the glass network implies that controlling its redox state and local environment can directly influence glass connectivity [20].

Over the past decades, vanadium bearing oxide glasses have been investigated for potential applications as cathode materials [21,22]. Vanadium oxide is used in various glass-ceramics as a coloring agent [23,24]. However, its impact on nucleation and crystal growth remains poorly understood [25,26]. Crystallization studies have shown that four coordinated pentavalent vanadium acts as a network former, increasing network polymerization and slowing down crystallization [27]. In addition, vanadium is of particular interest for nuclear waste

<footnote><sup>a</sup> Corning European Technology Center, Avon 77210, France</footnote>

<footnote><sup>b</sup> Sorbonne Université, Muséum National d'Histoire Naturelle, UMR CNRS 7590, Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, IMPMC, Paris 75005, France</footnote>

<footnote><sup>c</sup> Geomat Lab, CNRS-IPGP, Université de Paris, Paris 75005, France</footnote>

<footnote>* Corresponding author.
E-mail address: neuville@ipgp.fr (D.R. Neuville).</footnote>
applications as it greatly improves sulfur solubility within the glass matrix [28,29]. This increase in sulfur solubility has been linked to pentavalent vanadium acting as a network modifier, leading to glass depolymerization [28], though it is unclear if this is due to five or four coordinated pentavalent vanadium unit [29,30]. These contradicting examples highlight that the role of vanadium in oxide glasses is not yet fully understood, particularly regarding its redox behavior and local structure. It is possible that vanadium acts as a network former or modifier as a function of its redox state and of the glass composition. In order to better predict the properties of vanadium-doped aluminosilicate glass, it is necessary to determine vanadium redox state and identify the local structures corresponding to its various oxidation states.

In this study, XANES (X-ray Absorption Near-Edge Structure), Raman and optical absorption spectroscopy were used to determine vanadium redox state and local environments in a sodium aluminosilicate glass. The goal is to understand the reduction mechanism of vanadium and define the stability domain of its different oxidation states. Using those techniques, we measured the vanadium redox state along a reduction profile, enabling us to correlate local structure, redox state and sample color.

## 2. Experimental section

### 2.1. Glass synthesis

The base undoped glass used in this study, 68.2SiO₂ -12.5Al₂O₃-19.3Na₂O (mol %), is the same as in a previous investigation [31]. The obtained glass was finely ground in an agate mortar and mixed with V₂O₅ (99 % Supelco, Empura) to achieve a molar concentration of 1 mol % of VO₂. The mixture was then melted four times at 1600 °C, each time for two hours, using a Pt crucible. The sample obtained is labelled NASV with respect to previous studies [31]. Small amounts (<2 g) of NASV were remelted in a graphite crucible at 1500 °C for 15 min. The obtained sample is labelled NASV_C. Sample NASV exhibits a light yellow-green color, while sample NASV_C is of a dark brown color.

Fragments of the NASV sample were remelted in an alumina crucible (5 mm in diameter) at 1400 °C for 3 h. The alumina crucible was then placed inside a graphite crucible preheated to 1000 °C. The temperature was subsequently increased to 1400 °C and maintained for 4h15. The oxygen fugacity was below the detection limit of the zirconia electrode used which is 10⁻⁶ log(atm). The temperature was then reduced to 1000 °C in 1 hour and the alumina crucible was transferred to another furnace preheated at 550 °C, where it was annealed for 12 h. The furnace was then switched off and the sample was allowed to cool to room temperature over approximately ~6 h. The final sample was mirror-polished on both sides to a thickness of about 1 mm. A cross-section of the obtained sample, labelled NASV_profile, is presented in Fig. 1.

### 2.2. Chemical analysis

Electron Probe Micro-analysis (EPMA) measurements were performed on a Cameca SX-Five at the Camparis facility (Sorbonne Université, Paris). A voltage of 25 kV and a beam size of 20 µm were used for all measurements. For Si, Na and Al, a current intensity of 4 nA and a counting time of 10 s were used, while a current intensity of 200 nA and a counting time of 20 s were applied to measure the low concentrations of V and Fe. The standards used for calibration were Albite NaAlSi₃O₈ (Na, Si), Pyrope (Mg,Fe)₃Al₂(SiO₄)₃ (Al), FeS₂ (Fe) and Vanadinite (Pb₅(VO₄)₃Cl) for V. At least ten measurements were performed for each composition and the average values are reported in Table 1.

### 2.3. XANES V K-edge

V K-edge XANES spectra were recorded on the DiffAbs beamline located at the SOLEIL synchrotron facility (Saint-Aubin, France). During

Surface

<figure><img src="image_4.png" /><figcaption>Fig. 1. Cross-section of the sample obtained after melting in an alumina crucible under reducing condition. The vertical line indicates the profile performed in Raman and optical absorption spectroscopy.</figcaption></figure>

<caption>Table 1</caption>

Compositions (mol%) measured by EPMA. Uncertainties on the last digit are given between brackets.

<table><thead><tr><th>Sample</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>Na<sub>2</sub>O</th><th>FeO<sup>a</sup></th><th>VO<sub>2</sub><sup>b</sup></th></tr></thead><tbody><tr><td>NAS</td><td>68.2 (8)</td><td>12.5 (3)</td><td>19.3 (3)</td><td>&lt;0.005</td><td>0</td></tr><tr><td>NASV</td><td>68.0 (9)</td><td>12.1 (1)</td><td>19.4 (2)</td><td>&lt;0.005</td><td>1.016 (5)</td></tr></tbody></table>

a) Total iron given as FeO.

b) Total vanadium given as VO₂.

the measurements, SOLEIL operated at 2.75 GeV and 500 mA. A Double Crystal Monochromator equipped with two Si(111) crystals was used to tune the energy. XANES spectra were collected in fluorescence mode using a four-element silicon drift detector (SDD, Vortex-ME4®, Hitachi) set at 90° from the incident beam. The beam size was 200 × 260 µm². XANES spectra were collected between 5440 and 5700 eV with steps of 2 eV between 5440 and 5450, 1 eV between 5450 and 5460, 0.4 eV between 5460 and 5490, 1 eV between 5490 and 5520 and 2 eV between 5520 and 5700 eV. Energy was calibrated using a vanadium metal foil with the inflexion point of the obtained spectra set to 5465 eV. High temperature experiments were conducted by loading glass powder into a 500 µm hole of 90Pt-10Ir wires [32], which were heated by passing an electric current through them. Each wire was calibrated using the procedure detailed by Neuville and Mysen [33]. The temperature measurement error was conservatively set to ± 20 °C. Reference compounds for Vanadinite ((Pb₅(VO₄)₃Cl), V⁵⁺ in coordination 4) and Cavansite ((Ca(VO)Si₄O₁₀), V⁴⁺ in coordination 5) were obtained from the minerals collection of Sorbonne Université). All XANES spectra were processed using the Larch software [34].

### 2.4. Optical absorption spectroscopy

All optical absorption spectra were recorded using a Perkin-Elmer® Lambda 1050 UV-Visible-NIR spectrophotometer in transmission mode, with a studied range of 4000 cm⁻¹ to 40,000 cm⁻¹. Double-sided mirror-polished samples, with thicknesses of approximately 600 and 250 µm were used for samples NAS and NASV, respectively. To obtain the spectra for the profiles presented in the discussion section, a Schwarzschild-type Cassegrain microscope was used to improve the spatial resolution up to 100 × 50 µm². Details about this experimental setup can be found in Chassé et al [35].
## 2.5. Raman spectroscopy

Unpolarized Raman spectra, were acquired using a Labram HR Evolution spectrometer equipped with a Peltier-cooled CCD and an 1800 lines per mm grating. The samples were excited with a Coherent MX 488 nm solid-state laser focused through a × 50 Olympus objective (LMplanFl, WD: 10.6 mm) on the sample surface. The spectral resolution of the setup is ~1.7 cm⁻¹ and the spatial resolution is ~1 µm. The laser power at the exit was adjusted to 100 mW. Spectra were acquired from 20 cm⁻¹ to 1500 cm⁻¹ thanks to a ULF filter to attenuate the laser signal. A linear baseline was fitted over the 1350–1500 cm⁻¹ range. Spectra fitting was performed using the Fityk software [36].

# 3. Results

## 3.1. XANES spectroscopy at V K-edge

The XANES spectra at the V K-edge exhibit an intense pre-edge peak between 5469 and 5471 eV, which corresponds to the 1s→3d transition [37]. This pre-edge peak will be the only feature considered in this study. In order to extract the pre-edge peak from the spectra, a background modelled by a linear and a Lorentzian function is subtracted. This background was fitted from 5462.9 to 5483.9 eV while excluding

the 5468.7 to 5478.5 eV range. This allows to differentiate changes occurring in the pre-edge region from shifts of the white line absorption.

As shown in Fig. 2b, the pre-edge feature of the XANES spectra varies greatly with the oxidation state and coordination of vanadium. V⁵⁺ is present in both V₂O₅ and vanadinite (Pb₅(VO₄)₃Cl) whereas V⁴⁺ is present in Cavansite (Ca(VO)Si₄O₁₀). The higher the oxidation state of V, the higher the intensity of the pre-edge is. This trend is clearly seen for V⁵⁺ and V⁴⁺ in Fig. 1b and also holds for V³⁺ that has a lower pre-edge intensity than V⁴⁺ [39,40]. The position of the pre-edge centroid is sensitive to the V coordination. Compounds containing V in higher coordination states tend to have centroids at higher energies. For example, V⁵⁺ is 5-fold coordinated in V₂O₅ whereas it is 4-fold coordinated in vanadinite and their centroids are shifted by about 1 eV. The diagram presented in Fig. 2d shows the pre-edge intensity and the centroid for several compounds as well as the ones reported by Benzi et al. [38], the references considered here are simple vanadium oxides (V₂O₃, VO₂ and V₂O₅) as well as Cavansite and Vanadinite. As a first approximation, the centroid energy can be considered to depend only on the V coordination and the pre-edge intensity on its redox state. This approximation has been successfully applied in previous studies [14,41]. For a more precise approach that requires additional reference samples, readers are referred to the study of Chaurand et al. [42]. As evidenced in Fig. 2d, the pre-edge intensity of the V³⁺ reference is way lower than that of the

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** A) V K-edge XANES spectra of crystalline references and B) zoom in the pre-edge region. Spectra in A) are offset for clarity. (Vanadinite: V⁵⁺ in CN 4, Cavansite: V⁴⁺ in CN 5) C) Pre-edge regions for NASV sample measured at different temperatures as well as NASV_C sample at room temperature. D) Centroid intensity diagram with values reported from Benzi et al. [38] (red squares) as well as the references from this study (black circles).</figcaption></figure>
$V^{4+}$ one. The pre-edge intensity of the NASV_C sample is very close to the one of the $V^{3+}$ reference indicating that it is possible to obtain $V^{3+}$ in NAS glass by melting in a graphite crucible. The centroid energy is very different from the one of the crystalline $V^{3+}$ reference ($V_2O_3$) in which the $V^{3+}$ ion is in octahedral coordination. This suggests that $V^{3+}$ in NAS is not in octahedral coordination but rather in a lower coordination environment.

Fig. 2b shows that the XANES spectra of the NASV sample is different between room temperature and 600 °C (~Tg). This variation is due to the photo-reduction of V under the X-ray beam [43]. The NASV0_10 exhibited a brown spot where the beam was focused. The interaction between the beam and the sample, along with the use of an approximation in the data treatment, make the quantification of V redox state unreliable. This is especially true for the lower concentration of vanadium as shown in Figure S1 However, XANES will still provide valuable semi-quantitative information on both V redox state and coordination.

## 3.2. Optical absorption spectroscopy

### 3.2.1. Study of the base sample

Optical absorption spectroscopy is a widespread technique to study the oxidation state of vanadium in glasses [13,44]. In the case of the spectra of NASV, presented in Fig. 3, several transitions were associated with the presence of $V^{4+}$ and $V^{5+}$.

The oxygen metal charge transfer (OMCT) $O-V^{5+}$ is responsible for the intense absorption in the blue and UV range. As $V^{5+}$ has a $d^0$ electronic configuration, no *d-d* transitions are expected for this ion. The $V^{4+}$ ion has a $d^1$ configuration, therefore, only one transition is expected in octahedral symmetry. However, several studies have shown that three transitions can be associated to $V^{4+}$ [13,44]. This can be explained by the presence of a vanadyl moiety ($V = O$), shortening one of the bonds and distorting the site geometry. As shown in Fig. 3, three transitions can be observed at 9500, 16,500 and 21,500 cm$^{-1}$. The determination of the position of the third transition is unreliable because it overlaps with the $O-V^{5+}$ OMCT. The values for the first two transitions can be attributed to the vanadyl moiety as they are very close to those previously reported in a sodium disilicate glass, whereas the third $V^{4+}$ transition is reported at 24,000 cm$^{-1}$ [45,46].

According to previous works, the expected absorption bands for the $V^{3+}$ ions are centered around 14,000 and 22,000 cm$^{-1}$ with the one at 22,000 cm$^{-1}$ being the most intense of the two [44,46]. Because the $O-V^{5+}$ OMCT and the third *d-d* absorption of $V^{4+}$ fall near the 22,000 cm$^{-1}$ region, it is difficult to conclude if there is a $V^{3+}$ contribution or not. However, no absorption band is visible around 14,000 cm$^{-1}$, which indicates that the concentration of $V^{3+}$ ions in NAS glass is low. This is expected for a glass melted at 1600 °C under air [15,46]. However, Cicconi et al. [15] determined absorption contributions due to $V^{3+}$ in a

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Optical absorption spectra of samples NAS and NASV. Spectra were normalized to sample thickness.* The $V^{4+}$ absorption around 23,500 cm$^{-1}$ has been reported in literature [15,45]. Due to the wide $O-V^{5+}$ absorption band, it wasn't directly observed here.</figcaption></figure>

sodium aluminosilicate glass even for $V^{3+}$ concentrations as low as 0.09 mol %. Therefore, the absorption band at 21,500 cm$^{-1}$ can be attributed to a *d-d* absorption of the $V^{3+}$ ions overlapping with a contribution from the third *d-d* absorption band of $V^{4+}$.

### 3.2.b. Study along the reduction profile

As shown in Fig. 1, the sample obtained by remelting NASV in an alumina crucible under reducing conditions exhibits a color gradient. This gradient is directly linked to the oxygen fugacity gradient induced by the synthesis protocol. The color variation can be used as a visual indicator of the changes in the V redox state, with more reduced regions showing different colors compared to areas with higher oxidation states. The bottom of the sample is a pale greenish-yellow color, identical to that of NASV synthetized under air. The top part of the sample exhibits a dark green color. A distinct dark brown region, of about 1 mm in length, separated the green and pale yellow regions. Notably, the boundaries of the brown region do not form a straight line parallel to the bottom of the crucible. In fact, the brown area is wider near the edge of the crucible. We attribute this to the glass surface not being flat but rather exhibiting a concave meniscus due to capillarity (Fig. 1). Since the surface is not flat, the diffusion profile cannot be considered strictly unidimensional. It is also possible that the transport mechanism involves a small convective contribution. However, due to the small radius of the crucible, the convective contribution must be small. The optical absorption spectroscopy profile (Fig. 4) was performed along the vertical dotted line in Fig. 1, orthogonal to the bottom of the meniscus. Along this line, the diffusion can be considered to be unidimensional. The profile is expressed in depth representing the distance to the surface in µm.

As discussed before, optical absorption spectroscopy is highly sensitive to both $V^{4+}$ and $V^{5+}$ ions while still being able to detect $V^{3+}$ ions. However, the absorption coefficient of the $O-V^{5+}$ OMCT is very high, making it impossible to assess the $V^{5+}$ concentration, even qualitatively, using optical absorption spectroscopy. At depth greater than 4500 µm (i. e. at the bottom of the crucible), the absorption bands of $V^{4+}$ are weak to the point where only the one at 9500 cm$^{-1}$ is still visible (Fig. 4). The difference between the spectrum of NASV and the oxidized spectra for NASV_profile (orange and light blue curves in Fig. 4b) can be explained by a difference in synthesis temperature. Indeed, sample NASV was equilibrated at 1600 °C while NASV_profile was equilibrated at 1400 °C. It has been shown that melting temperature directly influences room temperature redox state [31,46]. A lower melting temperature implies a more oxidized glass. Therefore, the concentration of $V^{4+}$ in sample NASV_profile is lower than in NASV. This is also true for the low concentration of $V^{3+}$ in NASV which will be even lower for NASV_profile. This explains why the absorption bands attributed to $V^{4+}$ are less visible in NASV_profile spectra than in NASV spectrum. The strong absorption above 25,000 cm$^{-1}$ indicates the presence of $V^{5+}$ in this region. We can conclude that, for depths greater than 4500 cm$^{-1}$, the two species present are $V^{4+}$ in minority and $V^{5+}$ in majority.

At depth <3000 µm, the $V^{4+}$ absorption bands become very intense and the $O-V^{5+}$ OMCT disappears (red and dark blue spectra in Fig. 4b). The slight increase in the normalized area from 0 to 3000 µm can be attributed to a change in sample thickness. With the decrease in OMCT absorption, it is now possible to clearly identify the absorption at 22,500 cm$^{-1}$ which can be attributed to a *d-d* absorption of the $V^{3+}$ ion. The absence of the second, less intense, $V^{3+}$ absorption band near 14,000 cm$^{-1}$ indicates that the concentration of $V^{3+}$ is rather low. We can conclude that for depths lower than 3000 µm, the two species present are $V^{3+}$ in minority and $V^{4+}$ in majority.

In the 3200 to 4200 µm range, the optical absorption spectra changes from exhibiting strong $V^{3+}$ and $V^{4+}$ absorption to exhibiting a strong $O-V^{5+}$ OMCT with minor $V^{4+}$ absorption bands (Fig. 4a). In this range we observe simultaneously both $V^{4+}$ absorptions, the $V^{3+}$ absorption and the $O-V^{5+}$ OMCT (light blue spectra on Fig. 4a). The depths at which each species is found according to OAS is represented in Fig. 4c.

The most intense absorption band related to the $V^{4+}$ ions is centered
<figure><img src="image_7.png" /><figcaption>Fig. 4. A) Optical absorption spectra between 3200 and 4500 µm in depth. The spectra in red are spaced 300 µm apart. The light blue spectrum corresponds to a depth of 3700 µm. The spectra are not normalized and a simple constant baseline was fitted to all spectra. A change of detector is located at 11,615 cm⁻¹. B) Selected optical absorption spectra, with identified transitions highlighted, as well as a measurement artefact. The spectra correspond to the colored points in pattern A. The range considered in pattern A is highlighted by two black lines. C) Normalized integrated area between 6000 and 12,000 cm⁻¹ as a function of depth. The area at the lowest depth is set to 1 while the area at the highest depth is set to 0. Those values are used to normalize all data points.</figcaption></figure>

at 9500 cm⁻¹. By measuring the area in the 6000–12,000 cm⁻¹ range at different depths, it is possible to monitor the change in V⁴⁺ concentration along the reduction profile. The results are presented in Fig. 4c. The evolution of V⁴⁺ concentration as a function of depth follows an error function. The mathematical shape confirms that the mechanism at play is diffusive in nature, details will be given in Section 4.2. Moreover, it confirms that the main reaction occurring as depth increases is the oxidation of V⁴⁺ to V⁵⁺.

The green spectrum (circle) shown in Fig. 4b exhibits higher absorption in the 15,000 to 25,000 cm⁻¹ range compared to spectra at

lower depths, such as the red or blue ones in Figure 4.B. The maximum difference between the green and red spectra occurs at 19,500 cm⁻¹ (See Figure S2). This contribution appears to be very large as its approximate Half-Width at Half-Maximum (HWHM) is of 4000 cm⁻¹. No absorption bands are expected from V⁴⁺ or V⁵⁺ near 20,000 cm⁻¹ and the V³⁺ band was identified at 22,500 cm⁻¹ (Fig. 4b) while the second d-d transition for the V³⁺ ion is expected at 14,000 cm⁻¹ [15,46]. The nature of the band will be discussed in a later section.

### 3.3. Raman spectroscopy

#### 3.3.a. Study of the base sample

The interpretation of the Raman spectrum of NAS was performed in a previous study [31]. The addition of 10 mol % of VO₂ leads to the apparition of several contributions, highlighted in Fig. 5. Those contributions are centered at 250, 380, 860 and 920 cm⁻¹, with a shoulder also visible at 956 cm⁻¹. Similar contributions have been observed in various vanadium bearing glasses [47,48]. The Raman spectrum of NASV_C only exhibits the shoulder at 956 cm⁻¹ which becomes the dominant contribution in the 700–1200 cm⁻¹ region. The difference between the spectra of NASV and NASV_C highlights the two contributions at 860 and 920 cm⁻¹ that can easily be fitted by Gaussian functions. This shows that, out of the five bands observed, four can be attributed to an oxidized V species, while the shoulder at 956 cm⁻¹ can be attributed to a more reduced V species. Though more precise attributions will be discussed in Section 4.1, it is safe to assume that the first four contributions are due to V⁵⁺ while the shoulder at 956 cm⁻¹ is due to V⁴⁺.

#### 3.3.b. Study along the reduction profile

The Raman profile was performed on the same sample than starting from the bottom of the meniscus and continues orthogonally toward the bottom of the sample. Along this path, the diffusion profile can be considered as unidimensional.

As discussed in Section 3.3.a, five Raman contributions appear with the addition of V in NAS glass. Out of those five contributions, four can be attributed to V⁵⁺. The most intense of those contributions is located at 920 cm⁻¹. Therefore, by monitoring changes in the integrated area between 800 and 1000 cm⁻¹, it is possible to study the evolution of V⁵⁺ along the reduction profile (Fig. 6c). The shape of the profile follows an error function, which confirms that the underlying process is diffusive in nature.

For depths greater than 4500 µm (i.e. at the bottom of the crucible), the area between 800 and 1000 cm⁻¹ is very high. The light blue and orange Raman spectra shown in Fig. 6b both show the four contributions associated with V⁵⁺ and these spectra are very close to those of the sample obtained under air (Fig. 4). This confirms that, at the bottom of

<figure><img src="image_8.png" /><figcaption>Fig. 5. Unpolarized Raman spectra of undoped glass as well as V-doped glasses synthesized under air and reducing conditions. The difference between NASV and NASV_C spectra is shifted for clarity. The difference is fitted by two Gaussian functions, see text for details.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** A) Raman spectra between 2500 and 4500 µm in depth. The spectra in red are spaced 300 µm apart. The spectra are not normalized and a simple constant baseline was fitted to all spectra. B) Selected Raman spectra. The range considered in pattern C is highlighted by two black lines. C) Normalized integrated area between 800 and 1000 cm⁻¹ as a function of depth. The area at the lowest depth is set to 0 while the area at the highest depth is set to 1. Those values are used to normalize all data points. The colored points correspond to the spectra in Figure B.</figcaption></figure>

the crucible, the redox state is very close to the one equilibrated under air, meaning that diffusion was too slow to affect the entire sample. For depths between 200 and 2500 µm (i.e. near the surface, in the dark green region), the area between 800 and 1000 cm⁻¹ is much lower. The dark blue and green Raman spectra in Fig. 5b show no contribution of V⁵⁺, and they closely match the spectrum of the NASV_C sample (Fig. 4). We can conclude that at depths between 500 and 2500 µm, no V⁵⁺ is present. The first spectrum in the profile (red spectrum in Fig. 6b) shows some contribution of V⁵⁺ at 860 cm⁻¹. This is due to a slow reoxydation

during the cooling process, as the sample was not quenched. Therefore, when the sample was allowed to cool under air, a slow diffusive process partially reoxydized the first 200 µm of the sample.

The area between 800 and 1000 cm⁻¹ significantly increases in the 3000 to 4000 µm range. As shown in Fig. 6a, the contributions of V⁵⁺ progressively intensify with increasing depth. Notably, all four V⁵⁺ contributions increase at the same rate with increasing depth.

This Raman spectroscopy study allows us to divide the sample into three regions: the first, for depths greater than 4500 cm⁻¹, where both V⁴⁺ and V⁵⁺ are present; the second, between 3000 and 4000 µm, where V⁵⁺ is progressively reduced to V⁴⁺; and the third, where no V⁵⁺ remains. Raman spectroscopy shows only one reduced contribution (see Section 3.3.a) which we attribute to V⁴⁺. The presence of a V³⁺ Raman contribution for depths lower than 2500 µm will be discussed in Section 4.1. The regions of existence of the different vanadium redox state are shown in Fig. 6c.

# 4. Discussion

## 4.1. Determination of redox state and coordination of vanadium

The XANES spectrum of NASV (Fig. 2c) shows that the pre-edge peak intensity falls between those of tetravalent and pentavalent reference compounds. This indicates that the V redox state in the NASV glass is a mixture of V⁴⁺ and V⁵⁺. This mixed valence state is further confirmed by optical absorption spectroscopy (Fig. 3). However, optical absorption spectroscopy suggests that a small fraction of V³⁺ is also present. As discussed in section 3.2.a, optical absorption spectroscopy can detect V³⁺ concentrations as low as 0.09 mol %. This corresponds to a V³⁺ fraction of <1 % of the total V content in NASV which would not be detectable by XANES spectroscopy. The optical absorption measurements show that V³⁺, V⁴⁺ and V⁵⁺ coexist in NASV sample. This coexistence will be further discussed in Section 4.3.

The pre-edge peak centroid in NASV (Fig. 1.C) shows that pentavalent vanadium is present both as 5- and 4-fold coordinated species. The XANES data also indicate that tetravalent vanadium is closer to five-fold coordinated than six-fold coordinated. This is consistent with the optical absorption spectroscopy results, where three transitions can be attributed to V⁴⁺ in a lower symmetry site than an octahedron.

In NASV, vanadium exists in three different forms: [V⁵⁺O₄]³⁺, [V⁵⁺O₅]⁵⁻ and [V⁴⁺O₅]⁶⁻. Those species were previously reported by molecular dynamic simulations [16], though [V⁴⁺O₄]⁴⁻ was the favored entity for tetravalent vanadium in their study. Several other experimental studies have also highlighted the presence of two different coordination states for pentavalent vanadium in glass, as well as V⁴⁺ being in a lower symmetry site than octahedral [14,47].

The XANES spectrum for NASV_C (Fig. 2c) shows that the pre-edge intensity is below that of tetravalent vanadium references, indicating that the V redox equilibrium in NASV_C is a mixture of V³⁺ and V⁴⁺. This suggests that melting V-bearing glass in a graphite crucible creates sufficiently reducing conditions to form a mixture of V³⁺ and V⁴⁺. V²⁺ is only obtained under extremely reducing conditions in oxide glasses (fO₂ < 10⁻¹⁸) [41]. Even in a graphite crucible, those oxygen fugacity values are not reached, meaning that there are no V²⁺ ions in NASV_C.

Raman spectroscopy provides more insights on the local environment surrounding vanadium ions. As discussed in Section 3.3, five contributions are associated with the addition of vanadium. Out of those five, the ones at 250, 380, 860 and 920 cm⁻¹ can be attributed to oxidized V⁵⁺ ions. The two contributions at 250 and 380 cm⁻¹ are attributed to V⁵⁺O₅ species. More specifically, they can correspond to the internal bending modes of trigonal bipyramids [48,49]. The contribution at 860 cm⁻¹ is attributed to the A₁ stretching vibration mode of the V⁵⁺O₄ tetrahedra [28,29]. This vibration is observed in the Raman spectrum of Vanadinite, as shown in Figure S3. The contribution at 920 cm⁻¹ is assigned to the stretching of the V⁵⁺=O vanadyl bond. Because double bonds are very potent Raman vibrators, this
contribution is very intense, even with low vanadium concentration. This explains why this contribution is the most prominent upon vanadium addition. This same vibration is also seen in the Raman spectrum of V₂O₅ (Figure S4).

The attribution of the shoulder at 956 cm⁻¹ is more complex and has been discussed in previous studies [47]. Since it becomes the main contribution when the sample is reduced, it is likely due to a V⁴⁺ species. One possible attribution is the stretching vibration of V⁴⁺=O [49], as it is very intense and occurs near the V⁵⁺=O contribution. The V⁴⁺=O vanadyl moiety can be found in five-fold coordination as well as in distorted octahedral environments. Another possible attribution for this contribution is the symmetric stretching vibrations of V⁴⁺O₃ groups [29]. These vibrations come from O₄V⁴⁺-O-V⁴⁺O₄ linkages, where each vanadium is double bonded to oxygen. Such structures have been evidenced in minerals, such as Cavansite [50] and other compounds [51] where V is in five-fold coordination. Regardless of the attribution considered, a [V⁴⁺O₅]⁶⁻ structure can explain the 956 cm⁻¹ contribution. This is coherent with the XANES data presented in Fig. 1c where the centroid position suggests that tetravalent vanadium is five-fold coordinated. The attributions considered here are summarized in Table 2.

The contributions at 860 and 920 cm⁻¹ are attributed to different V⁵⁺ species. Therefore, Raman spectroscopy is able to monitor relative changes in the proportion of [V⁵⁺O₄]³⁻ and [V⁵⁺O₅]⁵⁻. The Raman profile shown in Fig. 6c shows that the 860 cm⁻¹ and 920 cm⁻¹ contributions increase at the same rate as the depth increases. This implies that [V⁵⁺O₄]³⁻ and [V⁵⁺O₅]⁵⁻ are likely formed under the same oxygen fugacity.

XANES spectroscopy shows that the redox state of NASV_C is a mixture of V³⁺ and V⁴⁺. However, no Raman contribution could be attributed to V³⁺ species, likely because the V³⁺ concentration is low and V³⁺ does not form Raman-active double bonds with oxygen. Two notable Raman modes of V₂O₃ is the A₁g located around 250 cm⁻¹ and the E₉ mode located at 210 cm⁻¹ [49,52]. Both of these modes are related to stretching vibrations within a [V³⁺O₆]⁹⁻ octahedron. No such contributions could be identified on the Raman spectra of NASV_C (Fig. 5) or on the Raman spectra of NASV_profile even though optical absorption spectroscopy of the reduced NASV_profile sample show the presence of V³⁺. This can indicate that V³⁺ in not in octahedral coordination in NAS glass. Considering the XANES spectrum of NASV_C, the centroid energy is much lower than the one reported for octahedral V₂O₃. As discussed in Section 3.1, a lower centroid energy indicates a lower coordination number. V³⁺ could be either in a four or five coordinated environment which would explain the absence of the aforementioned Raman contributions. The review of V K-edge XANES spectra in glass done by Sutton et al. [14] shows a wide dispersion of the centroid energy for V³⁺ bearing glasses. The centroid energy spans from the value for the octahedral V₂O₃ reference to about 1.2 eV less. The centroid energy of the NASV_C sample is 0.7 eV below the one of the V₂O₃ reference (Fig. 2d) and falls well within Sutton et al.'s range. Those XANES measurements suggest that V³⁺ ions occupy a wide variety of coordination sites in silicate glasses. There are few V³⁺ complexes in tetrahedral coordination in the literature [53,54]. Weber and Riseberg [54] assign absorption at 20,600 and 23,000 cm⁻¹ to transition to ¹T₂ and ³T₁ excited states of V³⁺ in tetrahedral coordination. The transition observed around 22,500 cm⁻¹ in the optical absorption spectra of NASV_profile could be explained by V³⁺ in tetrahedral coordination. However, the general consensus is that

<table><caption>**Table 2**
Attribution of the Raman contributions highlighted in Fig. 5.</caption><thead><tr><td>Raman shift (cm<sup>-1</sup>)</td><td>Local structure</td></tr></thead><tbody><tr><td>250</td><td>[V<sup>5+</sup>O<sub>5</sub>]<sup>5-</sup></td></tr><tr><td>380</td><td>[V<sup>5+</sup>O<sub>4</sub>]<sup>3-</sup></td></tr><tr><td>860</td><td>[V<sup>5+</sup>O<sub>4</sub>]<sup>3-</sup></td></tr><tr><td>920</td><td>[V<sup>5+</sup>O<sub>5</sub>]<sup>5-</sup></td></tr><tr><td>956</td><td>[V<sup>4+</sup>O<sub>5</sub>]<sup>6-</sup></td></tr></tbody></table>

V³⁺ occurs in octahedral coordination in silicate glasses [44,46,47]. Our experiments do not give enough information to draw definitive conclusions on the coordination of V³⁺ in sodium alumino-silicate glass.

It is important to note, that no contributions around 686 cm⁻¹ appear in the Raman spectra upon vanadium addition to the glass. Contributions in this range have been linked to V⁵⁺-O-V⁵⁺ groups, which are indicative of vanadium clustering [55]. Such a contribution is visible in the V₂O₅ spectrum presented in Figure S3. This suggests that the vanadium concentration is too low for significant clustering of V⁵⁺ to occur.

## 4.2. Diffusive mechanism

As shown by both the optical absorption and the Raman profiles (Fig. 4c and 6c), diffusion is the main process occurring in the NASV_- profile sample. The diffusion coefficient can be approximated by knowing the duration of the experiment and the depth affected by diffusion using Eq. (1):

$$D = \frac{d^2}{4 * t} \qquad (1)$$

with *t* the time of the experiment in seconds and *d* the depth affected by diffusion in meters. In our case, the time of the experiment was 4h15 or 15,300 s. The depth *d* is either 3 mm or 4 mm depending on whether we consider the beginning or the end of the sigmoid. The diffusion coefficient obtained is therefore comprised between 2.4 × 10⁻⁹ m²·s⁻¹ and 4.2 × 10⁻⁹ m²·s⁻¹. In order to compare these values with those of O²⁻, one can use the viscosity of the glass melt and apply the Eyring's law (Eq. (2)).

$$D = \frac{kT}{\lambda\eta} \qquad (2)$$

with *k* the Boltzmann constant, *T* the temperature in Kelvin, *λ* the distance travelled during one diffusion jump and *η* the viscosity. Because we are considering oxygen diffusion, *λ* will be taken as 0.28 nm which is an approximation of the O—O distance [56,57]. The viscosity measured at 1400 °C for the NAS glass is of 2.39 log(Pa·s) (Figure S5). Although the viscosity of the NASV sample was not measured, the influence of multivalent elements on viscosity is mostly visible at lower temperatures, as evidenced for Ce [58] and Fe [4]. Therefore, we can consider the viscosities of NAS and NASV to be sufficiently similar for the purpose of determining the order of magnitude of diffusion coefficients.

The obtained diffusion coefficient for O²⁻ at 1400 °C in NAS is 3.4 × 10⁻¹³ m²·s⁻¹. This is several orders of magnitude lower than the redox diffusion coefficient estimated from Eq. (1) and the spectroscopic profiles. The experimental diffusion coefficients are similar to those reported for molecular oxygen in comparable compositions [57]. This suggests that the diffusive species is molecular oxygen, which aligns with several previous studies that indicate that the diffusive mechanism for redox reactions involves the diffusion of oxygen at high temperatures [57,59,60]. It is important to note that, since the atmosphere is more reducing than for the original sample which was equilibrated under air, the diffusion mechanism occurs from the bulk of the sample towards the surface. Therefore, no CO or CO₂ enters the sample. This is confirmed by the Raman spectra in Fig. 6a where no contributions from these gases are detected nor from dissolved C species.

## 4.3. Optical absorption spectroscopy profile in the 12,000 to 20,000 cm⁻¹ range

The green spectrum shown in Fig. 4b exhibits higher absorption in the 15,000 to 25,000 cm⁻¹ range compared to spectra at lower depths, such as the red or blue ones in Fig. 4b. The maximum difference between the green and red spectra occurs at 19,500 cm⁻¹ (Figure S2). In order to better monitor the evolution of this 19,500 cm⁻¹ contribution, Fig. 7 reports the integrated area in the 12,000–20,000 cm⁻¹ range as a function of depth. The range limit is set at 20,000 cm⁻¹ to avoid taking into
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Normalized integrated area between 12,000 and 20,000 cm⁻¹ as a function of depth. The area at the lowest depth is set to 1 while the area at the highest depth is set to 0. These values are used to normalize all data points. Colored points will be mapped onto the CIE diagram in Fig. 9.</figcaption></figure>

account changes in the UV edge.

The area between 12,000 and 20,000 cm⁻¹ accounts for the V⁴⁺ absorption at 16,500 cm⁻¹ and partially for the V³⁺ absorption at 22,500 cm⁻¹. More importantly, it includes the absorption around 19,500 cm⁻¹. The normalized integrated area can therefore be seen as the sum of two distinct phenomena. The first one being the decrease in V³⁺ and V⁴⁺ concentration with increasing depths. This phenomenon is very well modeled by the profile shown in Fig. 4c and follows a sigmoidal pattern. The second phenomenon is the rise of the large contribution centered around 19,500 cm⁻¹ which will induce a deviation from the sigmoid pattern. This deviation from the sigmoid pattern can be seen in Fig. 8 where all profiles are directly compared.

The integrated area in the 6000–12,000 cm⁻¹ range and in the 12,000 cm⁻¹–20,000 cm⁻¹ range deviate from one another for depth comprised between 3100 and 4000 µm. In this region, the integrated area between 12,000 and 20,000 cm⁻¹ is significantly higher than the expected sigmoid behavior. When comparing this depth region to the existence domain of the different vanadium species, we can conclude that the additional absorption at 19,500 cm⁻¹ only occurs in the domain where V³⁺, V⁴⁺ and V⁵⁺ coexist. If it were a *d-d* transition of the V³⁺ ion, its intensity would linearly decrease with decreasing V³⁺ concentration. This means that we would observe a monotonous decrease in the 19,500

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Normalized integrated area obtained by Raman spectroscopy (black squares, 800–1000 cm⁻¹) and by optical absorption spectroscopy (red circles: 6000–12,000 cm⁻¹ and blue circles: 12,000–20,000 cm⁻¹). The domain of existence of the different V redox states are shown above the figure.</figcaption></figure>

cm⁻¹ absorption with increasing depth. This is not what is observed in Fig. 8. A similar reasoning can be used to rule out a *d-d* transition of the V⁴⁺ ion. Moreover, the V⁵⁺ ion does not exhibit any *d-d* transition as it has a *d*⁰ electronic configuration.

A possible attribution for this 19,500 cm⁻¹ absorption would be a change in the coordination of V³⁺ or V⁴⁺ occurring only over a limited range of oxygen fugacity. A change in environment would change the crystal field surrounding the ion and therefore the position of the absorption. However, the absorption spectra of [V⁴⁺O₄]⁴⁺ only shows absorption for wavenumbers lower than 14,000 cm⁻¹ [61]. Therefore tetrahedral V⁴⁺ cannot explain the transient absorption at 19,500 cm⁻¹. Since the absorption at 9500 cm⁻¹ is attributed to V⁴⁺ in five coordinated or distorted octahedral environment, a change in the coordination of V⁴⁺ would also be visible on this band. More specifically, the integrated intensity in the 6000–12,000 cm⁻¹ range would show a deviation from a sigmoid pattern which is not observed on Fig. 8.

The question of V³⁺ coordination has been discussed in Section 4.1. It is possible that for depths comprised between 3100 and 4000 µm, the coordination of V³⁺ is different than at lower depths. Trigonal distorted octahedral complexes of V³⁺ have shown wide absorptions over the 18,000–22,000 cm⁻¹ range [62] which could explain the absorption at 19,500 cm⁻¹. Moreover, the absorption coefficients of *d-d* transitions are higher for tetrahedral complexes than for octahedral ones with central symmetry. A lowering of the symmetry surrounding the V³⁺ ion can explain both the change in position and the relatively high absorption. However, there is no apparent reason for the change in coordination to occur only in the 3100 to 4000 µm.

Another explanation for this absorption could be intervalence charge transfers (IVCT) between different vanadium ions. The absorption coefficients of IVCTs are at least an order of magnitude higher than those of *d-d* transitions [2]. Moreover, they tend to give very large absorption bands located around 20,000 cm⁻¹ as is the case for the Fe²⁺/Fe³⁺ charge transfer [63] or the Cu⁺/Cu²⁺ one [64,65]. A charge transfer between V⁴⁺ and V⁵⁺ would most likely occur for all depths were V⁴⁺/V⁵⁺ coexist, i.e. for depths superior to 3100 µm. On the other end, a V³⁺/V⁴⁺ IVCT would occur for depths inferior to 4100 µm, which is not the case. Therefore, V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ IVCTs cannot explain why the absorption at 19,500 cm⁻¹ only occurs between 3100 and 4000 µm. A possible explanation would be a V³⁺/V⁵⁺ IVCT as such a transition would only occur in the overlap of the existence domain of V³⁺ and V⁵⁺ which is 3100–4000 µm. As shown is Fig. 8, the integrated area on the Raman profile starts increasing at 3000 µm. As discussed in Section 3.3. b, this profile is sensitive to V⁵⁺. This means that the V⁵⁺ concentration is non negligible for depths superior to 3000 µm. As shown in Fig. 8, the blue profile stops following the red profile at 3000 µm, which corresponds to the apparition of V⁵⁺ in the glass. Similarly, the optical absorption band for V³⁺ is no longer visible for depths superior to 4000 µm (Fig. 4a). This correlates to the depth at which the blue and red profiles fall within the error bars of one another. Even though some intervalence charge transfer involving V³⁺ have been identified with over transition metal ions such as Fe²⁺ [66], to our knowledge, no V³⁺-V⁵⁺ IVCT have been evidenced before. The possibility of a V³⁺-Fe²⁺IVCT seems highly unlikely given the low concentration of Fe measured by EPMA.

The existence of a V³⁺-V⁵⁺ IVCT supposes that both species can coexist in glass similar to what is observed for Mo³⁺-Mo⁵⁺ [67]. The coexistence of V³⁺ and V⁵⁺ is far from trivial as these species would be expected to react via symproportionation. While their coexistence in glasses has been evidenced by Johnston [68] and Weyl [69], it has not been prominently highlighted in recent studies.

The explanation for their coexistence lies in the standard potentials of the V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ redox couples. They have been measured in sodium disilicate glasses under air [9]. Using their value for the reduction enthalpies and entropies of both vanadium redox couple, it is possible to construct Fig. 9.

As shown in Fig. 9, V³⁺/V⁴⁺ and V⁵⁺ coexist at equilibrium in sodium disilicate under air for temperatures above 1500 °C. Below 1000
<figure><img src="image_12.png" /><figcaption>Fig. 9. Predominance diagram of vanadium species in sodium disilicate glass under air as a function of temperature calculated from Rüssell and Von der Gonna [9].</figcaption></figure>

°C, V⁵⁺ is the only predominant species. However, there is no temperature range where V⁴⁺ is the only stable redox state. Although Fig. 9 illustrates the high temperature equilibrium in glass, it has been shown that the high temperature redox state is effectively frozen during quenching [31], at least when the sample contains a single multivalent element as in the case here.

The coexistence of V³⁺ and V⁵⁺ has also been evidenced by XANES measurements performed on glasses equilibrated under different oxygen fugacities [14,41]. These studies report a single sigmoidal relationship between the pre-edge peak intensity and the oxygen fugacity. This indicates that the sigmoid curves for the V³⁺/V⁴⁺ and the V⁴⁺/V⁵⁺ equilibria are close enough to appear as one combined sigmoidal curve. This further supports that V³⁺ and V⁵⁺ can coexist in glass to a significant extent, making a V³⁺-V⁵⁺ IVCT possible.

The attribution of this wide 19,500 cm⁻¹ is not clear and the two hypotheses formulated here are the following:

- As the 3100–4000 µm region corresponds to the reduction front of the sample, it is where the reduction reaction was occurring when the sample was cooled. As such, the local environment surrounding the V³⁺ ion was frozen into a transition state and is not settled on its final coordination. Therefore, there is a variety of V³⁺ environments in the 3100–4000 µm range which give rise to a wide absorption band.

- The absorption corresponds to a V³⁺-V⁵⁺ IVCT that occurs over the oxygen fugacity range where V³⁺ and V⁵⁺ coexist.

### 4.4. Implication on glass color

As shown in Fig. 1, the sample NASV_profile exhibits three distinct color regions. By analyzing the optical absorption spectra presented in Fig. 4a, we can map the evolution of the sample color as a function of depth (Fig. 10).

As shown by the red dot in Fig. 10, the sample near the surface is green due to the presence of V⁴⁺, with smaller contributions of V³⁺ (from 200 to 3000 µm). For increasing depths, the color shifts from the red point to the dark blue point where the sample has a dark brown hue. In this region (3100 µm for the dark blue point), V⁵⁺ is also present in the glass. This leads to the rise of the O-V⁵⁺ OMCT absorbing in the blue region, as well as the additional contribution at 19,500 cm⁻¹. Both contributions increase the overall absorption, causing the sample to darken. This is reflected in the CIE diagram where the blue point is far for the center, meaning the sample is darker. When the depth is decreased from the dark blue to the green point (from 3100 to 3600 µm), the 19,500 cm⁻¹ contribution becomes less important. Regardless of the hypothesis considered for this absorption, its decrease is likely due to the decreasing V³⁺ concentration as the redox state becomes more oxidized.

<figure><img src="image_13.png" /><figcaption>Fig. 10. CIE diagram built from the optical absorption spectra in Fig. 4.A. The colored points correspond to those displayed in Fig. 7 and correspond to depths of 1700, 3100, 3600, 4100 and 5300 µm (red, dark blue, green, light blue and orange respectively). The diagram is built for a sample thickness of 1.367 mm. The colored points are reported on the cross-section of the sample.</figcaption></figure>

As a result, the sample gradually loses its dark brown color and shifts to a more greenish-yellow color. Compared to the red point, the green one (3600 µm) exhibits the O-V⁵⁺ OMCT which is responsible for the yellow color, while V⁴⁺ ion contribute to the blue color. When depth is further decreased, from the green point to the light blue and then the orange point, the color becomes progressively lighter. This is because the concentration of V⁴⁺, the main coloring ion in the glass, decreases.

### 4.5. Reduction mechanism

The Raman profile shows an increase in V⁵⁺ concentration with increasing depth, while the optical absorption profile in the 6000–12,000 cm⁻¹ range shows a decrease in V⁴⁺ concentration with increasing depth. As shown in Fig. 8, the two profiles mirror each other. A slight shift is observed between the disappearance of V⁵⁺ and the appearance of V⁴⁺, which can be explained by the difference in depth probed by the two techniques. Indeed, optical absorption measurements were performed in transmission mode, meaning that the entire thickness of the sample contributes to the spectra even if the diffusion profile is not perfectly unidirectional (see Section 4.2). Therefore, the optical absorption profile averages different redox states along the sample thickness, unlike the Raman profile, which provides more localized information. Nonetheless, the mirroring of the optical and Raman profiles confirms that the two reactions occurring are:

$$[V^{5+}O_5]^{5-} + \frac{1}{2}O^{2-} \rightleftharpoons [V^{4+}O_5]^{6-} + \frac{1}{4}O_{2(g)} \quad (3)$$

and

$$[V^{5+}O_4]^{3-} + \frac{1}{2}O^{2-} + \frac{1}{4}O_{2(g)} \rightleftharpoons [V^{4+}O_5]^{6-} \quad (4)$$

Because the Raman attribution shows that [V⁵⁺O₅]⁵⁻ and [V⁵⁺O₄]³⁻ are formed simultaneously, the overall reaction can be written as a combination of both Eqs. (3) and 4:

$$x[V^{5+}O_5]^{5-} + y[V^{5+}O_4]^{3-} + O^{2-} \rightleftharpoons 2[V^{4+}O_5]^{6-} + \frac{x-y}{4}O_{2(g)} \quad (5)$$

The amount of oxygen released during vanadium reduction depends on the relative proportions of tetra- and penta-coordinated V⁵⁺. Molecular dynamic simulations predict that about 60 % of V⁵⁺ is five-fold coordinated [16]. This means that the x and y coefficients in Eq. (5) are similar. Consequently, the amount of oxygen released should be small. Free oxygen diffusion can occur without the formation of oxygen bubbles as long as the amount of gas produced is small [70]. This may explain why no gas bubbles are observed at the edge of the reduction
profile in Fig. 1.

## 5. Conclusion

In this study, we investigated the vanadium redox state in an aluminosilicate glass under various oxidizing/reducing conditions. XANES and Raman spectroscopy reveal that the $V^{5+}$ ion exists as both $[V^{5+}O_4]^{3-}$ and $[V^{5+}O_5]^{5-}$ entities, without any evidence of clustering. Tetravalent vanadium is either five-fold coordinated or in a distorted octahedral configuration, as shown by optical absorption, Raman and XANES data. Our results show that Raman spectroscopy is highly effective to monitor changes in $V^{5+}$ concentration, while optical absorption spectroscopy is more suitable to monitor changes in $V^{4+}$ concentration. These two laboratory techniques are sufficient to assess vanadium redox changes.

The change in vanadium redox state is probed along a reduction profile. The redox states range from a mixture of $V^{3+}/V^{4+}$ at the reduced end to a mixture of $V^{4+}/V^{5+}$ at the oxidized end. The main reaction occurring along this profile is a reduction of vanadium from its pentavalent to its tetravalent state. $[V^{5+}O_4]^{3-}$ and $[V^{5+}O_5]^{5-}$ seem to be reduced to $V^{4+}$ at the same time. Along this reduction profile, there is a region where $V^{3+}/V^{4+}$ and $V^{5+}$ coexist. This leads to an additional contribution in the optical absorption spectra that is attributed to a dispersion in $V^{3+}$ coordination number or to a $V^{3+}-V^{5+}$ IVCT. This shows that the relationship between vanadium redox state and color is not trivial and that transient regimes occur over a limited range of oxygen fugacity. The results confirmed that the reduction reaction is limited by the diffusion of molecular oxygen from the sample towards the surface. The multiple possible coordination states for vanadium ions imply that the underlying redox mechanism is complex which could have implications on bubble release upon glass reduction.

## Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## CRediT authorship contribution statement

**Adrien Donatini:** Writing – review & editing, Writing – original draft, Visualization, Investigation, Data curation. **Peggy Georges:** Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization. **Tiphaine Fevre:** Writing – review & editing, Supervision. **Laurent Cormier:** Writing – review & editing, Writing – original draft, Supervision, Resources, Project administration, Funding acquisition, Conceptualization. **Daniel R. Neuville:** Writing – review & editing, Writing – original draft, Supervision, Resources, Project administration, Funding acquisition, Conceptualization.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Adrien Donatini reports financial support was provided by National Association of Technical Research. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

We thank SOLEIL (Gif-sur-Yvette, France) for provision of synchrotron facilities (project number 20230206) as well as Solenn Reguer for her help performing XANES experiments. This work was supported by ANRT, under the CIFRE Contract 2021/0986 and by Corning SAS. We would like to thank Sorbonne université's mineral collection for the loan

of mineral samples.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jnoncrysol.2025.123689.

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