# Perspectives on oxynitride glasses: Transparency, compositions, redox, composites, crystallization, and applications

Alexis Duval, Patrick Houizot, Tanguy Rouxel

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## To cite this version:

Alexis Duval, Patrick Houizot, Tanguy Rouxel. Perspectives on oxynitride glasses: Transparency, compositions, redox, composites, crystallization, and applications. Journal of the European Ceramic Society, 2025, 46 (3), pp.117848. 10.1016/j.jeurceramsoc.2025.117848. hal-05372723

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# Perspectives on oxynitride glasses: Transparency, compositions, redox, composites, crystallization, and applications

Alexis Duval<sup>* ,1,2 #</sup>, Patrick Houizot<sup>3</sup>, Tanguy Rouxel<sup>4</sup>

Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) - UMR 6251, Rennes F-35000, France

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

- Glass

- Glass-ceramic composite

- Oxynitride

- Redox chemistry

- Functionalization

## ABSTRACT

Oxynitride glasses are glasses wherein two-fold oxygen atoms are substituted by three-fold nitrogen atoms. This results in increased physical and mechanical properties, which have long driven the development of such materials. This article examines the reactivity of oxynitride melts, with the aim of deepening our knowledge of the formation of oxynitride glasses, of the reasons for their opacity, and of the influence of the often-overlooked redox state. The functionalization of oxynitride glasses through the crystallization of oxynitride and nitride crystals, and by the control of the particles size of the precipitating metal and nitride species (from the nano- to the micro-scale), is also discussed in the light of recent advances in the fields of optical and photonic materials. Our main objective is to provide insights and a renewed momentum to this field by providing new guidelines for future fundamental and applied investigations.

## 1. Introduction

Silicate oxynitride glasses were first synthesized by Mulfinger in 1966 by bubbling ammonia in soda-lime-silica melts [1]. They were evidenced ten years later by Jack in the intergranular glassy phases of silicon nitride ceramics sintered with additives (Al₂O₃, MgO, Y₂O₃) [2]. In 1983, Marchand reported the synthesis of phosphate oxynitride glasses obtained by the ammonolysis of phosphate melts [3]. Subsequent studies focused on changing the synthesis conditions so as to reach larger nitrogen contents (from few at% to over 10 at%), but also on broadening the spectrum of accessible glass compositions (introduction of alkali, alkaline earths, rare-earths), and finally on investigating their structure to get a better insight into their composition-structure-property relationship [4].

Indeed, these specialty glasses show exceptional properties, that are of strong interest for many applications. In oxynitride glasses, three-fold nitrogen substitutes for two-fold oxygen, resulting in increased cross-linking density, packing density and volume density of energy. As a consequence, they show unique physicomechanical properties with respect to their oxide counterparts, such as large glass transition tem-

perature (up to 1090 °C for RE-Si-O-N glass [5], where RE stands for rare-earth), density (5.83 g·cm⁻<fnref n="3" />, Er-Si-Al-O-N glass [6]), refractive index (2.31 at 633 nm, La-Si-O-N glass [5]), electrical conductivity (3·10⁻⁷ S·cm⁻<fnref n="1" /> at room temperature, Li-P-O-N glass [7]), and mechanical properties (elastic modulus, hardness, Poisson's ratio, and fracture toughness reaching approximately 190 GPa, 13 GPa, 0.33, and 1.59 MPa·√m respectively in Y-Si-Al-O-N and Mg-Si-Al-O-N glasses [8–13]), as depicted in Fig. 1 (a-f). Nevertheless, their use at large scale is limited by (i) their challenging and expensive synthesis conditions (high temperature, inert atmosphere, nitride reactants) and (ii) the loss of transparency, which becomes increasingly prominent with increasing nitrogen contents. Little success was achieved so far regarding this latter issue

As a matter of fact, oxynitride melts are complex reaction media, wherein multiple reactions occur concurrently. Indeed, not all oxides can be incorporated into oxynitride glasses. Some experience either reduction or nitriding, while polyvalent cations exhibit a propensity to adopt specific oxidation states. In this work, we present a thorough overview of the reactivity of oxynitride melts, in the light of previously reported studies. The emphasis is placed on the range of homogeneous

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: alexis.duval@uni-jena.de (A. Duval).</footnote>

<footnote n="1">¹ Present address: Otto Schott Institute of Materials Research, Friedrich Schiller University Jena, Lessingstraße 14, 07743 Jena, Germany.</footnote>

<footnote n="2">² https://orcid.org/0000-0001-9192-2454</footnote>

<footnote n="3">³ https://orcid.org/0000-0002-6360-6996</footnote>

<footnote n="4">⁴ https://orcid.org/0000-0002-9961-2458</footnote>
oxynitride glass compositions, redox state, and the elaboration of glass composites comprising metal or nitride particles. In addition, perspectives towards transparent oxynitride glasses, and crystallization of oxynitride and nitride phases to form oxynitride glass-ceramics, are discussed.

Finally, this study highlights several pending aspects regarding the chemistry and physics of oxynitride glasses (glass network structure, mechanical behavior, crystallization mechanisms). A prospect towards the future trajectories of oxynitride glass compositions (broader compositional range, larger nitrogen content, control over the redox state), investigating the potential to further tailor their properties, is addressed. Numerous innovative outlooks and functionalities of oxynitride glasses, glass-ceramics, and glass composites are presented, spotlighting their relevance for a wide range of cutting-edge technologies and contemporary applications.

## 2. Reactions in oxynitride glass melts

Ellingham diagrams serve as powerful tools to predict chemical reactions in both oxide and oxynitride melts, encompassing reactions among (i) the various oxides and nitrides, (ii) the melt and the crucible, and (iii) the melt and the atmosphere. Although these diagrams are only applicable to pure substances (where the activity coefficients are taken as unity), and cannot be used to predict the precise reduction rate of polyvalent cations in oxide glasses, they give a good and reliable estimation of the reactions occurring between nitride and oxide species within oxynitride melts (see Sections 3.1 and 3.3). In the Ellingham approximation, the Gibbs free energy $\Delta G^\circ$ associated with the oxidation reaction of a compound (whether it is a metal or a nitride) into its respective oxide, is calculated from the standard enthalpy of formation, $\Delta H^\circ$, and the standard molar entropy, $\Delta S^\circ$, $\Delta G^\circ(A_xO_y) = \Delta H^\circ - T\Delta S^\circ$, and both the standard reaction enthalpy, and the standard reaction entropy, are supposed temperature-independent over the temperature range under investigation. Then, provided that the system is at

thermodynamic equilibrium, one can predict the feasibility of various reactions within a melt. The reactions are normalized to 1 mol of $O_2$. Let us consider the oxidation reactions of two metals, generally termed M and A, into their corresponding oxides, $MO_y$ and $AO_i$, which write:

$$ \frac{2}{y}M + O_2 = \frac{2}{y}MO_y \quad (1) $$

$$ \frac{2}{i}A + O_2 = \frac{2}{i}AO_i \quad (2) $$

If the Gibbs free energy associated to a reaction is negative, the equilibrium is shifted to the right, resulting in the oxidation of the metal into its respective oxide. Conversely, whenever the Gibbs free energy becomes positive, the reaction inverses, and the oxide decomposes into its constituent metal and molecular oxygen.

From Eqs. (1) and (2), one can directly write the chemical reaction of a metal M with an oxide $AO_i$:

$$ \frac{2}{y}M + \frac{2}{i}AO_i = \frac{2}{i}A + \frac{2}{y}MO_y \quad (3) $$

It is noteworthy, however, that the Ellingham approach has some limitations, in particular when the temperature dependence of the specific heat of the products and reactants (Kirchoff's law) cannot be neglected. Oxidation reactions are mostly exothermal ($\Delta H^\circ < 0$) and since the specific heat of the products ($MO_y$) is usually larger than those of the reactants, $d\Delta H^\circ(T)/dT > 0$. But the change of $\Delta G^\circ$ is usually only of a few tens of J.mol$^{-1}$. The relative positions of the respective curves in the Ellingham diagrams allow to predict which chemical reactions occur throughout the melt. If, at a given temperature T, the Gibbs free energy associated with the $A/AO_i$ couple supersedes that of the $M/MO_y$ couple (i.e., if the curve associated to the $A/AO_i$ couple is above that of the $M/MO_y$ couple), then the reaction equilibrium is shifted to the right (i.e., $AO_i$ is reduced into A, and M is oxidized into $MO_y$). By way of comparison, whenever the temperature becomes smaller than T, the reaction

<figure><img src="image_4.png" /><figcaption>Fig. 1. (a) Young's modulus E, (b) hardness H (Vickers test), (c) fracture toughness $K_{tc}$ as measured by the indentation cracking method, (d) Poisson's ratio $\nu$, (e) coefficient of thermal expansion $\alpha$, and (f) refractive index $n$ of conventional glass families (oxynitrides, aluminosilicates, silicates, borates, phosphates, and chalcogenides) with varying glass transition temperature $T_g$ and density $\rho$. The data was collected using the SciGlass database. [14].</figcaption></figure>
equilibrium is shifted to the left. The difference between the two curves is the Gibbs free energy associated with reaction (3). The reaction is thermodynamically feasible when $ΔG° < 0$.

For the remainder of the study, let us more generally consider the following couples: M/MOx/MOy/MNz, and A/AO1/ANj. MNz and ANj are nitrides, and M exhibits various oxidation states (M$^{y+}$ and M$^{x+}$, where y > x). Typically, M encompasses transition metals or rare-earths, while A is an alkali or a p-block element (e.g., M = Ti, Nb, Eu, and A = Li, Al, Si). Hence, various reactions equilibria take place between MOx and MOy species, and it can be inferred that the difference in free Gibbs energy is the driving force governing which reaction occurs, and that the reaction with the largest change in Gibbs free energy is the most likely to occur. This is where the intrinsic error in the approach, which neglects any change of the formation enthalpies with temperatures, might be of importance. In particular, when the difference in Gibbs free energy

between the various reactions involved becomes small, the predominance of one or the other reaction is driven by other factors such as reaction kinetics, glass composition, and synthesis conditions (temperature, holding time). Based on Eqs. (1) and (2), such redox equilibria are generally expressed as:

$$ \frac{2}{y-x} \text{MO}_x + \text{O}_2 = \frac{2}{y-x} \text{MO}_y \quad (4) $$

In oxynitride melts, the introduction of nitrides in the glass batch (usually Si3N4 or AlN) as the nitrogen source induces several additional reactions. In the first place, a nitride ANj can undergo decomposition into its constituent metal and molecular nitrogen:

$$ \frac{2}{j} \text{AN}_j = \frac{2}{j} \text{A} + \text{N}_2 \quad (5) $$

<figure><img src="image_5.png" /><figcaption>Fig. 2. Gibbs free energies of various M/MOx couples: (a) alkali, (b) alkaline earths, (c) p-block elements, (d) rare-earths, (e) 3d elements, (f) 4d elements, (g) 5d elements, and (h) actinides. [16–20] Solid curves represent oxides unreactive towards Si3N4 (used as the nitrogen source in oxynitride glass synthesis), while dashed curves represent oxides reduced by Si3N4.</figcaption></figure>
It can also react with the ambient oxygen from the atmosphere, and oxidize into AOi:

$$ \frac{2}{i} AN_j + O_2 = \frac{2}{i} AO_i + \frac{j}{i} N_2 \quad (6) $$

It is worthy of mention that silicate and phosphate melts are insensitive towards molecular nitrogen [15].

From Eqs. (4) and (6), it stems that a nitride ANj can reduce an oxide MOy either into another oxide MOx:

$$ \frac{2}{y-x} MO_y + \frac{2}{i} AN_j = \frac{2}{y-x} MO_x + \frac{2}{i} AO_i + \frac{j}{i} N_2 \quad (7) $$

or into the metallic state M, according to Eqs. (1) and (6):

$$ \frac{2}{y} MO_y + \frac{2}{i} AN_j = \frac{2}{y} M + \frac{2}{i} AO_i + \frac{j}{i} N_2 \quad (8) $$

In rare occurrences, an oxide MOy can be nitrided into MNz by another nitride ANj, and it follows that:

$$ \frac{2}{y} MO_y + \frac{2}{i} AN_j = \frac{2}{y} MN_z + \frac{2}{i} AO_i + \left( \frac{j}{i} - \frac{z}{y} \right) N_2 \quad (9) $$

Si3N4 is the most utilized nitrogen source in silicate oxynitride glass synthesis. Consequently, we will discuss hereafter the reactivity between silicon-based compounds (A = Si) and other species, ranging from M = Li to M = Cf.

## 3. Oxynitride glass compositions and derived materials

### 3.1. Chemical systems

The Ellingham diagrams with the various M/MOy couples under investigation are represented in Fig. 2(a-h) [16–20]. For the sake of clarity, the M/MOy couples wherein the Gibbs free energies are positive over the 300–2000 K temperature range (such as the case of noble metals for example) are not shown. In instances where M can form multiple oxides, only the M/MOy couple showing the smallest Gibbs free energy across the considered temperature range, without decomposition, is indicated. As the substitution of oxygen for nitrogen increases the viscosity of glass melts, the synthesis temperatures of oxynitride glasses obtained by melt-quenching are mostly larger than those required for oxide glasses in order to allow for the pouring of the melt. The smallest temperature reported was 1300 °C for Li–Si–O–N glasses, thanks to the low-melting temperatures reachable in the Li2O–SiO2 chemical system. Hence, we will consider reactions occurring at temperatures larger than 1300 °C.

According to the obtained diagrams, alkaline earths (AE) and RE-based oxides do not react with Si3N4. This was confirmed experimentally, as both AE- (AE = Be, Mg, Ca, Sr, or Ba) and RE- (La–Lu) containing oxynitride glasses were described as homogenous [10,21–32]. The specific cases of europium and cerium oxides are discussed in Section 3.2.

Li2O is the sole alkali oxide that does not react with silicate oxynitride melts [33–35]. The possible incorporation of Na2O somewhat sparks controversy and seems to greatly depend on the synthesis conditions, as it sometimes violently reacts with the melt, whereas in other cases no elemental loss was observed [36,37] – suggesting a largely kinetically driven incorporation behavior. Other examples of kinetics-limited reactions include the case of titanium species (whether the reduction of Ti4+ to Ti3+, or the nitriding of TiO2 to TiN [38,39]), samarium and ytterbium species (where the reduction of Sm3+ and Yb3+ to Sm2+ and Yb2+ appears specific to the systems and processing conditions [23,40]), and the formation (or subsequent reactions) of silicon or silicide metallic particles (directly tied to the transparency of the

glasses, and strongly dependent upon the applied cooling rates [41]), as considered in Sections 3.2, 3.3, and 3.5. Na2O and K2O were successfully introduced in phosphate oxynitride glasses, where the phosphorous compositions were prepared by melting a crushed oxide glass at relatively low temperature (700–800 °C) typically in a graphite crucible in ammonia flow [42,43].

The research on 3d, 4d, and 5d transition metal oxides (TMO)-containing oxynitride glasses is rather confidential. Indeed, most experience either reduction or nitriding in oxynitride melts (discussed in detail in Section 3.3) [38,39,44]. A few exceptions include Sc2O3 [45], TiO2, Y2O3 [4,46], ZrO2 [47,48], and HfO2 [34]. It is noteworthy that the studied concentrations of Sc2O3, TiO2, and ZrO2 were usually small, as these oxides promote crystallization. Conversely, Y–Si–Al–O–N glasses were widely explored, owing to the large cationic field strength of Y3+ (CFS = z/r², where z and r are the valence and the ionic radius of a given cation, respectively). It effectively increases both the packing density and the stiffness of the glass network, thereby substantially increasing the mechanical properties of the resulting glasses. Although ZnO is expected to be reduced by Si3N4, which was highlighted in-situ by Laurent et al. [38,39], Luo et al. managed to synthesize Zn-containing oxynitride glasses [11]. Fe2O3- and ZnO-bearing phosphate oxynitride glasses were previously obtained [49,50].

In a similar manner, only a fraction of the oxides of p-block elements is compatible with silicate oxynitride glasses. On the one hand, aluminosilicate oxynitride glasses were reported as homogeneous, and were extensively studied. On the other hand, the elaboration of silicophosphate oxynitride glasses required a three-step synthesis to prevent phosphorus loss (detailed in ref. [51]), following:

$$ \frac{1}{3} Si_3N_4 + \frac{2}{5} P_2O_5 = SiO_2 + \frac{2}{5} P_2 + \frac{2}{3} N_2 \quad (10) $$

As with silicon, aluminum and phosphorus form bonds with nitrogen in the glass network. Experimental evidence reveals that PbO undergoes reduction to Pb [38]. Numerous efforts were devoted in achieving B-containing oxynitride glasses, in vain [52–54]. Indeed, (i) B2O3 react with Si3N4 to form BN [16–18], while (ii) the solubility of BN in glass-forming melts is particularly small (< 0.25 mol. %) [55,56]. This is corroborated by practical observations, where even small traces of boron nitride from the crucible cause a darkening of the resulting glass [57,58]. Consequently, no B-containing oxynitride glasses were obtained yet. On the other hand, Al2O3, GeO2, SnO, and PbO can be incorporated in phosphate oxynitride glasses [49,59,60].

Finally, the existing thermodynamic data pertaining to actinides primarily encompass early actinides, extending up to Cf. Late actinides are highly radioactive, and little is known regarding their thermodynamic properties [19,20]. Although oxynitride glasses containing actinide oxides were not reported, it is anticipated that early actinide oxides do not react with Si3N4, with the exception of Ac2O3, and are thus compatible with oxynitride glass compositions.

Regarding crucible materials – an essential step for producing oxynitride glasses with targeted chemical compositions while limiting the impurity content – Ellingham diagrams are also very helpful, as they allow to anticipate the plausible chemical reactions occurring in the typical synthesis conditions of these glasses (from 1300 to 1800 °C under protective atmospheres of N2 or Ar). Oxynitride glass-forming melts are very corrosive. It turns out that oxide-based (SiO2, Al2O3) and noble metal-based (Au, Pt, Ir) crucibles are found unsuitable [21,57,58, 61–71]. Hence, despite minor contamination arising from dissolution or side reactions, crucibles made from BN, C, Mo, Nb, and W remain largely used as they provide relative chemical inertness and cost effectiveness under these harsh processing conditions.

Fig. 3 provides a concise summary of the chemical compositions of oxynitride glasses detailed in existing literature [10–12,21–24,26–34, 38,39,42–46,49–52,55,56,59–61,68,72–84]. Notably, they cover a substantial range of the periodic table. It is clear that the chemical
<figure><img src="image_6.png" /><figcaption>Fig. 3. Reported cationic and anionic chemical compositions of oxynitride glasses. Colors indicate whether the elements are present as major components in silicate oxynitride glasses (green), nitrides (purple), metals (red), traces from crucible corrosion (yellow), or in -OH and -NH bonds (orange). Compositions only accessible in phosphate oxynitride glasses are shown in blue. [10–12,21–24,26–34,38,39,42–46,49–52,55,56,59–61,68,72–84].</figcaption></figure>

reactions predicted to occur in oxynitride melts, using Ellingham dia-
grams, align remarkably well with the literature data pertaining to
elemental loss, reduction, and nitriding of oxides. In a way, a connection
emerges in oxynitride glasses, wherein the Si-N and Si-O bonds can be
associated with Si₃N₄ and SiO₂, respectively. Moreover, the systemati-
cally larger free Gibbs energy (by approximately 120 kJ·mol⁻¹ over the
considered temperature range) of the oxidation reactions of Si₃N₄ to
SiO₂ with respect to that of P₃N₅ to P₂O₅ further supports the observa-
tion that a wider range of compositions is accessible for phosphate
compositions than for silicate ones.

A few cationic compositions remain either unexplored, or shallowly
investigated. These include silicate oxynitride glasses containing either
(i) Tm₂O₃ (although the chemical similarity among rare-earths elements
strongly suggests compatibility with oxynitride glasses), or (ii) radio-
active oxides (Pm₂O₃ and actinide oxides). Phosphate oxynitride glasses
offer a slightly broader scope of chemical compositions, encompassing
some oxides of p-block elements (GeO₂, SnO, PbO) as well as K₂O. It is
noteworthy that phosphate oxynitride glasses containing heavier alkali
oxides (Rb₂O, Cs₂O) were not reported. Surprisingly, only one article
focused on Ge-containing oxynitride glasses, despite the potential ad-
vantages of introducing nitrogen in germanate glasses for optical ap-
plications [59] notably in extreme environments (enhanced
temperature, chemical, and mechanical resistance).

As most cationic compositions were already explored, the past
decade saw a considerable attention put on the introduction of other
anions in oxynitride glasses. While oxycarbonitride glasses have
received limited consideration [79], extensive research endeavors were
conducted on the incidence of fluorine introduction on the glasses

structure and properties of both silicate and phosphate oxynitride
glasses. Fluorine increases the solubility of nitrogen and decreases the
glass transition temperature, with little changes in the mechanical
properties [85–87]. F-containing oxynitride glasses, thanks to their
biocompatibility, bioactivity, and large mechanical properties, emerge
as a promising avenue to address the issue of the poor compressive
strength of bioactive glasses [51]. It is noteworthy that no other halogen
elements beside fluorine were incorporated in oxynitride glasses. The
substitution of oxygen by both nitrogen and sulfur in glasses has also
been investigated in the area of batteries (electrolytes), with the aim to
combine the large ionic conductivity of sulfide glasses, and the chemical
durability of oxynitride glasses [82,83].

The nitrogen content of oxynitride glasses is commonly expressed in eq. % N, and is calculated from the valence of oxygen and nitrogen anions according to:

$$
\text{eq. \% } N = 100 \times \frac{3[N]}{3[N] + 2[O]} \qquad (11)
$$

where [N] and [O] refer to the atomic concentrations of nitrogen and oxygen, respectively. Typically, the maximum nitrogen content of oxynitride glasses reachable using conventional melt-quenching is close to 30 eq. % N. This primarily stems from the large synthesis temperatures required to obtain a homogeneous melt, and the propensity of N-rich oxynitride melts to crystallize during quenching, thereby hampering the possibility of achieving larger nitrogen contents. However, Ali et al. [26] introduced an innovative approach employing metals and hydrides as reactants, resulting in a remarkable increase of the nitrogen content up to ~ 60 eq. % N, without evidence of devitrification. This method was
later extended to AE- and RE-containing oxynitride glasses [32,88,89]. Furthermore, the utilization of aerodynamic levitation in conjunction with laser heating [90] suits particularly the synthesis of oxynitride glasses, as (i) few crucibles are unreactive towards oxynitride melts, and contamination from graphite or BN crucibles is common [58], (ii) large melting temperatures are required to obtain homogeneous melts, and (iii) small holding times effectively limit nitrogen and transparency losses [88,91]. Furthermore, a large cooling rate results in the expansion of the glass-forming region. Perspectives regarding this technique include densified oxynitride glasses [92], and overlooked aluminate oxynitride glasses [64,93,94]. It is noteworthy that, although ion-exchange in molten salts was never reported in the case of oxynitride glasses, it should result in significant increases of the resistance toward surface microcracking and strength, and induce large refractive index gradients (bear in mind that increasing nitrogen contents result in increased refractive indices [41,95,96]). At the same time, compositional gradients – in the form of nitrided surfaces obtained following heat treatments in an ammonia atmosphere slightly above the glass transition temperature – enable the achievement of multi-purpose glasses with controllable surface and bulk properties [97]. Taking a more unconventional approach, nitrides were used as foaming agents in glass batches by Lebullenger et al., enabling the design of glass foams with finely controllable apparent density, porosity (open or closed), and pore size [98]. These architectural materials are suitable for uses as photocatalysts, thermal insulators, and biomaterials. Moreover, the addition of nitrides to chalcogenide glass batches (referred to as chalconitride glasses) allowed to greatly increase the transmission in the IR range notably by removing numerous OH-related active bands [99].

Despite considerable progress, several questions remain regarding the atomic structure of oxynitride glasses, especially with respect to the interplay between the modifiers and nitrogen, as well as the distribution of nitrogen in the glass network [27,100–103]. The potential existence of nano-scaled phase separation [104,105], sometimes hinted in specific oxynitride glass compositions, albeit never unequivocally confirmed, could play a role regarding their high temperature behavior [4,106]. Only a limited number of neutron diffusion experiments, along with high-resolution transmission electron microscopy (HRTEM) coupled with energy-dispersive X-ray analysis (EDX) experiments, were performed on oxynitride glasses [27,107–109]. The challenge arises from the similarity in bond lengths between Si-O and Si-N bonds (1.61 and 1.72 Å [110], respectively), in addition to oxygen and nitrogen both being light elements.

Still, these specialty glasses exhibit superior corrosion resistance, increased glass transition temperature, thermal stability, density, electrical conductivity, dielectric constant, refractive index, and mechanical properties, as well as decreased thermal expansion coefficient, in comparison to their oxide counterparts [4,33,47,94,96,111–114]. Oxynitride glasses are therefore ideal candidates for a myriad of uses, ranging from protective coatings, reinforcement material, decorative enamels, joining or sealing material, to structural applications, optics, photonics, biomedicine, nuclear waste management, radiation shielding, and energy storage [4,115–118].

### 3.2. Redox state

The redox state of glasses containing polyvalent cations can be controlled to a certain extent through the fine tailoring of the synthesis temperature, time, and atmosphere [119–121]. Further thermal treatments also hold considerable sway over the redox state [122]. Moreover, the interplay between glass composition and optical basicity substantially comes into effect. Indeed, large optical basicities promote the higher oxidation state (except in the case of Cu⁺/Cu²⁺) [123,124]. While Al, C, or sugar are typically used as reducing agents [125,126], little to no efforts were devoted in using nitrides as reducing agents. Oxynitride glasses are systematically prepared under protective atmosphere (Ar, N₂) to prevent any nitrogen loss within the melt. Hence,

oxynitride melts emerge as favorable reaction media, offering a facile and quick means to tailor the redox state simply by weighing the right quantity of reducing agent (Si₃N₄, AlN), regardless of the cooling rate and glass optical basicity. The absence of molecular oxygen in the synthesis atmosphere enables to maintain the desired redox state once the reduction reaction is complete. On the one hand, the reduction rate ([M⁺]/[M⁻] + [M⁻]) can be tuned from 0 to near 100% [127] following Eq. (7). As this reaction occurs uniformly throughout the oxynitride melt, the redox state of glasses can accurately and effectively be defined not only on the surface of the melt if one were to use a reducing gas flow, but also in the bulk, even for large specimens (cm³ scale). On the other hand, cations with uncommon oxidation states for oxide and oxynitride glasses, usually present only in small quantities (e. g., Ti³⁺, W⁵⁺, or Bi⁺ [128,129]), can reliably be isolated. It is noteworthy that the ammonolysis of phosphate oxynitride melts also promotes the reduction of specific species (e.g., Eu³⁺ into Eu²⁺) [30].

Here again, Ellingham diagrams are a valuable tool for predicting the redox state in the melts. Indeed, the 95–100% complete reduction of both Ce⁴⁺ into Ce³⁺ [130] and Eu³⁺ into Eu²⁺ [23] was observed in oxynitride melts. According to these diagrams, the reduction reactions with Si₃N₄, such as the reduction of (i) CeO₂ into Ce₂O₃, and (ii) Eu₂O₃ into EuO, effectively become thermodynamically feasible starting from 540 and 1480 K, respectively [16–18]. In other words, one can associate the MOₓ/MOᵧ couples to the M⁺/M⁻ redox couples within oxynitride melts, e.g., EuO/Eu₂O₃ to Eu²⁺/Eu³⁺. However, the use of nitrides as reducing agents is not straightforward. Various chemical reactions compete in oxynitride melts, where MOᵧ can either be reduced into MOₓ or M, or nitrided into MNₓ. In addition, some elements present more than two different oxidation states (e.g., vanadium), so that multiple reactions have to be taken into account.

Let us consider the case of vanadium, depicted in Fig. 4(a-c). In a typical oxynitride glass synthesis (~ 2000 K) where Si₃N₄ is used as the nitrogen source, a competition ensues between (i) the reduction of V₂O₅ into VO₂, (ii) the reduction of V₂O₅ into V, and (iii) the nitriding of V₂O₅ into VN. Nevertheless, previous work revealed that V₂O₅-containing oxynitride batches yielded VN-containing oxynitride glasses [38,39], while metallic V was not observed. Consequently, it is essential to inspect step by step the predominance of each species. In this context, the reaction featuring the largest change in Gibbs free energy is that of the reduction of V₂O₅ into VO₂ (-537 kJ·mol⁻¹ at 2000 K). In a second step, a competition arises between (i) the reduction of VO₂ into V₂O₃, (ii) the reduction of VO₂ into V, and (iii) the nitriding of VO₂ into VN. This time, the reduction of VO₂ into V₂O₃ is the reaction with the largest change in free Gibbs energy, and it follows that VO₂ is preferentially reduced into V₂O₃. Finally, V₂O₃ is nitrided into VN. Vanadium nitride is stable and does not decompose into metallic vanadium at such temperatures (ΔG°ₗ/ VN < 0), so that no further reactions occur in the oxynitride melt. To summarize, V⁵⁺ first undergoes a reduction into V⁴⁺, which is subsequently reduced into V³⁺. Finally, V₂O₃ is nitrided into VN, which is unreactive towards the oxynitride melt. This approach is thus consistent with previously described experiments. In other words, the oxidation state of polyvalent cations can easily be adjusted without the precipitation of metals or nitrides, simply by choosing the appropriate quantity of batched Si₃N₄. The expected proportions of each species, depending on the Si₃N₄/V molar ratio in the batch, are shown in Fig. 4(d). According to Eq. (7), the reduction reaction of V₂O₅ into VO₂ writes:

$$4 \text{VO}_{5/2} + \text{SiN}_{4/3} = 4 \text{VO}_{2} + \text{SiO}_{2} + \frac{2}{3} \text{N}_{2} \quad (12)$$

For instance, if a glass initially contains 12 mol. % of VO₂/₂ (i.e., 6 mol. % of V₂O₅), then one must introduce $12 \times \frac{2/i}{2/(y-x)} = 12 \times \frac{1}{4} = 3$ mol. % of SiN₄/₃, i.e., 1 mol. % of Si₃N₄, so as to reach the complete reduction of V⁵⁺ into V⁴⁺. Hence, the required quantity of Si₃N₄ is particularly small with respect to that of the oxide to reduce. It is
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (a-c) Gibbs free energies of various oxidation and nitriding reactions of vanadium species. [16–18] Solid curves represent species unreactive towards Si₃N₄ (used as the nitrogen source in oxynitride glass synthesis), while dashed curves represent species reduced or nitrided by Si₃N₄. (d) Expected relative fractions of oxide and nitride vanadium species with an increasing Si₃N₄/V molar ratio in the batch.</figcaption></figure>

important to stress the fact that, despite molecular N₂ is formed during the reduction of polyvalent cations, no significant bubbling of the melt was reported, even for large concentrations of polyvalent cations (e.g., 15 at% of Eu [22,23,131]).

Nonetheless, this approach must be applied systematically to each polyvalent cation. Indeed, the differences in Gibbs free energies are sometimes rather similar between the various reactions involved. For instance, in the case of titanium species (see Fig. 5(a-b)), the change in Gibbs free energy associated with (i) the reduction reaction of Ti⁴⁺ into Ti³⁺, and (ii) the nitriding reaction of TiO₂ into TiN, is roughly similar at 2000 K. Preliminary experiments conducted by our group indicated that

small Si₃N₄ contents promote the reduction of Ti⁴⁺ into Ti³⁺, which is readily evidenced by the purple hue of the resulting glass [125]. Previous studies mentioned either the formation or the absence of TiN particles in Ti-containing oxynitride glasses [38,39].

Oxynitride melts are thus formidable reaction media, wherein by choosing the right synthesis conditions and reactants in the batch, one can directly define the oxidation state of numerous polyvalent cations within the glass (shown in Fig. 6(a-c)), without neither experiencing elemental loss nor loss of transparency. In addition, this approach provides a pathway to investigate the relaxation kinetics and mechanisms of specific mixed redox couples in glasses, e.g., Eu²⁺ + Ti⁴⁺ = Eu³⁺ + Ti³⁺.

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** (a-b) Gibbs free energies of various oxidation and nitriding reactions of titanium species. [16–18] Solid curves represent species unreactive towards Si₃N₄ (used as the nitrogen source in oxynitride glass synthesis), while dashed curves represent species reduced or nitrided by Si₃N₄.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Gibbs free energies of various MOₓ/MOᵧ couples: (a) 3d elements, (b) 4d and 5d elements, and (c) rare-earths [16–20]. Dashed curves represent oxides reduced by Si₃N₄ (used as the nitrogen source in oxynitride glass synthesis).</figcaption></figure>

An exhaustive list of redox couples, for which the reduction rate is directly tunable through Si₃N₄ addition, includes: Ti³⁺/Ti⁴⁺, V³⁺/V⁴⁺/V⁵⁺, Mn²⁺/Mn³⁺, Fe²⁺/Fe³⁺, Co²⁺/Co³⁺, Cu⁺/Cu²⁺, Nb⁴⁺/Nb⁵⁺, Mo⁴⁺/Mo⁵⁺/Mo⁶⁺, Ce³⁺/Ce⁴⁺, Eu²⁺/Eu³⁺, W⁵⁺/W⁶⁺, and Re⁴⁺/Re⁶⁺/Re⁷⁺. For the Cr²⁺/Cr³⁺/Cr⁶⁺ and Sn²⁺/Sn⁴⁺ redox couples, a competition occurs between the reduction of MOᵧ into either MOₓ or M. It is noteworthy that RE oxides are neither reduced into metallic species, nor nitrided, and are thus relatively stable as oxides in the oxynitride melts even at large nitrogen contents (> 15 eq. % N). In addition, it is believed that Sm²⁺ and Yb²⁺ were identified in Y-Si-Al-O-N glasses [40]. In the case of actinide oxides, the lower oxidation state is favored.

The redox state of glasses serves as a critical determinant of their properties. The most conspicuous change manifests in their color. In addition, light, thermal, and electrical excitation can induce structural alterations in the vicinity of polyvalent cations, as well as changes in their oxidation states, thereby affecting the optical properties of glasses. Moreover, the coexistence of polyvalent cations with different oxidation states can largely improve the glass electrical conductivity, as it provides an additional conduction pathway through polaron conduction (i.e., polaron hopping from the polyvalent cation with the lower oxidation state Mx⁺ to the polyvalent cation with the higher oxidation state My⁺ [132]). Furthermore, the redox state has great consequences for both the thermal (glass transition temperature, thermal expansion coefficient), mechanical (elastic moduli, hardness, fracture toughness, viscosity), and luminescent properties (emission wavelength, intensity, lifetime) of glasses [30,125,126,133–135]. Hence, a fine control over the reduction rate allows for the tailoring of these properties, enabling further functionalities through glass crystallization. An example, wherein the tuning of the Si₃N₄/Eu molar ratio in the batch allowed to adjust precisely the europium reduction rate, is given in ref. [136]. The control over the oxidation state of polyvalent species thus encompasses multiple perspectives for glasses, with potential uses not only as decorative glazes, functional windows, and smart glasses, but also in the fields of solid-state batteries, photonics, UV shielding, and dosimetry.

### 3.3. Composites

The reactivity of TMO in silicate oxynitride glasses was investigated by Turmel [38,39], and in phosphate oxynitride glasses by Le Sauze et al. and Tessier et al. [44,137]. In N-rich silicate oxynitride melts, it was evidenced that most 3d TMO experience reduction (Cr, Mn, Fe, Co, Ni, Cu) and/or nitriding (TiN, VN, Cr₂N), while ZnO and CdO are prone to volatilization. In phosphate oxynitride melts, metallic silver and copper were successfully precipitated within the glass matrix from the reduction of the corresponding oxides. Depending on the concentration of metallic and nitride particles, their size ranged from hundreds of nm to several µm.

Nevertheless, few metal/oxynitride glass and nitride/oxynitride glass composites were explored. Considering the thermal stability of the various nitrides, as well as the propensity of various metals to volatilize, one can infer from Fig. 2(a-h) the formation of the following metallic particles from the corresponding oxides in silicate oxynitride melts: Mn, Fe, Co, Ni, Cu, Ga, Ge, Mo, Ru, Rh, Pd, Ag, In, Sn, Sb, Te, W, Re, Tl, Pb, and Bi. Note that the formation of metallic silicon, and alloys with varying stoichiometries (e.g., Mn-, Fe-, Mo-, Ca-, Sr-, Ba-, La-, and Pr-silicides [32,39,41,61,138–141]), amorphous and crystalline [32,104,138,139], was also observed. They arise from various reactions involving silicon nitride (further discussed in Section 3.5). Moreover, the following nitrides are expected to form from the corresponding oxides in silicate oxynitride melts (see Fig. 7(a-c)): BN, TiN, VN, NbN, and TaN. In some instances, a competition may occur between the formation of (i) the metal and the nitride, or (ii) two nitrides of different stoichiometries. A notable example is Cr₂O₃, where the change in Gibbs free energies is similar for the formation of Cr, Cr₂N, and CrN. The predictions derived from Ellingham diagrams regarding the formation of metals and nitrides within oxynitride melts are consistent with the previously reported experimental observations.

The reduction and nitriding reactions occur with a loss of nitrogen, in the form of N₂. Depending on (i) the metal/oxide and nitride/oxide couples, and (ii) the concentration of the oxide in the glass batch, the released nitrogen can induce bubbling and foaming in the melt, resulting in expanded composite materials. In order to obtain TiN-containing
<figure><img src="image_10.png" /><figcaption>Fig. 7. Gibbs free energies of various MNz/MOy couples: (a) p-block elements, (b) 3d elements, and (c) 4d and 5d elements. [16–18] Colored solid curves represent oxides unreactive towards Si3N4 (used as the nitrogen source in oxynitride glass synthesis), while dashed curves represent oxides nitrided by Si3N4. The Gibbs free energy of the AlN/Al2O3 couple (where AlN is the other commonly used nitrogen source in oxynitride glass synthesis) is also represented.</figcaption></figure>

oxynitride glasses, Turmel preferably used a titanium oxynitride TiOxNy over TiO2 [38]. This strategy effectively limited the amount of molecular nitrogen produced, and consequently bulk nitride/oxynitride glass composites free from porosities were obtained. Nevertheless, certain oxides are nitrided without nitrogen loss, facilitating the achievement of bulk glass composites. Examples include B2O3, V2O3, and Ga2O3. In the case of B2O3, the nitriding reaction writes (see Eq. (9)):

$$2 \mathrm{B_2O_3} + \mathrm{Si_3N_4} = 4 \mathrm{BN} + 3 \mathrm{SiO_2} \quad (13)$$

In addition, little attention has been directed toward the control of metal and nitride particle sizes. As the reduction and nitriding reactions occur spontaneously inside the melt, this offers a prospect to finely adjust the size of the precipitated particles (from the nano- to the micro-scale) while ensuring a homogeneous distribution of these particles in the resulting glass composite. Transparent particulate glass matrix composites with improved mechanical performance and durability can be obtained by maintaining the particle size below 30 nm [142,143]. To accomplish this, it is imperative to precisely define (i) the synthesis conditions (temperature, holding time, melt viscosity), (ii) the particle/matrix formulations (suitable physical, optical, and thermal properties), and (iii) the concentration of the said particles. This methodology would enable the elaboration of glass composites featuring reinforcement particles intricately embedded in a resilient, corrosion-resistant, and inert glass matrix. Notably, by ensuring particle size below ~30 nm in radius, consequential improvement in mechanical properties are expected to be achieved – due to their inherently superior mechanical properties (e.g., elastic modulus and fracture toughness over three and ten times greater than the highest values reported for inorganic non-metallic glasses) – all while minimizing light scattering and thus preserving transparency, even with small concentration of reinforcement particles.

For instance, the introduction of 30 vol% of TiN or SiC particles into a Y-Si-Al-O-N glass matrix was followed by a 30 – 40 % increase in the elastic modulus, shear modulus, and hardness, while the fracture toughness remarkably increased by a factor 2 [38,144–148]. It is noteworthy that smaller particles yielded a more pronounced increase in

mechanical properties. Consequently, the precise tailoring of the particles concentration, size, shape, and distribution, holds promise to achieve substantial toughening of the resulting glass composites, all while preserving their transparency. Recently, the formation of nano-scale Te clusters by means of a controlled TeO2 reduction through the incorporation of AlN as a reductive agent allowed for the achievement of metal/oxide glass composites with tailored optical and luminescent properties, resulting notably in full visible-spectrum conversion [149]. It is worthy of mention that the crystallization of oxynitride glass composites, and the subsequent possibility of reactivity between the glass matrix and the second phase, which could potentially induce changes in the oxygen and nitrogen contents in both phases, was not studied. Metal/oxynitride glass and nitride/oxynitride glass composites are interesting for applications in areas such as photonics, energy, sensing, biomaterials, refractory coatings, and highly resistant windows to name a few of them [150–158].

### 3.4. Glass-ceramics

The conversion of an oxynitride glass into a glass-ceramic offers various advantages. Indeed, Si3N4 experiences partial decomposition during oxynitride glass synthesis (discussed in detail in Section 3.5), which results in the formation of nano- to micro-sized metallic particles (with diameters ranging from 10 nm to 10 µm [104,139,159,160]). These typically consist of iron silicides (FeSiα, where 0.33 < α < 3). Such heterogeneities are uniformly distributed within the whole glass specimens, and further act as preferential crystallization sites, allowing to obtain a uniform volume crystallization. This provides a singular pathway for attaining bulk crystallization in systems where, otherwise, crystallization is limited to the surface or can hardly be achieved. The distribution of silicide particles in the glass matrix can be estimated considering their velocity v in the field of gravity, following Stokes’s law:

$$v = \frac{D^2 g(\rho_{\mathrm{FeSi}_\alpha} - \rho_{\mathrm{melt}})}{18 \eta_{\mathrm{melt}}} \quad (14)$$
where D is the particle diameter, g is the gravitational acceleration, $\eta_{melt}$ is the viscosity of the melt, and $\rho_{melt}$ and $\rho_{FeSi_u}$ refer to the specific mass of the melt and of the silicide particle, respectively. Let us consider the synthesis of a common Y-Si-Al-O-N glass ($\rho_{glass} = 4 \text{ g.cm}^{-3}$) conducted at 1700 °C, where the melt viscosity reaches ~ $10^2 \text{ Pa.s}$, where 10 µm large FeSi particles are formed ($\rho_{FeSi} = 6.7 \text{ g.cm}^{-3}$). Thus, if one considers both $\rho_{melt}$ and $\rho_{FeSi}$ roughly independent on the temperature (i.e., $\rho_{melt} = \rho_{glass}$), it can be inferred that after a dwell time of 1 h, the movement of the silicide particles would be confined to only a few microns. By way of comparison, the N₂ bubbles resulting from the partial decomposition of silicon nitride would have traversed tens of cm within the same time-frame, allowing them to easily escape from the melt. As a consequence, oxynitride glasses may contain uniformly distributed silicide particles, while being free from gaseous heterogeneities. The crystal density within oxynitride glass-ceramics directly depends on the amount of silicide particles and, in fine, on the nitrogen content of the parent oxynitride glass and is typically as large as $10^{15} \text{ crystals.m}^{-3}$ for ~ 2.5 at % N. This makes them ideal candidates to reach a volumic and near complete crystallization of glasses into glass-ceramics, with large crystal densities even without prior nucleation heat-treatment and, consequently, small crystal sizes [136].

Previous studies dealing with oxynitride glass crystallization mostly focused on the identification of the crystalline phases [161–165]. Both oxide (SiO₂, LiAlSiO₄, CaAl₂Si₂O₈, Y₃Al₅O₁₂), oxynitride (MgAl₂·Si₄O₆N₄, Yb₄Si₂N₂O₇, Si₃Al₆O₁₂N₂, YSiO₂N), and nitride (ZrN, Si₃N₄) crystals were observed [24,166–169]. The emphasis was placed on evaluating the composition-dependent microstructure of the observed crystals, and on the subsequent incidence on the mechanical properties of the obtained glass-ceramics, where the elastic moduli, hardness, and fracture toughness approximately increase by ~ 25 %.

On the one hand, it is acknowledged from experimental data that crystal growth rates are directly linked to the increased glass network crosslinking density which follows from the substitution of 3 O²⁻ by 2 N³⁻ [161,163]. On the other hand, the nitrogen-content-dependent changes in crystallizing phases and their associated microstructures are not straightforward. This is exemplified by Höhe et al., who showed that the partial stabilization of tetragonal zirconia within oxynitride glass-ceramics did not strictly result from the incorporation of nitrogen, but rather from an indirect effect in which the spherical morphology of precipitated tetragonal zirconia at higher nitrogen contents suppresses the nucleation of the martensitic transformation [48]. In other words, our level of understanding of the role of nitrogen on atomic organization and crystallization remains relatively low. At small nitrogen content, whether nitrogen remains in the glass network or is incorporated into the newly formed oxide crystals remains unclear. This is an important issue as nitrogen-doping of oxide crystals significantly changes their properties (e.g., luminescent, optical, and electrical properties). Thorou gh investigations, combining spatially-resolved spectroscopies [170] (wavelength-dispersive spectroscopy (WDS), electron energy-loss spectroscopy (EELS), Raman spectroscopy), and in-situ temperature-resolved spectroscopies (¹⁵N nuclear magnetic resonance spectroscopy (NMR)), are essential to shed light on these intricate phenomena and effectively provide with a mechanistic description of the roles of nitrogen.

The introduction of nitrogen is found to extensively widen the range of accessible crystallizing phases resulting from glass to glass-ceramic conversion. A recent investigation conducted by our group utilized the reducing character of oxynitride melts so as to obtain $Eu^{2+}$ containing cm³ large oxynitride glass-ceramics, showing both persistent luminescence and mechanoluminescence [136]. Ternary and quaternary oxynitrides (and even nitrides) with desirable chemistries (O/N ratio), properties, and functionalities, e.g., piezoelectricity [171] or stress-sensitive light emission, might be elaborated by means of the glass-ceramic route. In the case of oxynitride glasses, this route allows to obtain large and homogeneous glass-ceramics in relatively short times (< 1 h).

Furthermore, the crystallization of glasses allows to promote the precipitation of non-equilibrium crystal phases, otherwise not accessible using conventional solid-state synthesis [172] where the most thermodynamically favorable phases are preferentially formed. Metastable oxynitride and nitride phases (e.g., TaOₓNᵧ), with innovative stimulus responsiveness, could be obtained this way. [173,174]

Oxynitride glass ceramics open a new realm of materials suitable for multifaceted applications, as photocatalysts, pigments, phosphors or white light-emitting diodes (LED) technologies, in various areas such as energy, environment friendly and optical materials [175–177].

### 3.5. Transparency

The transparency of glasses in the visible region is of primary importance for numerous optical applications (special windows, wear-resistant coatings) and the relative opacity of oxynitride glasses greatly reduces the scope of application of these glasses with exceptional mechanical properties. Several factors affecting the loss of transparency of oxynitride glasses were identified, such as (i) the presence of unmelted materials and bubbles in the melt, (ii) the precipitation of metallic and intermetallic compounds, (iii) the formation of interatomic bonds and structural units involving nitrogen which may partially absorb light in the visible range, and (iv) possible reactions with the crucible (whether SiO₂, Al₂O₃, or BN) [91,178,179]. Of course, these factors depend on the synthesis conditions (atmosphere, temperature, holding time, crucible material, and cooling rate). It is noteworthy that N-rich amorphous oxynitride thin films (notably in the Mg-Si-O-N, Ca-Si-O-N, and La-Si-O-N chemical systems) are optically transparent – in contrast to their bulk counterparts – highlighting the predominant role of bulk oxynitride glass processing in altering optical quality [104, 180–182].

The presence of unmelted materials cannot be predicted from the Ellingham approach as it chiefly relates to the kinetics of the chemical reaction. Temperature and holding times are of primary importance and this issue can mostly be solved by an iteration processing method. In a very similar way, bubbles can be removed by refining treatments, possibly facilitated by the addition of refining agents (for example sodium sulfate) in small quantities, which do not affect the final properties of the glass.

Regarding the precipitation of metallic and intermetallic particles, it stems from the partial decomposition during the synthesis of oxynitride glasses, which is evidenced by both an elemental loss of nitrogen (roughly 10–20 % of the nominal composition), and by the formation of silicon and silicide particles [138,139,178,179,183]. Metallic Si promptly reacts with several oxides in the melt, as exemplified in the following reactions:

$$5\text{Si} + 2\text{BaO} \rightarrow \text{SiO}_2 + 2 \text{BaSi}_2 \quad (15)$$

$$\frac{7}{3}\text{Si} + \frac{2}{3}\text{Fe}_2\text{O}_3 \rightarrow \text{SiO}_2 + \frac{4}{3}\text{FeSi} \quad (16)$$

These reactions are thermodynamically favorable in the usual temperature range of oxynitride glass synthesis ($\Delta G° = -33$ and $-462 \text{ kJ.mol}^{-1}$ at 2000 K, respectively) [16–18]. As a consequence, metallic Si is observed to a lesser extent with respect to silicide particles [104]. Due to their small melting temperature relative to the temperature range of oxynitride glass synthesis, these metallic nano- and micro-inclusions are subsequently observed in the form of spherical particles dispersed in the glass matrix. The decomposition of silicon nitride was attributed to various thermally activated reactions [58,65, 104,139,159,160,183–186], such as:

$$\frac{1}{2}\text{Si}_3\text{N}_4 \rightarrow \frac{3}{2}\text{Si} + \text{N}_2 \quad (17)$$

$$\text{SiO}_2 + \text{Si}_3\text{N}_4 \rightarrow 2\text{SiO} + 2\text{Si} + 2 \text{N}_2 \quad (18)$$
Consequently, the larger the Si₃N₄ content and the synthesis temperature are, the larger the volume fraction of metallic particles becomes. This severely deteriorates the optical transparency of the resulting glasses, progressively imparting a grayish hue, culminating in a complete opacification typically with nitrogen contents larger than 15 eq. % N [4,139]. Nevertheless, none of these reactions are thermodynamically feasible in the temperature range of oxynitride glass synthesis. Still, even at temperatures as small as 1300 °C, the introduction of nitrogen in Li₂O – SiO₂ binary glasses induced consequential darkening of the specimens, although the Gibbs free energies of reactions (17) and (18) are particularly large (115 and 371 kJ·mol⁻¹, respectively) and thus cannot account for this loss in nitrogen [16–18,161]. In addition, despite impurities (especially iron species) immediately react with silicon nitride to form various silicides, the use of high-purity reactants had little effects on the glass transparency [183]. Therefore, the source for the partial decomposition of Si₃N₄ remains unclear, and its relationship to the loss of transparency is not definitively established.

The incorporation of nitrogen in an oxide silicate glass results in a significant increase of the atomic packing density and tightens the atomic network. Although the incidence on the optical properties was little studied to date, there might be some correlation to the alteration of the transparency in the visible range. Further investigations are clearly needed in this area.

Various approaches were explored to prevent the opacification of oxynitride glasses, albeit with sparse success. It is noteworthy that in certain cases, slow cooling rates (~ 1 K·min⁻¹), [41] or further thermal treatments [61,159], can improve the optical transparency of oxynitride glasses. Still, these methods are limited to a narrow range of glass compositions, as they may induce partial to total crystallization of the glass specimens. Hence, it appears that a pathway towards both transparent and N-rich oxynitride glasses lies in achieving an appropriate balance between processing parameters (decreased cooling rates and working temperatures, increased holding times and N₂ pressures to promote the aggregation and sedimentation of metallic particles whilst limiting their formation), chemical compositions (particularly targeting highly devitrification-resistant compositions), and sample geometries (wherein smaller sample thicknesses can effectively mitigate the detrimental effects of the precipitating metallic particles).

Another pathway to prevent the loss of transparency of oxynitride glasses was but briefly mentioned by Messier et al. [187]. This approach lies in identifying a cation M⁺ (MO₃ compound) that is able to oxidize the formed Si particles into Si⁴⁺, and that is further reduced into another cation M⁺ (MO₃ compound). To achieve this, the MO₃ and MO₄ oxides must satisfy specific conditions, stemming from Eqs. (1) to (4) and (6) to (9):

(A) MO₃ must oxidize Si into SiO₂, and thus be reduced into MO₄. Therefore, the free Gibbs energy of the following reaction must be negative:

$$ \frac{2}{y-x} \text{MO}_3 + \text{Si} \rightarrow \frac{2}{y-x} \text{MO}_4 + \text{SiO}_2 \quad (19) $$

(B) Neither MO₃ nor MO₄ should be reduced into metals, which would remain in the oxynitride melt and, ultimately, in the resulting glass, hence decreasing the optical transparency. Consequently, the free Gibbs energies of the following reactions must be positive:

$$ \frac{2}{y} \text{MO}_3 + \text{Si} \rightarrow \frac{2}{y} \text{M} + \text{SiO}_2 \quad (20) $$

$$ \frac{2}{x} \text{MO}_4 + \text{Si} \rightarrow \frac{2}{x} \text{M} + \text{SiO}_2 \quad (21) $$

$$ \frac{2}{y} \text{MO}_3 + \frac{1}{3} \text{Si}_3\text{N}_4 \rightarrow \frac{2}{y} \text{M} + \text{SiO}_2 + \frac{2}{3} \text{N}_2 \quad (22) $$

$$ \frac{2}{x} \text{MO}_4 + \frac{1}{3} \text{Si}_3\text{N}_4 \rightarrow \frac{2}{x} \text{M} + \text{SiO}_2 + \frac{2}{3} \text{N}_2 \quad (23) $$

(C) Neither MO₃ nor MO₄ should be nitrided into MN₂. To maintain transparency, it is essential to prevent the occurrence of any micro- and macro-scopic heterogeneities, including both metal and nitride particles. As such, the free Gibbs energies of the following reactions must be positive:

$$ \frac{2}{y} \text{MO}_3 + \frac{1}{3} \text{Si}_3\text{N}_4 \rightarrow \frac{2}{y} \text{MN}_2 + \text{SiO}_2 + \left(\frac{2}{3} - \frac{z}{y}\right) \text{N}_2 \quad (24) $$

$$ \frac{2}{x} \text{MO}_3 + \frac{1}{3} \text{Si}_3\text{N}_4 \rightarrow \frac{2}{x} \text{MN}_2 + \text{SiO}_2 + \left(\frac{2}{3} - \frac{z}{x}\right) \text{N}_2 \quad (25) $$

(D) MO₃ must not be reduced into MO₄ by reacting with Si₃N₄. By doing so, MO₃ would directly react with Si₃N₄ and be consumed, so that no MO₃ species would remain to oxidize the metallic Si within the oxynitride melt. Thus, the free Gibbs energy of the following reaction must be positive:

$$ \frac{2}{y-x} \text{MO}_3 + \frac{1}{3} \text{Si}_3\text{N}_4 \rightarrow \frac{2}{y-x} \text{MO}_4 + \text{SiO}_2 + \frac{2}{3} \text{N}_2 \quad (26) $$

An ideal, but fictitious case, is illustrated in Fig. 8. It becomes evident that identifying an appropriate M/MO₄/MO₃/MN₂ couple meeting the previously stated requirements is rather challenging. Graphically, the Gibbs free energy associated with Eq. (4) must lie in the narrow green region, while the Gibbs free energies associated with Eqs. (1) and (6) must fall below it. In addition, the specified region becomes even narrower in the usual temperature range of oxynitride glass synthesis (> 1575 K).

Only two MO₄/MO₃ couples fit these criteria: Ce₂O₃/CeO₂ and EuO/Eu₂O₃ [16–19]. However, the said conditions are solely met up to 540 K and 1480 K, respectively. A plausible strategy entails defining an oxynitride glass composition with an especially small melting temperature (< 1480 K), e.g., by incorporating additional modifiers (Na₂O, MgO, or CaO) to low-melting-temperature chemical systems such as Li₂O – SiO₂ – Si₃N₄ glasses (recall that the Li₂O – SiO₂ binary system exhibits both several eutectic points, including one at ~ 1030 °C for a Li₂Si₂O₅ composition, as well as a glass-forming ability within a Li₂O content

<figure><img src="image_11.png" /><figcaption>Fig. 8. Gibbs free energies of various metal/oxide, nitride/oxide, and oxide/oxide couples. [16–18].</figcaption></figure>
range of approximately 20–40 mol. % [111,188,189]). If synthesis temperatures smaller than 1480 K can be reached, it effectively becomes possible to (i) substantially limit the partial decomposition of Si₃N₄, and (ii) re-oxidize the formed metallic Si by doping the glasses with Eu₂O₃. Although this latter example (Li-silicon oxynitride glasses) illustrates a promising approach for achieving transparent glasses even with large nitrogen contents, and is supported by recent works where Eu-containing oxynitride glasses showed improved transparency [127], its applicability is limited to a narrow range of glass compositions. In addition, RE oxides are particularly expensive, and Eu²⁺ slightly colors the resulting glasses with either a green (small Eu²⁺ content, ~ 1 at%) or an amber (large Eu²⁺ content, several at%) tint [131]. Furthermore, let's recall that the reactions that are considered in Fig. 8. might occur at very different rates (kinetics). In particular, the decomposition of Si₃N₄ into SiO₂ and N₂ is a very slow process below 1500 K. At such low temperatures, obtaining a homogenous melt free from unmelted raw materials may require prolonged heating – or the use of powders of smaller granulometry and thus larger surface area and associated reactivity.

### 3.6. Applications and research perspectives

Attention in this study has been devoted primarily to the chemical reactions occurring within oxynitride melts; however, it is also pertinent to highlight promising applications and avenues for future research – as schematically illustrated in Fig. 9. They encompass glassy systems – whether transparent or not – formulations that incorporate redox-active species, metal- and nitride-containing glass composites, and partially to fully crystallized materials. Prospects for applications are diverse, extending across energy technologies (energy storage, nuclear waste management, solid-state electrolytes for batteries), sensing (remote monitoring, dosimetry, stress and heat transfer sensing), optics and photonics (high-transmission fibers, high gradient-index-optics, photo-catalysts, radiation shielding, broadband wavelength conversion, stress-induced luminescence, phosphors and pigments), biomedicine (bio-imaging, mechanically strong prosthetic materials exhibiting both biocompatibility and antimicrobial properties), smart windows and displays (responsive to light, temperature, mechanical stress, electric field), as well as processing technologies (glass-metal sealing), niche and high-performance applications requiring specific couplings of phys-omechanical properties and material processing (wind turbine blades, decorative enamels, as well as hard, refractory, and functional coatings), but also architecturally tailored materials in the form of compositional gradients, mixed-matrix composites, or tailored porosity (thermal in-sulators, structural materials, and stimuli-responsive materials with adaptable mechanical, chemical, and luminescent behaviors).

In this context, a renewed interest in oxynitride glasses is fueled by advanced compositional and processing approaches aimed at enhancing mechanical performance (Al₂O₃ and RE₂O₃-rich compositions produced by aerodynamic levitation, ion exchange). Moreover, this study highlights promising research avenues, including probing the local nitrogen environment in glass networks and its evolution during crystallization, controlling environment-dependent redox equilibria in glass-forming melts, and exploring non-equilibrium crystal phase formation. Furthermore, new strategies are envisioned to achieve nitrogen-rich yet transparent bulk oxynitride glasses, as well as glass composites with reinforcement particles of controllable size, volume fraction, and distribution. Collectively, these directions aim to deepen the fundamental understanding of oxynitride systems' physicochemical behavior, thereby advancing their practical applications in light of emerging and environment-friendly technologies.

## 4. Conclusions

The reactivity of oxynitride melts was assessed in the light of Ellingham diagrams. The obtained results – notably regarding the range of accessible compositions for both phosphate and silicate oxynitride

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Summarized technological opportunities and emerging research axes for oxynitride glasses and derived materials (glass composites, glass-ceramics).</figcaption></figure>

systems – are consistent with previously reported experimental observations. This novel insight into the chemistry of oxynitride melts, and in particular the redox state of transition metals and rare-earth elements and the reduction and nitriding reactions, sheds some light on the source for the loss of transparency and on the directions to take to solve this problem – while offering unique means of glass functionalization.

Advances in materials science (emerging synthesis techniques, chemical strengthening) open up prospects for expanding the oxynitride glass forming regions and bring about new applications. However, fundamental investigations to understand the effect of nitrogen on the structure, on the atomic bonding chemistry, on the crystallization behavior and, ultimately, on transparency, remain to be carried out.

### CRediT authorship contribution statement

**Alexis Duval:** Writing – original draft, Methodology, Investigation, Data curation, Conceptualization. **Patrick Houizot:** Writing – review & editing, Supervision. **Tanguy Rouxel:** Writing – review & editing, Supervision.

### Declaration of Competing Interest

The authors 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 acknowledge financial support from Région Bretagne and from the European Research Council (ERC Adv. Grant n°320506 "DAMREG").

### Data Availability

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