Relationship of Metallophilic Interactions with Structural and Mechanical Properties of (1−x) (0.73GeSe₂-0.27Sb₂Se₃)-xAg₂Se Glasses

Abstract

The effect of Ag₂Se content on the structure and mechanical properties of (1−x) (0.73GeSe₂-0.27Sb₂Se₃)-xAg₂Se glasses is analyzed. The glass structure is studied using XRD and NMR analyses. A particular consideration relates to a multiple increase in plasticity with increasing silver selenide content in chalcogenide glasses. The observed effects are attributed to the formation of silver–silver metallophilic interactions.

1. Introduction

Flexible electronics are currently developing rapidly [1,2,3], expanding into new application areas. A particular example is the use of flexible electronic sensor devices directly on controlled moving objects, such as the human body or clothing [4,5]. To ensure the structural and functional stability of all components of flexible electronic devices, they must withstand repeated mechanical deformations, including the exploration of heating above room temperature [6,7,8]. For this reason, the development of inorganic materials with increased plasticity is an essential objective.

Most of the available inorganic semiconductors are brittle at room temperature [9,10,11]. The development of flexible electronics requires ductile semiconductors. The discovery of plasticity in the Ag₂S semiconductor [12,13,14] provides a breakthrough in resolving the contradiction between mechanical deformability and semiconductor properties and may facilitate rapid progress in flexible electronics [15,16,17]. Currently, one of the most promising materials for flexible sensors is ductile crystalline inorganic semiconductors based on Ag₂S, Ag₂Se, and Ag₂Te [18,19,20].

Until recently, three main groups of materials were used in flexible electronics, including inorganic nanocrystalline [21,22,23], inorganic amorphous [24,25,26], and organic compounds [27,28,29]. Nanocrystalline semiconductor materials exhibit relatively high stability of their functional properties, but low flexibility and plasticity. Organic semiconductors are quite flexible, but their properties are unstable over time. Inorganic amorphous semiconductors, represented primarily by amorphous silicon, feature an intermediate level of these performances and hardly adjustable electronic properties.

A special group of materials useful for flexible optoelectronics involves plastic glassy materials [30,31,32]. However, glassy materials commonly feature a high brittleness, resulting in their low resistance to mechanical stress and temperature fluctuations [33]. The same features relate to glassy semiconductors (chalcogenide glasses) used in IR optical fibers [34], micro-optics, and non-volatile memory devices based on phase-change materials [35], etc.

Silver-containing chalcogenide glasses have been studied in a series of research works. A high interest in these materials is primarily determined by their ionic conductivity and related properties [36,37]. In [38], (Ag₂S)_x(GeS₂)_100−x, (x ≤ 50) were characterized using neutron and X-ray diffraction techniques coupled with reverse Monte Carlo (RMC) modeling. Based on these studies, it was concluded that Ag ions are coordinated with three S atoms. Short (appr. 3.0 Å) Ag-Ag distances are observed in glasses enriched with Ag₂S. Glass transition temperatures (Tg) of glasses in the system Ag₂S–GeS₂–As(Sb)₂S₃ determined by differential thermal analysis (DTA) [39] indicate their high glass-formation ability. The bandgap Eg of these glasses was estimated on the basis of optical absorption spectra. It is noteworthy that none of the available publications present any discussions relating to the plasticity and existence of Ag-Ag metallophilic bonds in silver-containing chalcogenide glasses.

However, chalcogenide glasses with a high silver content exhibit increased plasticity compared to other chalcogenide glasses [40,41,42] due to the ability of silver atoms to form non-directional metallophilic bonds in the glass network.

Glasses have a number of advantages over crystals with respect to their use in flexible optoelectronics, including a gradual change in composition, higher ionic conductivity compared to crystals of the same composition, relatively low susceptibility to impurities due to being disordered systems, the possibility to fabricate products of almost any shape using modern glass technologies, etc. [43,44].

Glasses capable of incorporating at least 20 mol% Ag₂Se without losing their glass-forming capacity exhibit increased plasticity and may be promising materials for use in flexible electronics [41]. In [42], an analysis of the glass transition temperature of chalcogenide glasses was performed depending on the silver chalcogenide content. The observed changes in glass transition temperature are explained by the coexistence of silver–silver (Ag–Ag) metallophilic bonds in addition to silver–chalcogen (Ag–Ch) covalent bonds. When assessing the degree of connectivity of the chalcogenide glass network, it is conventionally assumed that the number of bonds formed by each atom coincides with its oxidation state. However, the analysis of a large number of studied silver-containing chalcogenide glass-forming systems has led us to the conclusion that the connectivity coefficient of silver in chalcogenide glasses significantly exceeds its formal oxidation state, since silver forms metallophilic bonds in addition to covalent bonds [38].

A more detailed analysis of the data presented in [42] suggests that the T_g value of the considered glass-forming system is primarily determined by germanium selenide GeSe₂, since this compound contains a metal with the highest coordination number in this system. It was also concluded that Ag₂Se in the studied glasses exerts the same effect on T_g as selenides of trivalent As and Sb.

The research [41] presents the dependence of the plasticity of glasses in the (1−x) (0.73GeSe₂-0.27Sb₂Se₃)-xAg₂Se system on the Ag₂Se concentration, calculated using the Milman relation [45] and measured by the load–unload method. The observed increase in plasticity may contribute to a significant improvement in functional properties, in particular, improved resistance to temperature fluctuations, and has good potential for use in flexible electronics. Based on the above, the (1−x) (0.73GeSe₂-0.27Sb₂Se₃)-xAg₂Se system, which features high plasticity and is promising for use in flexible electronics, was selected as an example for this study, aiming at the further development of the concept of metallophilic interactions of silver atoms in chalcogenide glasses and the study of their effect on the structure and stress relaxation process.

2. Materials and Methods

Glass synthesis. Chalcogenide glasses were synthesized from high-purity simple substances: Sb (99.995%), Se (99.997%), Ag (99.990%), and Ge (99.999%). The following (1−x) (0.73GeSe₂-0.27Sb₂Se₃)-xAg₂Se sample compositions were synthesized: x = 0.00; 0.05; 0.10; 0.15; 0.20; 0.25; 0.30; 0.35; 0.40; 0.45. For each case, the components of the corresponding composition were placed in fused silica ampoules, followed by their evacuation to a pressure of 10⁻⁴ mm Hg and sealing to guarantee constant glass composition during the synthesis. For all the compositions, the synthesis was carried out in a muffle furnace PM-10-20 (Teplopribor, Chelyabinsk, RF) at 900 °C for 3 h with permanent stirring. To increase the cooling rate, the ampoules were placed in ice water straight after the synthesis.

XRD measurement was carried out according to the methods described in [46] using a D2 Phaser automatic powder diffractometer (Bruker, Billerica, MA, USA) using Cu Kα monochromatic radiation in the range of 2θ angles from 6 to 100° with a scanning step of 0.02°.

NMR characterization of all the prepared chalcogenide glasses ground into powder was carried out using a Bruker Advance III 400 WB spectrometer (Bruker, Billerica, MA, USA) on ⁷⁷Se nuclei as described in [47]. The spectra were recorded using a standard Hann pulse sequence (90°-τ-180°- acquisition) with the 90° pulse duration 6 μs, delay time between excitation pulses 70 μs, and relaxation delay time between pulse series 20 s.

Mechanical properties were characterized with respect to the effect of Ag₂Se content in chalcogenide glasses on the relaxation of mechanical stresses. Cylindrical glass specimens with a diameter of 6 mm were cast in fused silica ampoules. After removal from the ampoules and mechanical treatment of the cylinder ends, their height was 7 mm. A compressive pressure of 10 N/mm² was applied to the cylindrical specimens. The position of the compressive surfaces was then recorded for a specified time (the first three cycles—150 s, subsequent cycles—300 s), and the change in pressure over time was measured, followed by repeating the cycle.

3. Results and Discussion

3.1. XRD Data and Analysis

The experimental XRD data were corrected, taking into account air scattering, polarization, and absorption, followed by normalization to derive the overall Faber–Zeeman structure factor S(Q) [48]. The resulting S(Q) data for the studied glasses are shown in Figure 1.

The dependence of the structure factor on the wave vector (Q = 4 π(sinΘ)/λ) for glasses forming a network structure is characterized by three peaks [49,50]. The peaks 1 and 2 (starting from small Q values) are usually called the first sharp diffraction peak (FSDP) and the principal peak (PP), respectively, while peak 3 is universal and determined by interatomic correlations between nearest neighbors. The PP is attributed to longer scales in real space. Its position in a purely tetrahedral system, such as germanium chalcogenides, is determined by the base-to-apex distance in the GeX₄ tetrahedron [50]. FSDP is associated with an even longer scale relating to the medium-range order and confirms its existence in glasses [51,52,53].

In recent years, numerous models have been proposed to explain the origin of FSDP, which can be divided into three main approaches, involving the cluster approach based on layered structures [54,55,56] and approaches considering intrastructural voids [57] and rings as elements of a disordered glass network [58].

The medium-range order in glass (derived from FSDP) is characterized by two values, the peak position (2π/Q is correlation length) and half-width (2π/ΔQ is coherence length) [59,60]. FSDP data for the studied glasses are presented in Figure 2. The baseline was subtracted from the structure factor vs. wave vector dependence, and the resulting curve was approximated by a Gaussian.

Upon introduction of Ag₂Se into the glass matrix, a decrease in the FSDP amplitude, correlation length, and coherence length occurs. A decrease in the FSDP amplitude relating to PP amplitude may be attributed to the disruption of the spatial network formed by the GeSe_4/2 and SbSe_3/2 structural units by silver. A disruption of the spatial covalently linked network and an increase in its lability should lead to a sharp decrease in the viscosity of the glass-forming melt with increasing temperature and an increase in the coefficient of thermal expansion. Such properties are typical for metallic glasses [61,62], for which no FSDP is commonly observed [50]. The absence of FSDP is also observed for colloidal glass-forming systems [50]. A disruption of the spatially covalently linked network and an increase in its mobility can occur for two reasons. The first one is a decrease in the degree of connectivity of the glass network (average coordination of the atoms forming the glass network). The second possible reason is the weakening of interatomic bonds, leading to their destruction with increasing temperature and transition to a molten state.

The decrease in correlation length L_cor and coherence length L_cog can also be associated with disturbances in the spatial network of the glass matrix by the introduction of Ag₂Se. Moreover, all these regularities are observed in the entire studied range of Ag₂Se concentrations. The determined correlation length values (Figure 2C) are between the values characteristic of pure glassy GeSe₂ (6.3 Å (Q_o = 1 Å⁻¹ [63])) and Sb₂Se₃ (5.4 Å (Q_o = 1.17 Å⁻¹ [64])).

The shift in L_cor values with increasing silver concentration from values more characteristic of a network formed by GeSe_4/2 structural units to those corresponding to the network formed by SbSe_3/2 units may indicate that the introduction of Ag₂Se primarily affects the network formed by GeSe_4/2 tetrahedra. This conclusion is consistent with the NMR spectral analysis results presented below. L_cog decreases quite significantly upon the introduction of Ag₂Se into the glass matrix (Figure 2C).

By Fourier transforming the presented data, we can obtain the total correlation function (T_x(r)):

$$T_X\left(r\right)=4\pi {\rho }_{0}r+{\displaystyle \frac{2}{\pi }}{\int }_0^{Q_{max}}Q\left[S_X\left(Q\right)-1\right]\sin \left(Qr\right)dr$$

where ρ_o is the atomic density.

The correlation functions obtained in this way are shown in Figure 3A.

The resulting value T_x(r) × r yields the radial distribution function (RDF), which shows the number of atoms located at a distance from r to r + dr from any given atom.

These data allow us to estimate the coordination numbers (CN) of the elements in the glass structure. Since the peaks corresponding to the first and second coordination spheres overlap, we assume that the boundary between them is at the minimum of the function. In this case, integration yields an average number of atoms in the closest vicinity of an arbitrary atom equal to 2.20 for the original glass and 3.1 for the composition containing 40 mol% Ag₂Se. A theoretical estimate, assuming an average coordination of Ge, Sb, Ag, and Se atoms of 4, 3, 3, and 2, respectively, gives values of 2.58 and 3.00, respectively. In addition, Figure 3B shows, as an example, the experimental dependence of RDF for a composition containing 40 mol% Ag₂Se and the theoretical partial contributions to RDF from GeSe_4/2, SbSe_3/2, and AgSe_3/2, jointly providing a good approximation of the first coordination sphere. The considered partial contributions were approximated by a Gaussian. The lengths of the corresponding bonds were taken as the positions of the maxima of the function: 2.37 Å (Ge-Se) [65]; 2.62 Å (Sb-Se) [64], 2.68 Å (Ag-Se) [66]. The areas of the Gaussian were calculated as the result of multiplying the corresponding coordination numbers and weighting coefficients. The obtained good agreement allows us to assume that Ge, Sb, and Ag are surrounded by 4, 3, and 3 Se atoms, respectively. It is noteworthy that the determined coordinations correspond to the formal oxidation states of all the involved elements, except silver. Coordination 3 is also characteristic of Ag in Ag₂S-GeS₂ glasses [38]. The same value was derived from the correlation between the glass transition temperature and the average coordination of atoms for nano-liquating chalcogenide glasses with Ag₂Se [67].

In a series of studies [68,69,70], the results of direct structural characterization methods (Isotopic Substitution Neutron Diffraction; Extended X-ray Absorption Fine Spectroscopy; and High-Resolution Neutron Diffraction) for chalcogenide glasses with high silver chalcogenide contents suggested that silver atoms are highly coordinated. Moreover, silver-to-silver coordination was found to be no lower than 2. The results of our studies are consistent with these data. Furthermore, approximation of the experimental RDF plot, taking into account the contribution from the second coordination sphere, requires the presence of an additional peak near 3 Å (Figure 3C). This approximation was carried out with fixed parameters (bond length and CN) relating to Ge-Se, Sb-Se, and AgSe bonds, while the peak in the region near 3 Å and the peak emulating the second coordination sphere were fitted with free parameters. According to the reference data for silver-containing chalcogenide glasses [38,71,72,73], the region near 3 Å corresponds to Ag–Ag interatomic correlations. In [74], this distance is considered between silver atoms in apexes AgX_3/2 pyramids connected by edges and formally relates to the second coordination sphere. For the glass containing 40 mol% Ag₂Se (Figure 3C), the obtained Ag–Ag distance is 3.03 Å, and CN = 1.8. The value CN = 1.8 is also consistent with data obtained for glasses of the Ag₂S–As₂S₃ and Ag₂S–GeS–GeS₂ systems [75].

The peak corresponding to the second coordination sphere is apparently determined by the secondary neighbors Ge–Ge and Se–Se. According to the reference data [65] for glassy GeSe₂, the corresponding values are 3.57 Å, CN = 3.2 for Ge-Ge and 3.89 Å, CN = 9.3 for Se-Se.

3.2. NMR Data and Analysis

In all ⁷⁷Se NMR spectra of the studied glasses, a non-uniformly broadened peak is observed with a maximum near 260 ppm. Moreover, the shape of the spectra changes with the variation of silver selenide content due to the redistribution of atoms (Ge, Sb, and Ag) located in the first coordination sphere of selenium atoms.

A detailed consideration of ⁷⁷Se NMR spectra for glasses with x = 0.00, 0.20, and 0.40 (Figure 4A–C) indicates that the observed spectral bands feature a quite complex shape and cannot be approximated by a single line. For their fitting, three bands were used at x = 0, and four lines for each of the other samples. The approximation allowed us to distinguish the components at 500, 320, 140, and −60 ppm in the studied spectra. According to [76,77,78,79,80], they can be attributed to Se atoms in Se-Se-Ge or Se-Se-Sb environment near 500 ppm, in Ge-Se-Ge environment (most likely, germanium selenide tetrahedra linked by vertices rather than edges) near 320 ppm, in Sb-Se-Sb environment near 140 ppm, and in Ge-Se-Ag environment near −60 ppm.

Furthermore, similar to the Ge-Se-Ge and Sb-Se-Sb pairs, it can be supposed that the signal from Sb-Se-Ag should relate to more negative chemical shift values (to the right in the spectra). However, there are no obvious signs of any detectable signals in this region.

The fraction of the first component near 500 ppm relating to Se atoms in Se-Se-Ge or Se-Se-Sb environment as a function of Ag₂Se concentration is shown in Figure 5.

Evidently, the probability of Se-Se bond formation significantly decreases with the increase in silver content. Furthermore, to explain the data in Figure 5A, we note that according to [65], the GeSe_4/2 tetrahedron is the main building block in similar glasses with a predominant GeSe₂ content, the regular chemical order of the glass is disrupted, and up to 25% of Ge and 20% of Se atoms participate in homopolar bonds. Since Ge has the highest coordination among glass-forming elements, it forms a rigid glass network with high average coordination, providing a stabilization of a significant concentration of short-range chemical order disturbances (Se-Se homo-bonds). In the glass with x = 0.0, almost a third of GeSe₂ is replaced by stibium chalcogenide, featuring a lower coordination of 3 according to XRD data. Therefore, the glass network becomes less rigid and stabilizes only about 10% of Se-Se bonds (Figure 5A). The introduction of Ag₂Se into the glass makes the network even less rigid. However, unlike the introduction of Sb₂Se₃ into GeSe₂, the average coordination number does not decrease in this case. In addition to the covalent bond with Se, Ag also forms metallophilic Ag-Ag bonds, which, unlike covalent bonds, are not directional and have a lower energy strength. As a result, the number of selenium atoms forming Se-Se homo-bonds decreases to 3%, i.e., only 1.5% of selenium valence bonds participate in the formation of homo-bonds. As discussed above, the existence of Ag-Ag metallophilic bonds is suggested by quantum-chemical calculations [20]. The presence of these bonds is further confirmed by an X-ray Photoelectron Spectroscopy (XPS) characterization of the glasses at the studied cross-section. The data presented in the “Supplementary” section indicate that the increase in Ag₂Se content in the glass matrix above the threshold concentration, providing the formation of metallophilic bonds, results in a growth of Ag bond energy. Hence, the formation of Ag-Ag metallophilic bonds in addition to Ag-Se covalent bonds leads to an increase in the bonding energy (see Figure S1).

Figure 5B, based on the NMR spectral results, shows a similar analysis concerning the effect of Ag₂Se concentration on the contribution of the second peak, with the maximum at 320 ppm relating to the Se atom surrounded by Ge (Ge-Se-Ge). These data show that the increase in Ag₂Se concentration results in a decrease in the integrated area of the first component by a factor of 3, while for the second component, it drops only by a factor of 1.2. This 1.2-fold reduction in the relative intensity of the second component is determined by the decrease in molecular fraction of germanium and antimony selenides in the glass composition with increasing x. This may indicate that the number of germanium selenide tetrahedra linked at their vertices remains virtually unchanged, and the glass matrix remains primarily composed of these structural elements.

Less significant changes with increasing Ag₂Se concentration are observed in the area of the third component, featuring a maximum at 140 ppm and attributed to selenium atoms in the Sb-Se-Sb environment. As shown in Figure 5C, the integrated area of this component does not change with Ag₂Se concentration. Although it seems unexpected since an increase in Ag₂Se content should lead to a decrease in the proportion of selenium bound to Sb, it should be taken into account that this band is associated with selenium not only in Sb-Se-Sb, but also in Ge-Se-Sb configuration. A decrease in the amount of selenium in the Ge-Se-Ge configuration (Figure 5B) is probably partly due to their transformation into Ge-Se-Sb. Therefore, the total amount of selenium in both configurations remains constant.

The percentage of the fourth component near −60 ppm relating to Se atoms in the Ge-Se-Ag environment as a function of Ag₂Se concentration is shown in Figure 5D. The addition of Ag₂Se in the amount up to 45 mol% evidently and expectedly leads to an almost linear increase (from appr. 0 to 20%) in the content of this component relating to Se atoms in the Ge-Se-Ag environment. Moreover, simple calculations show that the proportion of Se in Ag₂Se for glass with 40 mol% Ag₂Se is about 23%, confirming that Ag₂Se is primarily bound to GeSe₂.

3.3. Mechanical Properties of the Glasses

The effect of Ag₂Se content in the synthesized glasses upon their mechanical properties is illustrated by the following data. The stress relaxation plots for samples with different Ag₂Se contents are shown in Figure 6A with the indication of testing cycle numbers. The stress reduction curve over time was fitted by an exponential function (Figure 6B). The characteristic stress relaxation times for cycles 5, 6, and 7 determined from the corresponding exponents are shown in Figure 6C, depending on Ag₂Se content.

A prominent increase in relaxation time begins for compositions with an Ag₂Se content of more than 0.2. To verify the results obtained, a prolonged experiment consisting of a single cycle was performed for samples with x = 0 and x = 0.4. A stress of 50 N/mm² was applied to each sample for a short period of time (3 min) (Figure 7A), followed by the observation of the stress relaxation within 24 h (Figure 7B). Such a long relaxation time allowed us to obtain more informative plots, which could no longer be satisfactorily approximated by a single exponent. Therefore, the Kohlrausch function [81] was used to fit the following data:

$$\mathsf{\sigma }={\sigma }_{\infty }+ {\sigma }_0\cdot exp\left(-{\left({\displaystyle \frac{t}{{\tau }_0}}\right)}^{\beta }\right),$$

where τ₀ is an effective (averaged) relaxation time; σ₀ and σ_∞ are relaxable and non-relaxable parts of the initial stress; t is the process time; β ≤ 1 is the parameter characterizing the width of the relaxation time spectrum, the effective value of which is τ₀.

The decrease in β indicates broadening of the relaxation time spectrum. As an example, the approximation of the glass relaxation process for the glass x = 0.4 is shown in Figure 7C.

The effective (average) relaxation time τ₀ determined for glass without Ag₂Se (x = 0) is 24 s, while for glass with x = 0.4 it is 50 min. The stress applied to that glass decreases by half within 20 min, and by the end of relaxation decreases by a factor of 4. It should also be noted that for the Ag-free glass β = 0.21, while for glass with x = 0.4 it is 0.51. These data allow us to make the following conclusions. First, they confirm the conclusion that an increase in Ag₂Se content in the glass composition leads to an increase in the relaxation time based on the results of the experiment with multi-stage loading of the samples. This indicates a change in the relaxation mechanism of mechanical stresses. Simultaneously, a multiple increase in the pre-exponential factor σ₀ (the relaxed portion of the stress) is observed. Second, stress relaxation for Ag-free glass is caused by processes in a wider range of different structural units of the glass, while in glass with x = 0.4, unified structural units responsible for stress relaxation appear. These structural units may comprise fragments containing numerous labile metallophilic bonds.

One of the most concise definitions of plasticity is the ability to relax mechanical stress. Based on this concept, we can find the values of plasticity (δ) for the glasses with x = 0.0 and 0.4 using the following expression, affording the calculation of the relaxable part of stress initially arising in the sample:

$$\delta ={\displaystyle \frac{{\sigma }_0}{{\sigma }_0+{\sigma }_{\infty }}}$$

The calculation yields the values 0.1 and 0.7 for the glasses with x = 0.0 and 0.4, respectively, being in good agreement with the corresponding values δ₀ = 0.2 and δ₈ = 0.7 determined using the Milman equation [45]. This publication also states that the T_g value of the glass with x = 0.4 is 200 °C, which is 20 °C higher than that of conventional chalcogenide glasses such as As₂S₃ and As₂Se₃. Thus, the introduction of Ag₂Se into the chalcogenide glass increased its plasticity several times while maintaining a high softening point.

Memory elements based on phase-change materials (PCM) have been well-known for quite a long time, starting from Ge₂Sb₂Te₅. Although the range of related materials is currently being extended, particularly including metal selenides [64], the problem of high stresses at the glass–crystal interface is not fully addressed due to the rigidity of germanuum, gallium, and stibium chalcogenide structure [82]. The achieved plasticity growth from 0.1 to 0.7 upon the addition of Ag₂Se to Ge₂Se and Sb₂Se₃ is a promising approach to this objective.

According to NMR data, the increase in Ag content in the glass network leads to a decrease in a part of non-stoichiometric bonds like Se-Se and an increase in the part of Ge-Se-Ag bridging bonds replacing Ge-Se-Ge. Taking into account the data (particularly XRD) indicting that Ge forms GeSe4/2 with rigid covalent bonds and Ag features a reduced coordination and rigidity due to a higher contribution of electrostatic interactions, the replacement of Ge by Ag in the second coordination sphere should result in an increase in the structural lability, i.e., to mechanical stress relaxation, as confirmed by the study of mechanical properties. NMR results suggesting the increase in Ge-Se-Ag bridging bonds with the growth of Ag content correlate with XRD data indicating a decrease in FSDP amplitude, as well as a reduction in the correlation length and coherence length.

In order to study possible application areas of the prepared silver-containing glasses, some of their functional properties were characterized. The related experimental data and discussions are summarized in the “Supplementary” section. In particular, the dependences of the glass microhardness (Figure S2), the light fundamental absorption edge (Figure S3), and the crystallization stability of the glass (Figure S4) on the Ag₂Se content are presented. The difference between the crystallization temperature of the glass and the glass formation temperature is chosen as a measure of their crystallization stability.

4. Conclusions

According to XRD results, silver has a high coordination (3), which does not coincide with its formal valence. The observed disappearance of FSDP indicates an increase in glass network lability with increasing Ag₂Se content. Considering the high coordination of silver, the increase in the lability of the glass network cannot be explained by a decrease in the degree of its connectivity. Therefore, the cause is the formation of non-directional metallophilic Ag-Ag bonds with a softer interatomic interaction potential and lower bond energy compared to covalent interactions. The increased lability of the glass network also results in a decrease in the proportion of defective structural elements with disrupted short-range chemical order, i.e., covalent bonds between selenium atoms in stoichiometric glass. The measured mechanical properties also indicate an increase in glass lability and plasticity. Incorporating 40 mol% Ag₂Se into the glass increases plasticity from 0.1 to 0.7, and the characteristic stress relaxation time also increases from 24 s to 50 min, which means a change in the relaxation mechanism of mechanical stresses. This is accompanied by a decrease in the relaxation time spread, as evidenced by an increase in β parameter in the Kohlrausch equation. Thus, an increase in the microscopic and macroscopic lability of the glass while maintaining the average coordination of the atoms forming the glass network indicates a change in the nature of the chemical bonds. To ensure increased plasticity, these changes must lead to a decrease in the directionality and rigidity of the interatomic interaction potential. Such changes can ensure the formation of metallophilic Ag-Ag bonds.