Structure and crystallization behavior of binary silver tellurite glasses and glass ceramics

Using Raman spectroscopy in conjunction with X-ray diffraction (XRD), transmission electron micrographs (TEM-EDP), and scanning electron microscopy (SEM), the local environment of Te atoms and the crystallization behavior of glasses in the system of chemical formula xAg2O. (100 − x)TeO2 (25 ≤ x ≤ 50 mol%) have been examined. Crystalline structure was evident in the XRD spectra of glasses containing 40 and 50 mol% TeO2. On the other hand, an amorphous structure has been observed in glasses with lower Ag2O concentrations (25, 30, and 35 mol%). Thermal heating could be used to crystallize the amorphous glasses’ structure. Using DSC measurements, the treatment's temperature was controlled. The species with good crystallinity include Ag2Te4O9 and Ag2Te2O5. The results of TEM and EDP, as well as both SEM and XRD, revealed that in glasses enriched with Ag2O, crystalline clustered species were formed. Raman data proved that the crystalline clustered is improved as a result of the formation of TeO3 units enriched with nonbridging oxygen bonds. In compositions containing less than 40 mol%, Ag2O plays the role of a glass modifier. At higher Ag2O concentrations, it plays the role of building crystalline clusters of the Ag2Te4O9 and Ag2Te2O5 types.

The notation Q n m has been proposed for modified telluride glasses [13][14][15][16][17], where n signifies the amount of bridging oxygen (BO), and m determines the coordination number. Because the abundance of each tellurite polyhedron was supposed to be dependent on the species' charge, the number of non-charged tellurite polyhedra (Q 4 4 ) was thought to decrease as modifier oxide concentration increased. Alternatively, an increase in charged polyhedral (Q 3 4 ; Q 1 3 , Q 0 3 ) units was simply observed. The tellurium structural unit with the greatest TeO 2 concentration and lowest modifier oxide is a four-coordinated trigonal-bipyramid TeO 4 (Q 4 4 ), which may be generated in the paratellurite (a-TeO 2 ) polymorph of crystalline TeO 2 . Some of the stretched bonds can simple be broken when the TeO 2 concentration was lowered, resulting in a three-coordinated structure known as (Q 3 4 ; Q 1 3 ; Q 0 3 ). A similar approach was considered in alkalitellurite glass structures [15][16][17]. It was assumed that five tellurium polyhedra can be identified in tellurium glass systems modified by some extent of alkali or silver oxides [13][14][15][16][17].
Using 23 Na NMR spectroscopy, the formation of crystalline clusters produced from Na 2 TeO 3 may then be validated [17]. Sodium ions appear to be randomly distributed at low concentrations. At intermediate concentrations, there is also a strong middle-range order. The existence of some crystalline phases in the sodium tellurite system has been confirmed. In sodium-rich glasses, however, Na 4 Te 4 O 10 [13,17,18] and Na 2 TeO 3 may form. Recent studies have shown that Ag 2 O plays a similar role as played by Na 2 O in TeO 2 glasses since Ag 2 TeO 3 crystalline phases are confirmed to be formed in Ag 2 O-rich glasses.
The objective of this work is to determine the impact of both glass composition and thermal heat treatment processes on the structure of tellurite glasses. The findings of XRD, TEM-EDP, and Raman spectroscopy of silver telluride glasses and their crystalline derivatives are presented in this study. The transformation of amorphous to crystalline silver tellurite phases gives the material an extra advantage to be used in the biomedical field of application. Additionally ordered tellurite glasses can be used as high refractive indices and dielectric constants materials.

Sample preparation
The melt quenching technique was used to prepare glasses in the system xAg 2 O. (100x)TeO 2 , (25-50 mol%). Samples are obtained from reagent-grade mixtures of AgNO 3 and TeO 2 which have been melted in a Pt-Au crucible. To remove NO 2 , the specimens were heat-treated at a slow rate of 2°C/min from room temperature to 300°C and then melted for 20-30 min between 700 and 800°C before being quenched by pouring the melt between two metallic plates.

Sample characterization
A Shimadzu X-ray type Dx-30 diffractometer is used for X-ray diffraction measurements. The values of the maximum peak and intensity are used to determine the material type that is compared to patterns in the joint committee for powder diffraction 108 standards' international powder diffraction file (PDF) database (JCPDS). A transmission electron microscope (TEM) of type JEOL-JEM-1011 was used to determine the size and shape of the studied samples. Microstructural data were tested using the JSM-7500F field emission scanning electron microscope. The machine operated with an accelerated voltage of 25 kV. All samples were sputter-coated with a thin layer of gold (3-4 nm) to avoid sample charging and increase the signal-to-noise ratio. The samples containing 30 and 35 mol% Ag 2 O were heated in a muffle furnace (Heraeus KR170) with a temperature control of less than 2°C. The samples were heattreated at a temperature of 200°C for a treatment time interval of 10 h. After heating, the glasses were then kept in the furnace and held at the temperature of heat treatment for the desired time before cooling normally at room temperature. Using a Raman confocal microscope, WITec model Alpha 300, with laser power of 532 nm, and 10 mw in the range of 200-2000 cm -1 were recorded. The obtained spectra were deconvoluted to get information about the structural changes of the basic units in these glasses.

XRD results
Depending on the glass composition, both crystalline and amorphous natures can be possessed in the studied binary silver tellurite glasses. Glasses containing 25, 30, and 35 mol% Ag 2 O have the amorphous structure which is the most dominant, as reflected from XRD patterns presented in Fig. 1. Higher Ag 2 O (40 and 50 mol%) concentration leads to the formation of a specific type of crystalline species in the main tellurite glass network.
As shown in Fig. 1, the tendency for crystal formation increases as the Ag 2 O content increases from 35 to 50 mol%. There are weak diffraction peaks visible in the spectra of glasses containing 35 and 40% mol%. Conversely, glasses with a higher Ag 2 O content can resolve XRD peaks that are more intense and sharp. The crystalline aggregate of Ag ions in the tellurite network is confirmed to be the source of two distinct diffraction peaks, specifically those at 33°and 38°. This behavior suggests that other silver-containing species may have developed in high silver-containing glasses.
Adding Ag 2 O at the expense of TeO 2 can directly convert TeO 4 to other units containing NBO atoms [15][16][17]. High modification levels, on the other hand, can result in the formation of more NBO as well as additional structurally clustered species in the telluride network. At extremely higher Ag 2 O content (40 and 50 mol%), as in the present study, most Ag 2 O can be consumed to build crystalline cluster species of type Ag 2 TeO 3 [12][13][14]. This role was identified to be dependent on the bond type formed between Ag cation and oxygen ions. These findings suggest that at high Ag 2 O concentrations [13,14], it would be difficult to confirm its modifier role only in the binarymodified glasses, but it can play an additional role that appears in cluster formation.
Based on the above arguments, it can be concluded that TeO 4 tetrahedral units are the main forming species in the telluride network. Some ordered TeO 4 groups can be converted to TeO 3 units after the addition of a modifier [12][13][14][15][16][17][18][19]. Extremely high modifier contents can result in extra TeO 3 groups. (The tellurium unit contains one and two nonbridging oxygen atoms.) Furthermore, some crystalline species of type Ag 2 TeO 3 are formed in silverrich telluride glasses as well as Na 2 TeO 3 crystalline clusters which have been formed in silver and sodium-rich telluride glasses [14,17,19]. Figure 2 presents the TEM micrograph with its electron diffraction pattern (EDP) for two different compassions (30 and 50 mol% Ag 2 O) that were introduced as examples for the amorphous and crystallized samples, respectively. As shown in Figs. 2(a and b), the main structural groups are distributed separately with an irregular repetition. Furthermore, in the composition of higher Ag 2 O content (50 mol% Ag 2 O), the specific types of clusters are seen to be distributed as aggregates or clusters in the host main network. The above arguments agree well with the general concept of cluster formation [8][9][10][11]. In such a situation, the term ''cluster'' is generally defined as an aggregate or accumulation of bound atoms or molecules that are between the sizes of a molecule and a bulk solid [8,9]. This argument was supported to a great extent by experimental studies of Ag-rich telluride glasses, which revealed the presence of Ag clusters when the concentration of Ag 2 O is relatively high (50 mol% or more), see Fig. 2. In the sample of composition 50 mol% Ag 2 O, the highly crystallized telluride and Ag metallic phases are the dominant phases.

SEM-EDS spectra
On a selection of compositions, SEM-EDS analyses were performed to determine how composition affected morphology. Figures 3 and 4 display the SEM images and EDS spectra of glasses with 30 and 50 mol% Ag 2 O, respectively. There are tellurium, oxygen, and silver peaks to be seen. With an increase in Ag 2 O content, Ag's silver peak intensity increases. The two peaks for Ag and Te atoms are virtually similar at 30 mol% Ag 2 O, indicating that Ag 2 O is only used in TeO 2 modification. However, the sample with a 50 mol% Ag content had a higher peak intensity of Ag than Te. The XRD and TEM-EDP results are both confirming the higher level of modification of Ag to form accumulated crystalline clusters. As an alternative, it is also thought that the higher loading of Ag at the expense of Te atoms is the reason for the increase in relative intensity.

Effect of thermal heating
It can be realized from XRD patterns (Fig. 1) that samples of 25, 30, and 35 mol% Ag 2 O possessed an amorphous structure. Therefore, the thermal heat treatment at 200°C provided by the DSC curves (Fig. 5) was shown to have a significant impact on the structure of the presented compositions. The amorphous structure of 25, 30, and 35 mol% Ag 2 O-containing glasses could be changed into a more regular structure as seen from XRD spectra (Fig. 6) of samples treated thermally at 200°C for 10 h. The pattern from untreated samples is compared with the XRD patterns in Fig. 6. At 200°C, the sample is almost completely crystalline, with a little amount of glass phase visible in the background. The ordering of the Ag cations in the primary tellurite network is the polymorph of Ag 2 Te 4 O 9 [20][21][22]. This crystalline species is thought to be a metastable silver-like compound found in silver telluride samples with compositions of 30 and 35 mol% Ag 2 O.

Raman spectra
Raman spectral data are a complementary measurement that may be used for the investigation of the vibrational modes of the molecules that relies upon inelastic scattering [23,24].
It can be shown from Fig. 7 that both the intensity and relative area of the Raman bands at about (400-500 cm -1 ) and 600-700 cm -1 ) assigned to TeO 4 and TeO 3?1 showed a decreasing trend. Such smearing bands can be assured through the deconvolution analysis technique (DAT) previously discussed by different authors [25][26][27][28][29]. Figure 8 is an example of the deconvoluted spectra for two samples of glasses. The analyzed values of the relative area for each band are listed in Table 1

Conclusions
The XRD spectra of glasses in the xAg 2  Author Contribution GED suggests the object of study, interpreted the data and wrote early drafts of the manuscript. WAM performed in situ measurements including Raman and XRD spectroscopy. MS and AA conceptualized the experiment and revised the drafts and final version of the manuscript. All authors read and approved the final manuscript.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Declarations
Conflict of interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, and there is no professional or other personal interest of any nature or kind in any product, service, or company that could be construed as influencing the position presented in or the review of, the manuscript entitled.
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