**1.1 Composition of tellurite glass**

The selection of tellurite glass components when used in binary combinations with other materials is very important. It directly affects the glass-forming ability, thermal stability, refractive index, rare earth ion doping concentration, and spectral characteristics. **Table 1** lists the range of TeO2 glass formation in several binary tellurite glass systems. **Table 1** shows that TeO2 exhibits the largest glass formation range in the case of the three binary systems TeO2-ZnO (100–52 mol%), TeO2-WO3 (94.7–61.3 mol%), and TeO2-TiO2 (100–52 mol%).

The structure of tellurite glass is always generally based on binary systems. The ternary and multivariate tellurite glass systems have been generally used as rare earth-doped substrates for the investigation of the waveguide spectral properties. The diversity of components has helped to improve the chemical and thermal stability of tellurite glass-based devices. **Table 2** [14] includes the composition of the tellurite glass systems that have been reported in recent years. In all cases TeO2 was used as the glass-forming material, and its content was generally higher than 50 mol% (as shown in **Table 1**). Other oxides were generally used as modified bodies. From **Table 2** [14], it can be seen that the research objects of the binary system were more diversified. In addition to the common monovalent alkali metal oxides and divalent alkaline earth metal oxides, many other oxide components were involved, including CeO2, SmO2, V2O5, etc. It should be pointed out that in the case of the ternary tellurite glass systems, the TeO2-ZnO-RmOn and TeO2-WO3-RmOn glass systems were the most widely investigated, because these two systems possessed a wide range of glass formation regions and a wide range of adjustable components.

In this polyhedron, one Te atom is surrounded by four oxygen atoms, of which two oxygen atoms Oeq are at the equator position and the other two oxygen atoms Oax at the axial position. The Te atoms are linked by Oax or Oeq into TedOdTe; an apex of the tetrahedron on the equatorial plane remains unoccupied by oxygen atoms and is

**Binary system Ternary system Multicomponent glass**

TeO2-M3O4 (M = Co) [21] TeO2-WO3-R2O (R = Li, Na, K) [22] TeO2-ZnO-B2O3-GeO2-

TeO2-M2O3 (M = Sm, La) [21] TeO2-WO3-BaO [15] TeO2-ZnO-Na2O-Bi2O3

TeO2-CeO2 [21] TeO2-WO3-Bi2O3 [15] TeO2-ZnO-WO3-TiO2-

La, Bi) [16]

TeO2-B2O3-M2O3 (M = Al, Ga, Sc,

TeO2-PbF2 [33] TeO2-K2O-La2O3 [34] TeO2-ZnO-Nb2O5-Gd2O3

**system**

[20]

[24]

[29]

[32]

[35]

Na2O [23]

Na2O [25]

TeO2-Li2O-Nb2O5-K2O

TeO2-ZnO-R2O (R = Li, Na, K) [16] TeO2-ZnO-B2O3-K2O [10]

TeO2-ZnO-RO (R = Ba, Mg, Sr) [19] TeO2-ZnO-GeO2-Na2O

TeO2-WO3-Nb2O5 [28] TeO2-ZnO-Nb2O5-Nb2O3

chemical bonding were different from the traditional glass-forming bodies (B2O3, SiO2, GeO2, and P2O5), which determined the specificity of the tellurite glass

research, the general laws could be classified as follows:

Some scholars used various testing methods to conduct research and analysis on tellurite glasses, especially binary system tellurite glass. Jha et al. [37] considered that the main structural units of tellurite glass were TeO4 double triangular bipyramids (tbp's) and TeO3 bipyramids (bp's) triangular pyramids. In 1995, Neov et al. [36] were the first to perform neutron diffraction analysis on lithium tellurite glass and pointed out that in addition to the TeO4 structural unit, a deformed double triangular pyramid TeO3+1 existed in the glass network. One of the TedO bonds was significantly longer than the other three. Due to the short-range similarity between the glass and crystal structures, the structure of tellurite glasses can be studied and analyzed based on the structure of tellurite crystals with the same composition. Sakida et al. [32] compared the Raman spectra of alkali tellurite crystals, pure tellurite glasses, and alkali tellurite glasses. The resulting Raman spectra were considered to correspond to structural elements in the glass. TeO4 (tbp's) double triangular bipyramids were finally transformed into a TeO3 (bp's) triangular pyramid by TeO3+1. Tatsumisago et al. [38] studied the change of tellurite glass structure with temperature using Raman spectroscopy. Throughout the above

1. It was generally considered that there were two kinds of structural units that form a tellurite glass network. One was TeO4 (tbp's) double triangular pyramid in which the Te atoms were arranged as a four ligand, and the other

s lone electron pair [16]. This special polyhedral structure and the

occupied by Te<sup>0</sup>

TeO2-R2O (R = Li, Na, K, Rb, Cs,

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

TeO2-MO (M = Zn [17], Ba, Pb)

TeO2-M2O5 (M = P [21], V [26],

TeO2-MO3 (M = W [30],

*Tellurite glass systems [14].*

Ti) [15]

Nb [27])

Mo [31])

**Table 2.**

[18]

structure.

**263**

#### **1.2 The structure of tellurite glasses**

Early research was reported to suggest that the molecular structure of pure tellurite glass molecules comprised TeO4 double triangular bipyramids (tbp's) [36].


#### **Table 1.**

*The formation range of binary system tellurite glass.*



**Table 2.**

relatively broadband gain spectrum, which led to it attracting a great deal of research attention which has persisted up to the present. Currently, many worldclass university-based research institutions and industrial companies have investigated the potential of tellurite glass for use in fibers, and this has resulted in rapid progress. In this section, the composition, structure, and thermal stability of

The selection of tellurite glass components when used in binary combinations with other materials is very important. It directly affects the glass-forming ability, thermal stability, refractive index, rare earth ion doping concentration, and spectral characteristics. **Table 1** lists the range of TeO2 glass formation in several binary tellurite glass systems. **Table 1** shows that TeO2 exhibits the largest glass formation range in the case of the three binary systems TeO2-ZnO (100–52 mol%), TeO2-WO3

The structure of tellurite glass is always generally based on binary systems. The ternary and multivariate tellurite glass systems have been generally used as rare earth-doped substrates for the investigation of the waveguide spectral properties. The diversity of components has helped to improve the chemical and thermal stability of tellurite glass-based devices. **Table 2** [14] includes the composition of the tellurite glass systems that have been reported in recent years. In all cases TeO2 was used as the glass-forming material, and its content was generally higher than 50 mol% (as shown in **Table 1**). Other oxides were generally used as modified bodies. From **Table 2** [14], it can be seen that the research objects of the binary system were more diversified. In addition to the common monovalent alkali metal oxides and divalent alkaline earth metal oxides, many other oxide components were involved, including CeO2, SmO2, V2O5, etc. It should be pointed out that in the case of the ternary tellurite glass systems, the TeO2-ZnO-RmOn and TeO2-WO3-RmOn glass systems were the most widely investigated, because these two systems possessed a wide range of glass formation regions and a wide range of adjustable

Early research was reported to suggest that the molecular structure of pure tellurite glass molecules comprised TeO4 double triangular bipyramids (tbp's) [36].

Cs2O 98.0–87.5 ZnO 100–52.5 Rb2O 96.5–73.0 CdO 60.0–48.0 K2O 95.5–77.0 PbO 60.0–48.0 Na2O 91.5–59.5 Bi2O3 66–60 Li2O 87.0–69.5 WO3 94.7–61.3 BaO 93.0–80.0 Nb2O5 100–73.2

**Composition Glass formation range**

**TeO2 mol%**

tellurite glasses will be considered.

*Advanced Functional Materials*

**1.1 Composition of tellurite glass**

components.

**Table 1.**

**262**

**1.2 The structure of tellurite glasses**

**Composition Glass formation range**

TiO2 100–52.5

*The formation range of binary system tellurite glass.*

**TeO2 mol%**

(94.7–61.3 mol%), and TeO2-TiO2 (100–52 mol%).

*Tellurite glass systems [14].*

In this polyhedron, one Te atom is surrounded by four oxygen atoms, of which two oxygen atoms Oeq are at the equator position and the other two oxygen atoms Oax at the axial position. The Te atoms are linked by Oax or Oeq into TedOdTe; an apex of the tetrahedron on the equatorial plane remains unoccupied by oxygen atoms and is occupied by Te<sup>0</sup> s lone electron pair [16]. This special polyhedral structure and the chemical bonding were different from the traditional glass-forming bodies (B2O3, SiO2, GeO2, and P2O5), which determined the specificity of the tellurite glass structure.

Some scholars used various testing methods to conduct research and analysis on tellurite glasses, especially binary system tellurite glass. Jha et al. [37] considered that the main structural units of tellurite glass were TeO4 double triangular bipyramids (tbp's) and TeO3 bipyramids (bp's) triangular pyramids. In 1995, Neov et al. [36] were the first to perform neutron diffraction analysis on lithium tellurite glass and pointed out that in addition to the TeO4 structural unit, a deformed double triangular pyramid TeO3+1 existed in the glass network. One of the TedO bonds was significantly longer than the other three. Due to the short-range similarity between the glass and crystal structures, the structure of tellurite glasses can be studied and analyzed based on the structure of tellurite crystals with the same composition. Sakida et al. [32] compared the Raman spectra of alkali tellurite crystals, pure tellurite glasses, and alkali tellurite glasses. The resulting Raman spectra were considered to correspond to structural elements in the glass. TeO4 (tbp's) double triangular bipyramids were finally transformed into a TeO3 (bp's) triangular pyramid by TeO3+1. Tatsumisago et al. [38] studied the change of tellurite glass structure with temperature using Raman spectroscopy. Throughout the above research, the general laws could be classified as follows:

1. It was generally considered that there were two kinds of structural units that form a tellurite glass network. One was TeO4 (tbp's) double triangular pyramid in which the Te atoms were arranged as a four ligand, and the other was TeO3 (bp's) triangular pyramid in which the Te atoms were in a triple coordination. It was considered that there were generally five kinds of structural units in alkali tellurite crystals, as shown in **Figure 1(a–e)**. Q<sup>n</sup> <sup>m</sup> can be used to represent the structural unit in **Figure 1**, where n is the number of bridge oxygen molecules in the [TeO4] group and m represents the number of covalent bonds. Research on the distribution of various structural units (a–e) in tellurite glass has become a significant focus of research in this field.

2.When an alkali metal oxide or alkaline earth metal oxide was introduced into tellurite glass as a network modifier, the original glass network structure was destroyed. TeO4 (tbp's) double triangular pyramid was finally transformed into TeO3 (bp's) triangular pyramid by TeO3+1. Sekiya [39] investigated the TeO2-MO1/2 binary system and considered that when the alkali metal oxide content was low, the glass was composed of TeO4 (tbp's) double triangular pyramid and TeO3+1 polyhedron. When the alkali content was less than 20 mol %, the number of TeO3+1 polyhedra increased with the increase of the alkali metal oxide content. When the alkali content was between 20 and 30 mol%, TeO3 (bp's) triangular pyramids with non-bridged oxygen bonds appeared in the glass network structure, and the numbers of TeO4 (tbp's) and TeO3+1 decreased accordingly. When the alkali metal oxide content exceeded 30 mol %, the Te2O5 <sup>2</sup> polyhedron was formed in the network structure. When the alkali metal oxide content was greater than 50 mol%, it was considered that the glass network structure at this time was composed of TeO3 (bp's) polyhedrons, TeO3+1 polyhedrons, and independent Te2O5 <sup>2</sup> and TeO3 <sup>2</sup>. At this time, the number of TeO4 in the glass was very small, and the glass structure had become extremely complex.

**1.3 Thermal properties of tellurite glass**

*Transformation of glass structure during heating.*

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

**Figure 2.**

**Table 3.**

**265**

of the tellurite glass also increases.

*Characteristic temperature of tellurite glasses.*

The thermal stability of tellurite glass is primarily dictated by composition and the doping concentration of rare earth ions. The characteristic glass temperature values include glass transition temperature Tg, incipient crystallization temperature

The thermal stability of glass is usually expressed by ΔT, which is the differential value between Tx and Tg. A higher value of ΔT generally means that the glass has good thermal stability. If the value of the Tx is close to Tf, it will lead to crystallization during a fiber drawing process which leads to an increase in the loss (attenuation) of the resulting glass fiber. **Table 3** includes a listing of several kinds of tellurite glass with good thermal stability together with their characterized glass temperatures (Tg, Tx, and ΔT). In the case of TeO2-R2O (R = Li, Na, K, or other alkali metal) tellurite glass systems, as the content of the alkali metal oxide increases, Tg gradually increases, while Tx remains almost unchanged. Consequently, the ΔT increases correspondingly, and the resistance against crystallization

In addition, the introduction of rare earth ions also has an influence on the thermal stability of tellurite glasses. For example, 1 wt% Pr2O3 introduced to a

**Glass component Tg (°C) Tx (°C) Tx Tg (°C)** 85TeO2-15Na2O 277 447 170 70TeO2-10ZnO-20Li2O 265 392 127 70TeO2-20ZnO-10BaO [40] 339 495 156 82.5TeO2–7.5WO3-10Nb2O5 [41] 391 562 171 80TeO2-10WO3-10Nb2O5-1Yb2O3 [42] 404 566 162 60TeO2-20ZnO-7.5B2O3-7.5GeO2-5K2O 200 5 378 2 178

Tx, peak crystallization temperature Tc, and glass-melting temperature Tm.

3.Temperature also affects the structure of tellurite glass. For example, when the glass temperature was gradually increased and exceeded the melting temperature, the TeO4 (tbp's) double triangular pyramid would also be transformed into a TeO3 (bp's) triangular pyramid. This is mainly due to the fact that Te-Oax is caused by fracture with increasing temperature, and its structural transformation process is shown in **Figure 2**.

**Figure 1.**

*(a–e) Five basic structural units in alkali tellurite crystals. (f) Deformed bitriangular cone TeO3+1.*

*Tellurite Glass and Its Application in Lasers DOI: http://dx.doi.org/10.5772/intechopen.91338*

was TeO3 (bp's) triangular pyramid in which the Te atoms were in a triple coordination. It was considered that there were generally five kinds of structural units in alkali tellurite crystals, as shown in **Figure 1(a–e)**. Q<sup>n</sup>

be used to represent the structural unit in **Figure 1**, where n is the number of bridge oxygen molecules in the [TeO4] group and m represents the number of covalent bonds. Research on the distribution of various structural units (a–e) in tellurite glass has become a significant focus of research in this field.

2.When an alkali metal oxide or alkaline earth metal oxide was introduced into tellurite glass as a network modifier, the original glass network structure was destroyed. TeO4 (tbp's) double triangular pyramid was finally transformed into TeO3 (bp's) triangular pyramid by TeO3+1. Sekiya [39] investigated the TeO2-MO1/2 binary system and considered that when the alkali metal oxide content was low, the glass was composed of TeO4 (tbp's) double triangular pyramid and TeO3+1 polyhedron. When the alkali content was less than 20 mol %, the number of TeO3+1 polyhedra increased with the increase of the alkali metal oxide content. When the alkali content was between 20 and 30 mol%, TeO3 (bp's) triangular pyramids with non-bridged oxygen bonds appeared in the glass network structure, and the numbers of TeO4 (tbp's) and TeO3+1 decreased accordingly. When the alkali metal oxide content exceeded 30 mol

<sup>2</sup> polyhedron was formed in the network structure. When the

<sup>2</sup> and TeO3

<sup>2</sup>. At this time, the

alkali metal oxide content was greater than 50 mol%, it was considered that the glass network structure at this time was composed of TeO3 (bp's) polyhedrons,

number of TeO4 in the glass was very small, and the glass structure had

glass temperature was gradually increased and exceeded the melting temperature, the TeO4 (tbp's) double triangular pyramid would also be transformed into a TeO3 (bp's) triangular pyramid. This is mainly due to the fact that Te-Oax is caused by fracture with increasing temperature, and its

*(a–e) Five basic structural units in alkali tellurite crystals. (f) Deformed bitriangular cone TeO3+1.*

structural transformation process is shown in **Figure 2**.

3.Temperature also affects the structure of tellurite glass. For example, when the

%, the Te2O5

*Advanced Functional Materials*

**Figure 1.**

**264**

TeO3+1 polyhedrons, and independent Te2O5

become extremely complex.

<sup>m</sup> can

**Figure 2.** *Transformation of glass structure during heating.*

#### **1.3 Thermal properties of tellurite glass**

The thermal stability of tellurite glass is primarily dictated by composition and the doping concentration of rare earth ions. The characteristic glass temperature values include glass transition temperature Tg, incipient crystallization temperature Tx, peak crystallization temperature Tc, and glass-melting temperature Tm.

The thermal stability of glass is usually expressed by ΔT, which is the differential value between Tx and Tg. A higher value of ΔT generally means that the glass has good thermal stability. If the value of the Tx is close to Tf, it will lead to crystallization during a fiber drawing process which leads to an increase in the loss (attenuation) of the resulting glass fiber. **Table 3** includes a listing of several kinds of tellurite glass with good thermal stability together with their characterized glass temperatures (Tg, Tx, and ΔT). In the case of TeO2-R2O (R = Li, Na, K, or other alkali metal) tellurite glass systems, as the content of the alkali metal oxide increases, Tg gradually increases, while Tx remains almost unchanged. Consequently, the ΔT increases correspondingly, and the resistance against crystallization of the tellurite glass also increases.

In addition, the introduction of rare earth ions also has an influence on the thermal stability of tellurite glasses. For example, 1 wt% Pr2O3 introduced to a

