Tellurite glasses

5. SIMPLE INORGANIC OXIDE GLASSES

5.3 Tellurite glasses

39 Despite the known phase separation of binary corophosphate or phosphor-silicate systems is glass formation reported for ternary and quanternary systems containing only netwprk former oxides. The modifier free boro-phospho-silicate glass system, containing only the network forming oxides B2O3, P2O5 and SiO2 has been studied with multinuclear NMR Spectroscopy by Uesbeck et al. (2017). The quarternary, which also contains Al2O3, had been reported by Liu et al. (2018).

Borophosphate glasses containing transition metal and post-transition metal ions are studied for their large glass forming region, and their ability to stabilize high fractions of highly polarizable ions. Such glasses are of interest for their non-linear optical properties and electric field induced second harmonic generation (Dussauze et al. 2006).

5.2.10 Borate glasses applications

Applications of borate glasses range from shielding materials to sealing glasses to bio glasses (Bengisu 2016). Borate glass fibers are used in wound healing (Zhao et al. 2015), showing great potential for vascular regrowth.

For the technologically so important field of borosilicate glasses, we refer once again to Youngman (2022, this volume).

40 5.3.2 Structure of tellurite glasses

Contrary to silicate and the previously discussed oxide glass formers, tellurite units exhibit a lone electron pair that takes no part in bonding. Like tin or lead ions, the lone electron pair occupies one side of the basic polyhedra, and in terms of network bonding the lone electron pair can be seen as a non-bridging side at one corner of the pseudo-polyhedra and is one reason for the low Tg

(Tagiara et al. 2017) (See Fig. 22).

The basics of this structure has been confirmed by Raman (Tagiara et al. 2017), neutron diffraction (McLaughlin et al. 2001; Barney et al. 2013), and NMR spectroscopy (McLaughlin et al. 2001; Garaga et al. 2017). The lone electron pair at the Tellurium ion distorts the fundamental TeOn polyhedral structure and some ambiguity exists on the nature of the shortest Te-O bonds (see Fig. 22 for a depiction of various tellurite species). Depending on the analytical method used, the short bond might be seen as Te=O double bond or as being weakly bonded to a neighboring tellurium ion as in a Te-O- -Te type of bond, as depicted in Figure 22b as TeO3+1 unit. Diffraction studies seem to indicate a significant fraction of such terminal units (Barney et al. 2013); however, Raman spectroscopy does not see a Te=O double bond with a signal at ca. 850 cm^-1 for pure TeO2

(Tagiara et al. 2017). On the other hand, if the glass is melted in SiO2 crucibles, the tellurite network is modified and a Te=O bond is indeed observable in the Raman spectra (Tagiara et al.

2017).

Crucible dissolution has been discussed for the case of phosphate glasses before, and as shown by Tagiara et al. (2017), melting pure TeO2 in Al2O3 crucibles will enhance glass formation through the dissolution and uptake of Al2O3 from the crucible. The uptake modifies the network significantly creating non-bridging oxygen atoms (see Fig. 22 for the tellurite species seen in depolymerized glasses) but increasing Tg through the strong cross-linking of Al³⁺ ions.

Figure 22. Depiction of the tellurite polyhedra known from crystals and glasses, (a) TeØ4 or 𝑸𝑸_{𝑻𝑻𝑻𝑻𝑻𝑻}^𝑻𝑻 trigonal bipyramid (tbp), (b) TeØ3+1 an intermediate form between the four and three fold coordinated tellurite form with long and short bonds, (c) TeØ3O^- or 𝑸𝑸_{𝑻𝑻𝑻𝑻𝑻𝑻}^𝟑𝟑 (tbp), the non-bridging oxygen can be the long or the short bonded one, (d) TeØ2O or

𝑸𝑸_{𝑻𝑻𝑻𝑻𝟑𝟑}^𝟐𝟐 trigonal pyramid (tp), (e) TeØO2- or 𝑸𝑸_{𝑻𝑻𝑻𝑻𝟑𝟑}^{𝟏𝟏 (tp)}, (e) TeO32- or 𝑸𝑸_{𝑻𝑻𝑻𝑻𝟑𝟑}^{𝟎𝟎 (tp)}, after McLaughlin et al. (2000).

Like in many other glasses, depolymerization occurs with the addition of modifier oxides to pure TeO2. From a CN=4 fully polymerized tellurite network with four-fold coordinated entities,

Te O O

O

(a) O (b) (c)

Te O O

O O Te

O O

O Te

O O

O O-

Te O O

O

(d) (e) (f)

Te O- O

O

Te O- O

O-

..

41 𝑄𝑄_{𝑇𝑇𝑇𝑇4}⁴ , modifier addition creates first TeO3+1 (trigonal bi-pyramid) units, where the 4^th oxygen is loosely bound to another Tellurite entity, while one oxygen atom is bridging and 2 oxygen atoms are terminal, sharing an excess electron and a double bond. Further modifier oxide addition creates three-fold TeO3 groups with 1 or two nbO (trigonal pyramid, with the lone electron pair at the apex), that are however delocalized with the double bond of the 3^rd terminal oxygen atom. Figure 22 depicts the five tellurite polyhedra that are known from crystals and the TeO3+1 group that is an important intermediate species in glasses (McLaughlin et al. 2000).

It is easy to follow these structural changes by IR and Raman spectroscopy. Interestingly, the change in the Raman spectra is very different for addition of the intermediate oxide Nb2O5 or when adding modifier oxides such as Tl2O, ZnO or Al2O3 e.g. (Tagiara et al. 2017). Thomas suggested that niobium ions have a similar bond strength to oxygen as tellurium ions and that therefore a solid solution forms between the Te- and Nb-oxides. This is reflected for the (1-x)TeO2-xNb2O5

series in almost unchanged Raman spectra as Nb2O5 is added to TeO2. This behavior is contrary to the (1-x)TeO2-xTl2Oseries, where the weaker thallium ions act as network modifier, changing the connectivity and coordination of the tellurite polyhedra, which in turn is apparent in significant changes in the Raman spectra (Mirgorodsky et al. 2012). When TeO2 is combined with another compound that exhibits even stronger oxygen bonds than Te-O, TeO2 now acts as modifier oxide e.g. in (1-x)TeO2-xWO3 (Mirgorodsky et al. 2012) or when combined with borate as in the Li2O- x(2TeO2)-(1-x)B2O3 glasses (Chatzipanagis et al. 2019).

TeO2 reacts also readily with other typical glass former oxides, though the mixed B-, Ge- and Si-systems often show phase separation which is not described for phosphorus-tellurite glasses (Vogel 1992), but was for example reported for (1-x)TeO2-xWO3 (Mirgorodsky et al. 2012).

Phase separation or a low degree of mixing is often the reason for a strong negative mixed network former effect, as for example observed for Li2O-x(2TeO2)-(1-x)B2O3 glasses. (de Oliveira et al. 2018).

When studying the Raman spectra, it should be noted, that Raman spectroscopy has the highest sensitivity for high polarizable ions such as Te⁴⁺, and if combined with low polarizable classical network former such as B, Si or P, those bands are therefore often hidden under the Te- related Raman bands (Chatzipanagis et al. 2019).

For mixed tellurite-germanate glasses see section 5.4.

5.3.3 Properties and applications

Applications of tellurite glasses are especially focused to optics and photonics for their high polarizability resulting in high refractive index and high third order susceptibility, which is of interest for non-linear optics (NLO) (Weber 2006; Barbosa et al. 2017). Tellurites have a high transparency in the IR, as can be seen from Figure 8, the highest for oxide glasses as low as 5 to 7 µm (Vogel 1992). On the other hand, the high polarizability of tellurium decreases the transparency in the UV-visible wavelength region, shifting the band gap to lower energies (same figure). The low energy phonon side bands are advantageous for fluorescence, as is the high solubility of rare earth ions in tellurite glasses (Wang et al. 1994). The same authors also include a detailed study on the formability of tellurite glasses including fiber drawing and extrusion.

Other applications include laser materials and energy conversion, but also radiation shielding because of the high density and even biomedical applications (El-Mallawany 2018).

42 Many publications can be found on the structures and properties of tellurite glasses, including the “Tellurite Glasses Handbook: Physical Properties and Data” by R. A. H. El-Mallawany(Vogel 1992; El-Mallawany 2012). The melting temperatures are low, between 700-900 °C, and as a consequence, they also display a low Tg. Density and coefficient of thermal expansion are high while mechanical strength and hardness are relatively low compared with other oxide glasses (Stanworth 1952; Tagiara et al. 2017). The latter can be improved by crystallization, and transparent glass ceramics for optical applications have been successfully prepared e.g. by complete crystallization of the 75 TeO2 – 12.5 Nb2O5 – 12.5 Bi2O3 glasses (Bertrand et al. 2016).

Researchers that have worked excessively with Tellurium containing glasses might have experienced the typical garlic-odor breath, as the body converts any tellurium taken up, to dimethyl telluride (CH³)²Te (Chasteen and Bentley 2003).

5.3.4 Antiglass

The term “anti-glass” was defined by Burckhardt and Trömel (1983) when describing a very small group of oxide materials from the tellurite system. The definition of anti-glass refers to a solid, with cationic long-range order but lacking any anionic short-range order. This definition is exactly contrary to glasses with short-range order and a lack of long-range order, hence the name.

Many reported anti-glass structures derive from tellurites and are based on a CaF2 fluorite structure with Te⁴⁺ and other metal ions such as for example Bi³⁺ or Sr²⁺, statistically distributed at the available cation positions while not all crystallographic anion positions are occupied by oxygen (Bertrand et al. 2015). Reported anti-glass compositions include SrTe5O11 (Burckhardt and Trömel 1983) or glass ceramic from the TeO2−Nb2O5−Bi2O3 system (Bertrand et al. 2015).