CHEMICAL BONDS IN VITREOUS MATERIALS

From:

Alfred Margaryan

“Ligands and Modifiers in Vitreous Materials: Spectnoscopy of condensed Systems”

Publisher: World Scientific, Singapore, New Jersey, London, Honk Hong 1999

 

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All materials in nature exist in three states of aggregation: gas, liquid and solid. Gases can exist as the usually familiar gas or as plasma (ionized gas); liquids can exist in the form commonly known as liquid or as liquid crystals; and solids can be amorphous or have different crystalline forms.

 

The solid amorphous materials exist as powder, film, gel, resin and glass form.

 

The basic form of an amorphous state is the glass forming state for inorganic materials, and resin forming for organic materials.

 

Materials in the glass forming or vitreous state are solid, homogeneous, fragile and transparent matter. The glass forming state is located between the crystalline and liquid state.

 

All materials in the glass forming state have some general physico-chemical characteristics. Typical glass forming subjects are:

 

1.      Isotrop; which means that all properties are same on all sides of matter.

2.      Glasses during heat process do not melt as crystals, but gradually soften and pass from fragile to a high viscosity and liquid state (Figure 1.1). In this process all physico-chemical properties are changed uninterrupted.

 

 

Figure 1.1: Nature of cooling of the melts crystalline (curve 1), and glass forming (curve 2)

materials.

 

3. Glasses melts and solidify in reverse (Figure 1.1), which means that repeated process of melting and cooling of glass takes the same temperature regime.

 

Reverse process of properties indicates, that glass forming melts and solid glasses are true liquids, therefore the solid glass state is supercooled liquid.

 

Special scientific interest must be presented for evaluation of the glass forming process by from point the perspective of view of the nature of the chemical bonds, which are formed between individual elements of the structure.

 

The nature of the chemical bonds is one of the dominant factors in the glass forming process. However, in scientific literature there are few publications on this subject.

 

The investigation of chemical bonds in glass forming melt and in the solid glass are one of the general problems of the material sciences, especially for glass forming materials [1-9].

 

Winter-Klein [10-12] recommended the replace conception about glass forming elements by the conception of the glass forming bonds, accomplished by P-electrons.

 

Britton [13] and Rawson [14] established that for glass formation the first factor is the nature of the chemical bonds between particles of existing cells. Glass forming is made possible by the presence of a variety of mixed bonds ionic-covalency.

 

Dietzel [15] determined the position of glass forming oxides in the group of existing oxides and showed dependence of the melting points of some oxides at strength of the fields as Z/a² (Z-charge, a-distance of center between both ions). This dependence is presented on Figure 1.2. It shows the curve passed through fields of oxides, which have high melting point (2000-3000°C). These types of oxides have a predominantly ionic bond.

 

After the curve passes through the field of glass forming oxides (SiO₂, GeO₂, B₂O₃, P₂O₅) the end of the curve lies down in the field of oxides having very low melting point (0-400°C) which is characterized with covalence bonds. In glass forming oxides there are intermediate types of ionic-covalent bonds.

 

 

Figure 1.2: Melting points of some oxides function at strength of cations fields Z/a² (after

ref. [15]).

 

In the glass forming or vitreous state there is a large quantities of inorganic materials, from individual elements to complicated multicomponent systems.

 

Inorganic glasses are classified by several types:

 

1.      Elementary or monoatomic glass, which consists of one base element (S, Se, As, P).

2.      Oxide glass where typical glass forming components are B₂O₃, SiO₂, GeO₂, P₂O₅. Other oxides become to glassy state in small quantities under fast cooling conditions (As₂O₃, Sb₂O₃, TeO₂, V₂O₅). Some oxides cannot become to vitreous form independently (A1₂O₃, Ga₂O₃, Bi₂O₃, TiO₂, MoO₃, WO₃), however in combination with different components in binary and multicomponent systems develop glass-forming abilities.

 

Finally we have the types of silicate, borax, phosphate, germanate, tellurate, aluminate and other oxide glasses.

 

3. Halide glass [16] on base BeF₂, ZnF₂, ZrF₄, HfF₄, InF₄ with a combination of fluorides MeF, MeF₂, MeF₃ is named fluoride glass [17].

 

From chlorides in glass forming states we have a lot of data regarding the physico-chemical properties of Zinc chlorides (ZnCl₂).

 

There is some interest in glass systems MeF_n-HF, where the glass forming component is HF.

 

4. Chalcogan types of glass are formed on the basis of sulphides, selenides and tellurides. Glass forming elements in these types of systems are S, Se, and Te.

 

In binary systems, general glass forming components are selenides of arsenic, germanium, and phosphorus (As₂Se₃, GeSe₂, P₂Se₃) and sulphides of arsenic As₂S₃ and germanium GeS₂.

 

In general, chalcogan glass is very complex and different in composition.

 

5.      Special types of glass, such as groups of nitrate, acetate and sulphate glasses have low melting points. These types of glass are chemically unstable.

6.      A mixed type of glass formed by using a mix of previously glass formed components: oxides and Halides, oxides and chalcogans, chalcogans and halides.

 

Particular types of glass are nitride and oxide-nitride glasses on the base Si₃N₄ (MP-1900°C), AlN (MP-2200°C), BN (MP-3000°C), Be₃N₂ (MP-2200°C) [19]. These types of glasses are absolutely new. They have a very high chemical resistance and a high melting point.

 

In recent years we are finding in scientific literature some results about metallic glass (Fe, Pd, Zr, Ni, Cu) [20-22].

 

Petrovski [23,34] after conclusion of all presentations on glass forming states showed on the sample of beryllium fluorine glasses that covalence bonds favorable for glass forming, but ionic bonds for crystallization.

 

Generally the glass forming state of materials depend on the existence of mixed types of bonds ionic-covalence characters between elements of structure.

 

Spectroscopic investigation of transition elements in glass forming and crystalline materials are giving us the opportunity to evaluate the change of degree of covalency and strength of ligands field in above matrices.

 

In fundamental works Bethe [25] and Van Vleck [26-29] successfully developed the theory of crystalline field regarding crystalline materials.

 

The second period development of theory on crystalline field begins with the progress of spectrophotometry and electron paramagnetic resonance [30-35].

 

During this period the theory of fields of ligands was formed, it combined the theory of crystalline fields with the Mulliken [36] method of molecular orbitals.

 

Consequently, positive results of the theory of chemical bonding and the theory of crystalline fields were included in theory field of ligands.

 

One of the successful results of theory field of ligands are energetic diagram types of Tanabe-Sugano [37,83]. These diagram types are presenting the influence of the nature of ligands on the parameters of crystalline fields (spectrochemical and nephelauxetic lines) [39-43]. Margaryan et al. [44-47] showed that in vitreous materials the following chemical bonds exist between D-L-G-L-M, where D-dopant (ions of transition elements dⁿ, ¦ⁿ), L-ligand, G-glass forming atom near coordination sphere, M-modifier.

 

In the degree of covalency of L-G is higher (O-Si, O-Ge, O-B, O-P or F-Be) the degree of covalency between D-L (L-dⁿ or L-¦ⁿ) will be lower. The differences in covalency are described by the changes in the polarization of the ligands.

 

In glass forming (vitreous), the material elements of structure (L-G) have to be predominantly in covalent bonding, owing to spⁿ-hybridization of electron orbitals between the glass forming atom (G) and ligand (L). The opposite occurs in the crystalline form of matter.

 

Allen and Nebert [48,49] discovered that the EPR spectra of transition metals, in particular Mn(II), within organic and inorganic solvents show finer structures when the sample is in the glass forming phase rather than a polycrystalline phase. Figures 1.3 and 1.4 show the EPR spectra of Mn(II) in methanol (CH₃OH) and 12N HCl. The hyperfine structure becomes quite different when the sample changes from a transparent glass phase (curve 1) to a polycrystalline phase (curve 2).

 

 

Figure 1.3: EPR spectra of Mn(II) in vitreous (curve 1) and polycrystalline (curve 2)

methanol (after ref. [48]).

 

 

Figure 1.4: EPR spectra of Mn(II) in vitreous (curve 1) and polycrystalline (curve 2) 12N

HCl (after ref. [48]).

 

Margaryan et al. [50-58] investigated the EPR spectra of the paramagnetic Mn(II) ion in glass forming and crystalline (hexagonal form) germanium dioxide. The compositions of glass forming-dopant and crystal-dopant have a significant importance in determining the structure and character of bonds in vitreous-glassy (isotropic) and crystalline (anisotropic) systems. The important parameters are: the degree of covalency in Ge0₂ of the electron orbitals of Mn(II) and ligands, the field intensity of the ligands, the coordination position of Mn(II), regularity of the structure of Ge0₂ in the vitreous and crystalline states.

 

Figure 1.5 shows EPR spectra Mn(II) in vitreous (curves 1 and 2) and crystalline (curves 3 and 4) Ge0₂. Any change in the solvate cloud will lead to some difference in the EPR spectra of Mn(II) (see Figure 1.5).

 

 

Figure 1.5: EPR spectra of Mn(II) in vitreous (curves 1 and 2) and crystalline (hexagonal

forms) (curves 3 and 4) Ge0₂ (after refs. [50,51]).

 

An important role is played by the proportion of one or another type of bonding existing between the ligand and glassformer ions (L-G), and the ligand and dopant ions (L-D) in the structures.

 

A comparison of the EPR spectra Mn(II) in vitreous (curves 1 and 2) and crystalline (curves 3 and 4) germanium dioxide (Figure 1.5) shows identical data in [48,49] (Figures 1.3 and 1.4).

 

From the EPR data for Mn(II), we can conclude that the degree of hyperfine splitting (hfs) in a covalent ligand-dopant interaction is lower than that in an ionic bond. The hyperfine splitting for Mn(II) is directly proportional to the degree of ionic bonding in the ligand-dopant bond [50-52].

 

Previous interpretations received their confirmation for vitreous and crystalline form of fluorophosphates type of 45P₂O₅.55BaF₂ (in mol%), when investigated EPR spectra of Mn(II) respectively [54].

 

Figure 1.6 illustrates EPR spectra of Mn(II) in vitreous(curves 1 and 2) and crystalline (curves 3 and 4) form of fluorophosphates. It shows significant difference in spectra EPR and means of hyperfine splitting (hfs). Constant of hyperfine splitting for vitreous fluorophosphate (curves 1 and 2) equals to 94.33oe in crystalline form of the same composition (curves 3 and 4) A=89.98oe (Figure 1.6)

 

 

Figure 1.6: EPR spectra of Mn(II) in glasses of composition (mol%): 45P₂O₅.55BaF₂; Mn(II)

0.05 wt% (curve 1), 0.1 wt% (curve 2), and in their crystalline forms; Mn(II) 0.05 wt% (curve 3), 0.1 wt% (curve 4) (after ref. [54]).

 

Figure 1.6 shows that strength of the ligand field on Mn(II) in crystallized samples are significantly higher than in glass. These types of bonds, ligand-Mn(II) in glass forming and crystalline phases, have a different means of covalency. Under the crystallizing processing of glass the bond of covalency between ligand-Mn(II) is increases (A=89.98oe, Figure 1.6).

 

Margaryan et al. [44-47] show that the similarity of chemical bonds between ligands and dopants (L-D) in the different natures of glasses (berylliumfluorine, fluorophosphate, phosphate, silicate, germanate, borax and others) causes an analogy in their spectrochemical and spectroscopic parameters; even though they have the crystallochemist similarities of some other glass-formers (BeF₂, Si0₂, Ge0₂).

 

Authors [45,46] demonstrated that closest bonds of ligand-dopant in the berylliumfluorine, fluorophosphate and phosphate glasses are the main reasons for their similarity in spectral properties, although the nature of glass from a crystallochemical point of view are different (BeF₂ and P₂0₅).

 

Spectrochemical parameters Racha B and C, nephelauxetic relation b=B/B₀, positions of levels of ⁴T_1g (G) and ⁴T_2g (G) of transition elements in vitreous and crystalline phases giving us an opportunity to evaluate the changes of covalency and strength of ligand fields in the showing matters.

 

The electron spectra of absorption Mn(II) in minerals and glasses are useful for the expressive bands of transitions ⁶A_lg (S) ® ⁴E_g (G) and ⁶A_1g (S) ® ⁴E_g (D) can be used for correct calculations of electrostatic parameters Racha B and C from equation Tanae-Sugano [37,38]:

 

⁶A1g (S) ® ⁴Eg (G) = 10B+5C

 

⁶A1g (S) ® ⁴Eg (D) = 17B+5C

 

Racha parameter B evaluated interelectronic interaction of matrices with the electron orbitals of the dⁿ transition elements.

 

Introduction of Mn(II) ion in glass or other matrices leads to the decrease of Racha parameter B. Value of B in glass is reduced to 600 cm^-1, compared with 860 cm^-1 for free ions of manganese (B₀). From this fact we can conclude that in glass forming materials there exists a high degree of covalency between dopant and ligands [44].

 

Racha parameter B and nephelauxetic relation (b=B/B_o) is a measure of bond of covalency ligand-dopant.

 

The covalency increases when the values of B and b are reduced [59-61].

 

Table 1.1 demonstrates spectroscopic parameters for manganese minerals and glasses, and also counts parameters for B and b=B/B_o [62].

 

Table 1.1 shows that most of the low covalency between ligand-dopant observed for monocrystal MnF₂, where B=721 cm^-1, b=84% and berylliumfluorine glasses, where B=693 cm^-1, b=80%.

 

High covalency between ligand-Mn(II) appears in Mn₂Si0₄ when B=577 cm^-1 and b=67%; and silicate glasses when B=600 cm^-1 and b=69%.

 

Table 1.1

Some Spectroscopic Parameters of Manganese Minerals and Glasses (from [62])

 

 

According to the data of Margaryan [62,66] Figure 1.7 presents the energetic levels of the ⁶A_1g (S) ® ⁴T_1g (G) band and regular growth of covalency ligand-Mn(II) in some manganese minerals and glasses.

 

 

Figure 1.7: Position of ⁴T_1g (G) band in manganese minerals and glasses (after refs. [62,66]).

 

Therefore, the estimated degree of covalency or ionicity between dⁿ-electrons and ligands in crystalline, glass forming and liquid matrices it is possible to use the data of Racha parameter B, nephelauxetic relation (b=B/B_o) and the position of transition ⁴T_1g (G) and ⁴T_2g (G) levels.

 

Covalency of the ligand-Mn(II) is increasing in the line of glasses: Berylliumfluorine (⁴T_1g =21150 cm^-1), fluorophosphate (borax) (⁴T_1g=20000 cm^-1), phosphate (⁴T_1g =19850 cm^-1), germaniumfluorophosphate (⁴T_1g=19200 cm^-1), germaniumphosphate (⁴T_1g =19150 cm^-1), germaniumoxifluorate (⁴T_1g=19000 cm^-1), silicate (⁴T_1g=15450 cm^-1).

 

Amosov et. al [68] studied optical spectra of absorption of Co(II) in crystalline quartz (Si0₂) and the vitreous form of silicon dioxide (Si0₂) (Figure 1.8).

 

 

Figure 1.8: Co(II) absorption spectra in crystalline quartz (curve 1), and glass forming silica

(curve 2) (after ref. [8]).

 

Spectra of absorption of the Co(II) consists of two maximums 7000 and 17000 cm^-1. The band 7000 cm^-1 allows three maximums: in crystalline quartz is 7700, 6700 and 5700 cm^-1, and in the quartz glass is 8000, 6300 and 5300 cm^-1. In the visible parts of the spectra near 17000 cm^-1 exist hyperfine structures also.

 

The spectra of absorption of the Co(II) in crystalline quartz allows three maximums: 18500, 17100 and 15600 cm^-1 and in the quartz glass: 19700, 16500 and 14500cm^-1.

 

Therefore the rate of splitting in the quartz glass increases almost twice. The intensity of absorption of Co(II) in the quartz glass is significantly higher than in crystalline quartz.

 

At the time of transition from quartz glass to crystalline quartz the rate of splitting of bands increases. This fact is attributed to the nature of chemical bonds between Co(II) and ligands (oxygen) in crystalline, and vitreous forms of Si0₂.

 

These results are also confirmed when we compare the electrostatic parameter Racha B in crystalline and glass forming Si0₂. The parameter of Racha B depends on nature of the chemical bonds ligand-dopant and may be determined by using optical spectra of absorption of Co(II) for each concrete matters [69]. For Co(II) in crystalline quartz the rate of B=864 cm^-1, and respectively for quartz glass B=884 cm^-1 [68].

 

Ebendorff-Heidepriem and Ehrt [70] studied the relationships between local structure of rare earth doped fluoride phosphate and phosphate glasses and the spectroscopic properties of Eu(III) and Tb(III) ions.

 

The nephelauxetic shift of Tb(III) and Eu(III) f ® d absorption bands are used to measure the covalency difference between of rare earth ions and ligands. The differences in covalency are ascribed to changes in the polarizability of the ligands.

 

With increasing phosphate content the covalency between rare earth ions and surrounding ligands increases due to the substitution of fluorine ions by oxygen ions having higher electron polarizability. In fluoride phosphate glasses, the diphosphate groups dominate the spectroscopic properties of the rare earth ions [71].

 

Formulation of the scientific and experimental bases of evaluation on the nature of chemical bonds, created by ligands, modifiers, and dopants on the vitreous phase, and their influence on the spectrochemical and spectroscopic parameters of glasses have both an important scientific and practical interest in connection with the interpretation of the dynamics of the glass forming state and the development of sensitized and controllable lasers, as well as the creation of the wide type of amplifiers on the base of inorganic glass.