SPECTROSCOPY AND STRUCTURE OF SOME NON-SILICATE 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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5.l X-Ray and Neutron Diffraction

Subsequent chapters use data obtained by different experimental techniques in discussions of the spectroscopic properties of vitreous (glassy) materials fabricated from single to multicomponent systems. These discussions attempt to show that certain characteristics are inextricably related to the type of structure formed by a system, as well as to the bonding and type of molecules.

 

X-ray diffraction is used extensively in the characterization and determination of the structures of solid and liquid states. As previously discussed, although glasses are amorphous in structure, they can still posses short range order and, in many cases, as verified by diffraction techniques, medium range order.

 

The structure of liquid, vitreous and amorphous states can be described by means of a radial distribution function, for structures containing only one kind of atom. The radial distribution function (RDF) is valuable because it represents the average number of atomic centers in a spherical shell of radius, centered on any atom of the structure, and within a spherical shell of thickness. Density function commonly called the pair correlation function. For multicomponent systems, a partial pair correlation function defines pairs of individual atoms. Thus, describing a two component system consisting of atoms of type A and type B requires three pair correlation functions. The number of functions needed increases very rapidly with the number of components. In fact, it becomes impractical to use this technique for multicomponent systems with a large number of different types of atoms. When the correlation function is obtained from X-ray scattering, it is the electronic correlation function, and electrons are responsible for the diffraction effects observed. For neutrons, the atomic nucleii are the scatterers and the observed function relates the atomic centers. The scattering of the neutron is also independent of the scattered angle, simplifying interpretation. For a liquid or amorphous substance, Zernike and Prins [1] obtained the equation for the RDF.

 

Warren, Kruter and Morningstar [2] extend results to heteroatomic structures. The RDF approach is useful principally because the peaks are related to interatomic distances in the structure while the area under peaks is used to determine an average coordination number.

 

Zarzycki[3] performed X-ray scattering studies on vitreous GeO₂ at 20°C and liquid GeO₂ at 1200°C.

 

Figure 5.1 shows the X-ray scattering spectra and Figure 5.2 shows the resulting radial distribution functions (RDF). From his results, Zarzyeki concludes that GeO₂ has a tetrahedral coordination and consists of a [GeO₄] network.

 

Figure 5.1: X-ray scattering spectra from GeO₂ in liquid and solid phases (after [3]).

 

Figure 5.2: GeO₂ radial distribution function (RDF) derived from the spectra shown in Figure 5.1 (after [3]).

 

In the study of heteroatomic structures, it is difficult to interpret the radial distribution function. Various techniques evolved to circumvent the difficulty of assigning peak positions. If one assumes that the structure does not significantly change when a heavier atom is substituted for the one whose position is being ascertained, as heavier atoms are used, the RDF shows a more pronounced peak. This method must be used with caution because, as Holloway [4] shows, larger peaks can result from superpositioning several different distance separations. Moss and Price [5] studied the appearance of a sharp diffraction peak in many glasses at approximately 1.0-1.5 Å^-1. They concluded that the diffraction peak results from the random packing of structural units (the units can be single atoms or molecular units), regardless of the fact that structural units have different roles in different types of glasses. Other methods, such as joint use of X-ray and neutron diffraction, attempted early in the study of glasses [6-8], turned out to be very difficult to perform.

5.2 Infrared Spectroscopy

Radiation absorption in the region of 10⁴-10²cm^-1 can be exploited to obtain information about the structure of a substance. These regions of absorption depend on the interatomic forces and structural arrangement of the constituent atoms, which, in turn, affect the vibrational modes. Quantum mechanical selection rules determine which transitions occur as a result of the incident stimulus. It should be noted that, even for crystalline materials, a priori determination of the crystal structure is not feasible without some additional information. Spectral analysis, based on group theoretical methods, is usually performed to assign the spectra. Also, the structure is determined from X-ray diffraction, to provide some information on the expected vibrational modes.

 

A very common approach is to compare results from a glass to the crystalline phase. The two types of infrared (IR) spectroscopy commonly used for glasses are Rayleigh scattering and the Raman effect. Because the selection rules are different for the same material. Rayleigh scattering is reradiation of the incident electromagnetic wave: the scattering forces the electrons to vibrate, and the vibration occurs at the same frequency. However, if coupling occurs between the vibrational modes and electric polarization tensor, the energy of the scattered photons is changed (increasing or decreasing, according to the coupling), creating two additional spectral lines. In the Roman effect, increased energy is attributed to the Stokes line and decreased energy to the anti-Stokes line. Because of the high temperatures, accurate spectroscopic measurements on glass melts are difficult to perform. Nevertheless studies have been done to determine coordination numbers in a melt. Seifert et al.[9] studied the coordination number of Ge in GeO₂ as it is heated. Figure 5.3 shows their Roman spectra.

Figure 5.3: Spectra of Raman scattering from (1) GeO₂ glass, (2) and (3) GeO₂ melts (after [9]).

 

The coordination number of Ge, 4, remains constant. However, a strong broadening of the band centered at 500cm^-1 occurs, resulting from an increase in observable defects in the melt.

 

IR spectroscopy can determine the type of vibrations that the atoms in a substance experience. IR absorption and the Raman effect are caused by different types of vibrations, which are determined in part by the electric dipole moment and the dielectric polarization. For atoms or ions in a crystal, arranged in spatially periodic structures, vibrational states can be calculated (based on theoretical considerations), and thus the amount of IR absorption can be predicted. No theory can truly describe the lattice vibrations in glasses because the vibrations lack long-range order. Thus, it is not uncommon to base interpretations of the vibrational states of glass forming compound on the spectra of isolated component molecules. These vibrational states are observed not only in gases but in liquids and solids as well. However, the influence of neighboring molecules can change the structure and position of the absorption lines. Energy considerations dictate that each atom vibrate about a given position, the atoms displacement being governed by the position and direction of motion. Unsymmetric valence bond vibrations have higher vibrational frequencies and larger IR intensities, which are used to identify the molecules. Another characteristics used for identification is the form that the lower frequency vibrations take when the valence angle changes (deformational or elastic vibrations) [10,11].

 

IR absorption spectra of glasses are diffuse, featureless, and wider than those of crystals, due to the larger vibrational amplitudes of glasses. Comparing a compound’s spectra in glass forming and crystalline states shows the inherent glass structure. Systematically varying the composition of a glass provides information about the structural changes in the glass former.

 

Figure 5.4 shows the IR transmission spectra of glass forming GeO₂ and two forms of modifications of crystalline GeO₂ [10,12]. The shape of the glass forming GeO₂ spectra and the hexagonal modifications indicate the existence of analogous lattice structures in the matrices of both materials.

 

Figure 5.4: IR spectra of GeO₂ modifications (after [10,12]): (1) glass forming GeO₂, (2) hexagonal GeO₂, (3) tetragonal GeO_2.

 

 

Zarzycki [13] studied the IR spectra from vitreous GeO₂ and compared it to several of its crystalline variations. Figure 5.5 shows the resulting IR spectra. In hexagonal GeO₂, Ge has a coordination number of 4; in insoluble tetragonal GeO₂, Ge has a coordination number of 6; and, in the vitreous form of GeO₂, Ge has a coordination number of 4.

 

Figure 5.5: IR spectra for crystalline allotropes of GeO₂ and vitreous GeO₂ (after [13]).

 

5.3 Electron Paramagnetic Resonance Measurements

Whereas in NMR a magnetic field is used to study transitions between energy levels of the nuclear magnetic moment, in electron paramagnetic resonance (EPR) a magnetic field is used to study the transitions between electron levels. This method is also referred to as electron spin resonance (ESR). The g-factor is a number whose value is determined by the total spin coupling. For a free electron, the value of g is 2.002322, when quantum corrections are included.

 

EPR spectroscopy provides a means for studying many aspects of the structure of the glass state. In a glass, EPR is also used to study substitutional impurities, such as rare earth or transition metals and paramagnetic centers produced by radiation [14-17].

 

A considerable body of literature exists on the EPR study of paramagnetic ions. Many of the studies involve paramagnetic centers in glasses of different compositions. A limited number of EPR studies involve the glass forming and crystalline modifications of germanium dioxide. Weeks and Purcell [14] made considerable contributions in this area with their EPR studies of g- and b- irradiated modifications of GeO₂. They obtained the EPR spectra of GeO₂ in the hexagonal, tetragonal and glassy forms. Figure 5.6 and Figure 5.7 show the EPR spectra of gamma and electron irradiated modifications of GeO₂. In the glass forming GeO₂, three resonance lines exist at 1.9957, 2.006, and 2.008 that have been identified as being caused by internal paramagnetism of the compound. The intens line for g=1.9957 characterizes a disordered arrangement of centers with axial symmetry, arising from electrons centered on oxygen vacancies of the [GeO₄]^4- tetrahedra. The position of the g=1.9957 line and its width, in the glass forming GeO₂, coincide with the spectra of the hexagonal GeO₂ modification (Figure 5.6).

 

Figure 5.6: EPR spectra of gamma-irradiated GeO₂ samples (after[14]): (1) glass forming

GeO₂, 10⁷r dose; (2) hexagonal GeO_2, 6x10⁶r dose. Measurements at 78°K.

 

 

Figure 5.7: EPR spectra of tetragonal GeO_2, irradiated by an electron beam dose of 10¹⁷ electrons (after [14]).

The existence of such identical paramagnetic states indicates a correspondence in the structure of the glass and hexagonal forms of GeO₂. The spectral line from the crystalline tetragonal form of GeO₂ (Figure 5.7) differs both in shape and position (g=2.0037-2.0040) from the corresponding line in the spectra of hexagonal and glass forming GeO₂.

 

Weeks and Purcell [14] showed, through a series of studies of defects in single crystals and EPR studies of crystalline modifications of GeO₂ and glass forming GeO₂, that the main structural defect of glass forming GeO₂ is the oxygen vacancy in [GeO₄]^{4 -} tetrahedra. The concentration of oxygen vacancies is considerably greater in glass forming GeO₂ (the lower limit is approximately 1%) than in quartz glass, in which the upper limit is approximately 1% [18]. Thus, the germanium dioxide lattice is characterized by a higher defect density than silicon dioxide. Irradiating glass forming GeO₂ ionizes it and electrons localize around the oxygen vacancy. Thus, the defect becomes paramagnetic.

 

Margaryan et al. [15-17] investigated the EPR spectra of the paramagnetic Mn(II) ion in glass forming and crystalline germanium dioxide (see Figure 1.5). The composition of glass former dopant and crystal dopant are of special interest in determining the structure and character of chemical bonds in vitreous (isotropic) and crystalline (anisotropic) systems. The EPR spectra can provide information about the role of the dopant in these matrices. In an earlier work, Margaryan et al. [17, 19] investigated activation of the SiO₂-Mn(II) matrix. The group found that the short-range order around Mn(II) is retained in glass forming and crystalline states of SiO₂ and that the EPR spectra can be described by the same parameters. Very important parameters are: the degree of covalency in GeO₂ of the electron orbitals of Mn(II) and ligands, the field intensity of the ligands, the coordination position of Mn(II), and regularity of the structure of GeO₂ in the vitreous and crystalline states.

 

EPR study can provide the local environment around different structural compositions of GeO₂. Effects on the solute charge distribution, Mn(II), leads to distinct spectra. Figure 1.5 (see chapter No.1) shows EPR spectra of Mn(II) in glass forming and crystalline GeO₂. These results show that the d-orbitals of the low concentration manganese ion do not interact in the same way with the orbitals of the coordinating glass forming and crystalline GeO₂. The bond type in the structure, between the orbitals of ligand and glass former atom and ligand-dopant, plays an important part.

 

In the glass-forming phase, the components are primarily bound covalently through SPⁿ hybridization between the electron orbitals of the glass former and the ligands. The opposite occurs in the crystalline phase. In glass forming GeO₂ character of the bonding between the ligands and manganese ions is predominantly ionic character [20].

 

The form of the Mn(II) EPR spectra in crystalline (hexagonal) GeO₂ does not change when the manganese concentration increases (Figure 1.5), due to the presence of the regular form of [GeO₄]^4- tetrahedra around a paramagnetic center and the covalent character of the bond between ligands and manganese [15, 16, 17, 20].

 

The EPR spectra of Mn(II) in glass forming GeO₂ (Figure 1.5) show not only hyperfine splitting for g=1.99 but also the existence of other solvated structures around Mn(II) for the tensor values of g=2.67 and g=4.14. This confirms the oxygen fluctuations in glass forming germanium dioxide within the germanium sphere of influence. The existence of various compositional forms of oxygenated germanium modifies the field surrounding the paramagnetic manganese ion.

Figure 5.8 shows the EPR spectra of gamma-irradiated PbO-GeO₂ glasses [21]. An intens signal exists for g=1.996, with a shape characteristic of axially symmetric paramagnetic centers of spin s=1/2. The signal intensity of the g=1.996 line changes very little for PbO in the range of 5-20mol%. Increasing the PbO content in the glass causes a slow decrease in the signal amplitude; when the PbO content is 40mol%, the signal disappears. The signal, observed in the lead germanate glass with g=1.996, coincides in shape, width, and form with the signal observed in vitreous form GeO₂[14]. The signal is related to a self-paramagnetic defect that occurs as a result of a shortage of oxygen in the crystalline, the hexagonally modified GeO₂, and the vitreous GeO₂ [14, 20].

 

Figure 5.8: EPR spectra of irradiated PbO-GeO₂ glasses (after[21]).

 

Increasing the PbO concentration in the glass reduces the oxygen deficiency and, consequently, the number of self-paramagnetic centers decreases. Margaryan et al. [20, 22] studied the change in Mn(II) EPR spectra as a function of PbO concentration (up to 80mol%) in the PbO-GeO₂ glass system. Figure 5.9 shows the characteristic EPR spectra of lead germanate glasses.

 

Figure 5.9: EPR spectra of Mn(II) in the PbO-GeO₂ glass system, 0.03mass% Mn(II) (after[20]).

 

The spectra, with concentrations of 1, 2, and 5mol%, have two intense lines, finely split for g=4.14 (allowed) and g=1.99 (weakly allowed).Increasing the PbO concentration slowly decreases the intensity of the g=1.99 line and results, finally, in a finely split single intese line for g=4.14.

 

From the results described above, we can infer that glasses containing 5-80mol% PbO are formed by a mixture of single type of lead germanate structure, which might possess Ge-O-Pb bonds.

 

5.4 Fluorine Containing Germanate Vitreous Materials

Present needs for specialized optical materials require the creation of vitreous systems capable of considerable variation in their physicochemical and optical properties. This goal can be achieved through the use of glasses that are based on germanium dioxide and fluorides from the second and third groups of the periodic table.

 

A number of authors [23-26] have obtained transparent glasses for the IR region based on fluoride containing glass systems PbF₂-PbO-GeO₂,

AlF₃-PbO-GeO₂, and BaGeO₃-Ga₂O₃-RF_x. Sun [27] synthesized glasses for optical components using the RF-TiO₂-GeO₂ system. The refractive index of these glasses is in the range n_D=1.50-1.75 and the dispersion coefficient is between 25-34.

 

Margaryan [20, 28-40] and Chalilev [41-48] have performed systematic and detailed studies of various properties of fluoride containing germanate glasses.

 

5.4.1 R’Ge₄O₉-RF₂ (RO) Vitreous Systems

Synthesis of transparent glasses in simple binary systems of the type GeO₂-RF_x is difficult to accomplish. The high melt temperature, 1100-1300°C, form a number of germanium fluorides that are highly volatile and thus change the stoichiometry. The result is highly absorbing multicrystalline melt. To reduce this effect, GeO₂ is introduced into the melt as a metagermanate (RGeO₃), tetragermanate (RGe₄O₉), or complex compound of some other element. This procedure allows synthesis of transparent pseudobinary and pseudoternary fluoride containing glasses.

 

Table 5.1 lists glass formation bounds for some binary compounds. As a result of the introduction of GeO₂, in the form of tetragermanates of the alkaline earth elements, the glasses sustains stabilized retention in the molten phase and in the presence of fluorides [20, 28, 29].

 

As the information in Table 5.1 indicates, the domain of glass formation widens in fluoride-containing pseudobinary systems in the order of the fluorides BaF₂®SrF₂®CaF₂®MgF₂. This order is the same for the increasing field of the cations. The opposite behavior occurs in glasses containing alkaline earth oxides [20, 28-32].

Table 5.1

Limits of Glass Formation (mol%) (after [20, 28, 29])

1.                   

System	R
	Mg	Ca	Sr	Ba
CaGe4O9-RF2	77	75	67	63
CaGe4O9-RO	52	75	85	87
SrGe4O9-RF2	78	72	70	50
SrGe4O9-RO	50	74	80	83
BaGe4O9-RF2	75	75	70	65
BaGe4O9-RO	45	80	80	88

Interesting changes observed in density are produced by the content of RF₂ and RO in glasses R’Ge₄O₉-RF₂ and R’Ge₄O₉-RO [20, 28, 29]. The density function is monotonically linear in CaGe₄O₉ and SrGe₄O₉ germanates, as a function of fluoride content. Introducing fluorides into the calcium and strontium tetragermanates lowers the density, due to the fact that the average atomic weight of the fluorides is less than that of CaGe₄O₉ and SrGe₄O₉. Analysis also shows that these fluoride groups are able to fit into the structure of the glass and do not significantly change the underlying structure [20, 28, 29, 41, 42].

 

The density curves of CaGe₄O₉-RO and SrGe₄O₉-RO glasses have peaks that are displaced, relative to one another, depending on the alkaline earth cation type [20, 28, 29]. The presently accepted behavior for the germanates is for the germanium coordination number to change with oxygen content. Another explanation is that introduction of alkaline earth oxides forces the germanate tetrahedra to reorient themselves, thus producing the nonmonotonic, density dependence [20, 48].

 

The IR spectra of the systems CaGe₄O₉-RF₂ (Figure 5.10), SrGe₄O₉-RF₂ (Figure 5.11), and BaGe₄O₉-RF₂ (Figure 5.12) clearly show the existence of characteristic bands for the glass forming germanates. Large regions of absorption exist between 900-800cm^-1 (Ge-O-Ge) and 650-400cm^-1. Introduction of alkaline earth fluorides into the germanate melt does not disrupt the [GeO₄] structure. Fluoride or oxyfluoride groups of the type [MeF₄], [MeF₆], or [Me (O, F)₆] fit interstitially into the glass structure and change the physicochemical properties of fluorogemanate glasses, depending on the amount of fluoride introduced.

 

Figure 5.10: IR spectra of CaGe₄O₉ – RF₂ system glasses (after [20, 28,29]). MgF₂(mol%):

(1) 20, (2) 40, (3) 50. CaF₂: (4) 40, (5) 50. SrF₂: (6) 20, (7) 40, (8) 50. BaF₂:

(9) 20, (10) 40.

 

Figure 5.11: IR spectra of SrGe₄O₉ – RF₂ system glasses (after [20, 28, 29]). (1) SrGe₄O₉

(crystalline). MgF₂(mol%): (2) 20, (3) 40. SrF₂: (4) 40, (5) 50.

 

 

Figure 5.12: IR spectra of BaGe₄O₉ – RF₂ system glasses (after [20,28, 29]). (1) BaGe₄O₉

(crystalline). MgF₂ (mol%): (2) 20, (3) 40, (4) 60. BaF₂: (5) 20, (6) 40, (7) 60.

 

According to experimental results [49, 50] the wide spectral band in the region of 650-400cm^-1 can also be attributed to vibrations of groups of the type [Me(O, F)₆]. The formation of anions of the types [GeO₆]^8- or [GeF₆]^2- in fluoride-containing pseudobinary system R’Ge₄O₉-RF₂ is very unlikely, since this spectral range contains spectral bands resulting from vibrations of perturbed structures of Ge-O-Ge in [GeO₄] or [Me(O, F]₆]. The structure of fluorogermanate glasses consists of tetrahedra of [GeO₄], which undergo a monotonic change in spatial extent when alkaline earth fluorides are introduced.

2.                   

5.4.2 BaGeO₃-RF₂ and PbGeO₃-PbF₂ Vitreous Systems

3.                  The limit of glass formation in pseudobinary system (in mol%) are as follows: BaGeO₃-RF₂ systems, when RF₂=0.45CaF₂ ^. 0.55MgF₂ is a calcium-magnesium fluoride eutectic from 10-50% [41]; when RF₂=0.25MgF₂ ^. 0.75YF₃, it is a magnesium-yttrium fluoride eutectic from 5-65% [45]. Transparent glasses are obtained in BaGeO₃-MgF₂ systems when the content of MgF₂ is 15-65% [20].

 

In the pseudobinary system PbGeO₃-PbF₂, the range of glass formation is determined by the amount of fluoride introduced, which is 40% [46].

 

Table 5.2 summarizes some properties of BaGeO₃-(0.25MgF₂ ^. 0.75YF₃) glasses, as a function of the composition. Introducing eutectic fluoride (0.25MgF₂ ^. 0.75YF₃) instead of barium germanate results in an increase in the coefficient of thermal expansion and a lowering of the values of other properties.

Table 5.2

Some Physicochemical Properties of Glass System

BaGeO₃-(0.25MgF₂ ^. 0.75YF₃) [20, 45]

 

Glass Composition(mol %)
BaGeO3	0.25MgF2 . 0.75YF3	Density g/cm3	Refractive Index nD	Molecular Refraction R (cm3)
90	10	4.961	1.738	19.72
85	15	4.945	-	-
80	20	4.931	1.713	18.35
75	25	-	-	-
70	30	4.909	1.703	17.23
60	40	4.893	1.685	15.63
50	50	4.848	1.659	14.55
40	60	4.831	1.638	12.98

 

5.4.3 R’Ge₄O₉-Ba(PO₃)₂-RF₂ Vitreous Systems

Glasses that possess two glass forming components are of special interest to scientists. These pseudoternary systems are characterized, as a rule, by a very wide range of glass formation. This characteristic permits researchers to vary the mass content of forming elements and to make glasses with unusual amounts of modifiers so they can obtain desired physicochemical and optical properties [20, 30-40].

 

The domain of glass formation was determined for CaGe₄O₉-Ba(PO₃)₂-RF₂, SrGe₄O₉-Ba(PO₃)₂-RF₂, and BaGe₄O₉-Ba(PO₃)₂-RF₂, where RF₂ can be MgF₂, CaF₂, SrF₂ or BaF₂ [20]. The glass forming ability is most distinctive in GeO₂ and P₂O₅. Introduction of these compounds is not recommended because the melt loses considerable mass due to their evaporation.

 

GeO₂ was introduced into the glass in the form of a previously synthesized compound of tetragermanate of alkaline earth elements (R’Ge₄O₉), which is assures the retention of GeO₂ in the glass melt and essentially eliminates the loss of GeO₂ in the presence of alkaline earth fluorides. The second component, P₂O₅, was introduced, by means of barium metaphosphates-Ba(PO₃)₂. Thus, the use of alkaline earth tetragermanate and barium metaphosphate compounds in the synthesis of fluoride containing germanium phosphate glasses reduced losses to 1.0-1.5% by mass [20, 30, 31].

 

In fluoride containing germanium phosphate systems, the range of glass formation increases in the order BaF₂®SrF₂®CaF₂®MgF₂, which follows the increasing strength of the cation field. This behavior is the opposite of the behavior of systems containing alkaline earth oxides [20, 32-40]. Glasses with magnesium fluoride (MgF₂) are especially distinct in their ability to form glasses with up to 80-85mol% concentrations. This behavior also has a part in forming the elemental cell structure of the melt [20].

 

Figure 5.13, 5.14, and 5.15 show the IR spectra of fluoride containing germanium phosphate glasses of the systems CaGe₄O₉-Ba(PO₃)₂-RF₂, SrGe₄O₉-Ba(PO₃)₂-RF₂, and BaGe₄O₉-Ba(PO₃)₂-RF₂. The glasses contain 20-70mol% fluorides. In glasses, the concentration of R’Ge₄O₉ varies from about 10-60mol%, while that of Ba(PO₃)₂ varies from 10-70mol%.

 

The IR spectra of these corresponding systems were obtained with the RF₂ concentration as a parameter. The general features of the IR spectra are reminiscent of the superposed spectra of glasses of germanates, metaphosphates, and orthophosphates or the radicals [Me(O, F)₆] and [MeF₄].

 

Figure 5.13: IR spectra of the system CaGe₄O₉ – Ba(PO₃)₂ – RF₂ (from [20, 32]).

(a) 20MgF₂ ^. XBa(PO₃)₂ ^. (80-x)CaGe₄O₉: (1) x=20,

(2) x=50. 40MgF₂ ^. XBa(PO₃)₂ ^. (60-x)CaGe₄O₉:

(3) x=20, (4) x=40. 50MgF₂ ^. XBa(PO₃)₂ ^. (50-x)CaGe₄O₉:

(5) x=20, (6) x=40. 60MgF₂ ^. XBa(PO₃)₂ ^. (40-x)CaGe₄O₉:

(7) x=20, (8) x=30. (9) 70MgF₂ ^. 10Ba(PO₃)₂ ^. 20CaGe₄O₉.

(b) 20CaF₂ ^. XBa(PO₃)₂ ^. (80-x)CaGe₄O₉:

(10) x=20, (11) x=50, (12) x=70. 40CaF₂ ^. XBa(PO₃)₂ ^. (60-x)CaGe₄O₉:

(13) x=20,

(14) x=40. 20SrF₂ ^. XBa(PO₃)₂ ^. (80-x)CaGe₄O₉: (15) x=20, (16) x=40.

(c) (17) x=70. 30SrF₂ ^. XBa(PO₃)₂ ^. (70-x)CaGe₄O₉: (18) x=40, 919) x=50,

(20) x=60. 20BaF₂ ^. XBa(PO₃)₂ ^. (80-x)CaGe₄O₉: (21) x=20, (22) x=40,

(23) x=70.

 

Figure 5.13 (continued)

 

 

 

 

Figure 5.14: IR spectra of glasses of the system SrGe₄O₉ – Ba(PO₃)₂ – RF₂ (from [20, 52]).

(a) 20MgF₂ ^. XBa(PO₃)₂ ^. (80-x)SrGe₄O₉:

(1) x=20, (2) x=40. 40MgF₂ ^. XBa(PO₃)₂ ^. (60-x)SrGe₄O₉:

(3) x=20, (4) x=40. 50MgF₂ ^. XBa(PO₃)₂ ^. (50-x)SrGe₄O₉:

(5) x=20, (6) x=40. 20CaF₂ ^. XBa(PO₃)₂ ^. (80-x)SrGe₄O₉:

(7) x=20, (8) x=40. (9) x=50, (10) x=70.

(b) 20SrF₂ ^. XBa(PO₃)₂ ^. (80-x)SrGe₄O₉:

(11) x=20, (12) x=40, (13) x=50. 20BaF₂ ^. XBa(PO₃)₂ ^. (80-x)SrGe₄O₉:

(14) x=20,

(15) x=50, (16) x=60, (17) x=70. 40BaF₂ ^. XBa(PO₃)₂ ^. (60-x)SrGe₄O₉:

(18) x=20, (19) x=30.

 

 

 

 

 

 

Figure 5.15: IR spectra of glasses of the system BaGe₄O₉–Ba(PO₃)₂–RF₂ (from [20, 33]).

20MgF₂ ^. XBa(PO₃)₂ ^. (80-x)BaGe₄O₉:

(a) (1) x=20, (2) x=40, (3) x=60, (4) x=70. 40MgF₂ ^. XBa(PO₃)₂ ^.

^. (60-x)BaGe₄O₉:

(5) x=40, (6) x=50. 60MgF₂ ^. XBa(PO₃)₂ ^. (40-x)BaGe₄O₉:

(7) x=10, (8) x=30. 20BaF₂ ^. XBa(PO₃)₂ ^. (80-x)BaGe₄O₉:

(9) x=10, (10) x=20.

(b) (11) x=30, (12) x=40, (13) x=60, (14) x=70. 40BaF₂ ^. XBa(PO₃)₂ ^.

^. (60-x)BaGe₄O₉: (15) x=10, (16) x=20, (17) x=30,

(18) x=40. 60BaF₂ ^. XBa(PO₃)₂ ^. (40-x)BaGe₄O₉: (19) x=10, (20) x=20.

 

 

 

Intense lines are observed between the wave numbers 900-800cm^-1, 1300-1000cm^-1, and 650-400cm^-1. The bands observed in the region of 900-800cm^-1 are due to Ge-O-Ge in [GeO₄]^4-. The absorption maxima within 1300-1000cm^-1 are due to oscillations of PO₂, POP in the metaphospate ion (PO₃)_n^1-. Oscillations by groups of the types [Me(O, F)₆], [MeF₄], and [PO₂], in metaphosphate or tetrametaphosphate anions, are responsible for the bands in the region 650-400cm^-1. The Ge-O-Ge band is sensitive to the composition of the glass and shifts in the direction of higher frequencies when increasing the Ba(PO₃)₂ content in exchange for R’Ge₄O₉.

 

Fluoride containing germanium phosphate glasses that contain 50-60mol% Ba(PO₃)₂ have fluorophosphate and germanium phosphate as forming components. Bonds of the form Ge-O-P are found in the regions 1110-1100cm^-1 and 1070-1050cm^-1. This confirms the formation of germanium orthophosphate structures, Ge₃(PO₄)₄ [51]. Glasses that contain more than 60mol% Ba(PO₃)₂ are formed by fluorophosphate and pure germanate components. In the latter case, IR spectra at 890, 900, 905, and 910cm^-1 indicate antisymmetric oscillations of Ge-O-Ge bonds in [GeO₄]^4– [20, 28, 29]. It becomes clear, then, that fluoride containing germanium phosphate glasses are formed from component compounds of the form [GeO₄]^4-,

(PO₃)_n^1-, [Ge-(-O-P-)₄ ], [Me(O, F)₆] or [MeF₄]^2-.

 

5.4.4 BaGeO₃-BaB₂O₄-RF₂ Vitreous Systems

The glass in the BaGeO₃-BaB₂O₄-RF₂ systems that is of most interest to researchers is BaGeO₃-BaB₂O₄-MgF₂ [53]. Authors [54] produced fluoride containing borogermanate glasses from the system BaGeO₃-BaB₂O₄-RF₂, where R=Mg, Ca, Sr and Ba. The researchers determined the limits of glass formation and physicochemical properties. Introducing fluoride instead of oxide results in an increase in the domain of glass formation. Glasses containing MgO crystallize more readily than glasses containing MgF₂.

 

The refractive index for glasses containing 0-40mol% MgO varies from 1.713-1.710. Replacing MgO with MgF₂ results in lowering of the refractive index from 1.713 to 1.596. This reduction is attributed to the increased concentration of low polarizability in the fluorine ions [20].

 

The density of glasses containing MgF₂(0-60mol%) changes from 4.600 to 4.186g/cm³. The density of glasses containing MgO(0-40mol%) changes from 4.600 to 4.200g/cm³. Thus, replacing MgO with

leads to a widening of the domain of glass formation and a lowering of the refractive index.

 

5.4.5 BaGeO₃-Ga₂O₃-RF_x Vitreous Systems

The system BaGeO₃-Ga₂O₃-RF_x is the basis of a low crystallinity glass that is transparent in the near IR domain to 6 microns [55].

Table 5.3 shows some properties of the system of glasses BaGeO₃ - (0.25MgF₂ ^. 0.75YF₃) - Ga₂O₃ as functions of composition.

 

Table 5.3

Some Optical properties of Glasses of the

BaGeO₃-(0.25MgF₂ ^. 0.75YF₃)-Ga₂O₃ system (from [45])

 

Glass Composition (mol %)			Density g/cm3	Refractive Index nD	Molecular Refraction R(cm3)
BaGeO3	0.25MgF2 . 0.75YF3	Ga2O3
68.5	29.0	2.5	-	-	-
67.0	28.0	5.0	4.905	1.704	17.15
63.0	27.0	10.0	4.898	1.708	17.01
60.5	27.0	12.5	4.891	1.708	17.01
60.0	25.0	15.0	4.887	1.711	15.05
57.0	23.0	20.0	4.879	1.708	16.96
53.0	22.0	25.0	4.873	1.711	16.93

 

Figure 5.16 shows the IR spectra of fluoride containing barium gallium germanate glasses

.

Figure 5.16: IR spectra of glasses from the BaGeO₃ – (0.25MgF₂ ^. 0.75YF₃) – Ga₂O₃ system,

with a constant molar ration BaGeO₃ / (0.25MgF₂ ^. 0.75YF₃) = 7/3 (from [20, 45]). Corresponding to curves 1-6, the concentration of Ga₂O₃(mol%) is 0, 5, 10, 15, 20, and 25. Curve 7 corresponds to the spectrum of the gallium-germanate (Ga₆Ge₂O₁₃).

 

Proceeding from the spectra of 70BaGeO₃ ^. 30(0.25MgF₂ ^. 0.75YF₃), curve 1, bands can be observed between 800-700cm^-1 and 600-400cm^-1, resulting from valence bond and deformation of Ge-O-Ge bonds in [GeO₄] fragments of the germanate glass [20]. Introducing up to 15mol% of Ga₂O₃ produces a small change in the width of the band centered at approximately 770cm^-1. Increasing the Ga₂O₃ above 15mol% shifts the band at 560cm^-1 to 525cm^-1, and an absorption band appears at 1065cm^-1 and gets larger with an increasing concentration of Ga₂O₃. The glass spectra have an appearance that is very much like that of Ga₆Ge₂O₁₃ (Figure 5.16, curve 7) and that has absorption bands at 1055, 800, 700 and 525cm^-1. The structure of the latter gallium germanate is also characterized by a disordered arrangement of polyhedra of [GeO₄], [GaO₄], and [GaO₆] [20].

 

Several authors [56-58] have shown that [GaO₆] can become part of the cell structure of the glass. Depending on the amount of Ga₂O₃, groups of the form [GaO₆] appear within the structure. When the Ga₂O₃ content is not greater than approximately 15mol%, gallium exists in and reinforces the structure in the form [GaO₄]. For very large concentrations of Ga₂O₃, the glass structure undergoes spatial changes. Gallium polyhedra start to form their own structures, which consist of gallium germanate groups, and thus [GaO₄] and [GaO₆] coexist with [GeO₄]. Glasses that contain 15-20mol% of Ga₂O₃ can form two types of structures: structures that are mostly germanate and structures that are mostly galliate. The IR spectra (Figure 5.16) show that the coordination of gallium with the oxygen can be tetrahedral or octahedral. The concentration of [GaO₆] increases when there is an increase of Ga₂O₃ [20].

 

5.4.6 BaGeO₃-Al₂O₃-RF_x Vitreous Systems

In the systems BaGeO₃-Al₂O₃-RF_x the maximum Al₂O₃ content is approximately 20mol%. The physicochemical properties of these glasses, where

RF_x=0.25MgF₂ ^. 0.75YF₃, are very similar to the properties of the Ga₂O₃ system.

 

Figure 5.17 shows the IR spectra of fluoride containing barium aluminum germanate glasses. The IR spectrum for 70BaGeO₃ ^. 30(0.25MgF₂ ^. 0.75YF₃) (Figure 5.17, curve 1) is representative for this system and has regions of absorption in the ranges 800-700cm^-1 (minimum at 770cm^-1) and 600-400cm^-1 (minimum at 560cm^-1), due to valence and deformational vibrations of Ge-O bonds in [GeO₄] groups that form the glass structure.

 

Figure 5.17: IR spectra of glasses from the system BaGeO₃ – Al₂O₃ - (0.25MgF₂ ^. 0.75YF₃).

The constant molar ratio is BaGeO₃ / (0.25MgF₂ ^. 0.75YF₃) = 7/3 (from [59]). Curves 1-6 correspond to Al₂O₃ concentrations of 0, 5, 10, 12.5, 15, and 20mol%.

 

Introducing up to 10mol% Al₂O₃ causes insignificant changes in the spectrum. Starting at approximately 12.5mol% of Al₂O₃, the minimum at 770cm^-1 shifts to 795cm^-1 and 800cm^-1, while the minimum at 560cm^-1 shifts to 615cm^-1 (curves 4-6).

 

The character of the IR spectra (Figure 5.17) indicates that the coordination number of germanium atoms does not change in the glass studied. The shift from 560cm^-1 to 615cm^-1 shows that there are transitions of the type [AlO₆]«[AlO₄] [59]. The quadruple coordinated atoms of aluminum begin to form their own structure, which consists mostly of aluminate groups [59].

5.4.7 BaGeO₃ - Ca₂Al₂GeO₇ - RF₂ and BaGeO₃ - CaTiGeO₅ - RF₂ Vitreous Systems.

Authors [41] studied the glass formation and properties of BaGeO₃-RF₂(0.45CaF₂ ^. 0.55MgF₂)-Ca₂Al₂GeO₇ and BaGeO₃-RF₂(0.45CaF₂ ^. 0.55MgF₂)-CaTiGeO₅. Ca₂Al₂GeO₇ and CaTiGeO₅ were selected because of their crystallochemical compatibility with GeO₂ and SiO₂ and because of the existence of analogous silicates Ca₂Al₂SiO₇ and CaTiSiO₅.

 

The domain of glass formation in pseudobinary systems is in the following ranges: from 10-50mol% RF₂ in BaGeO₃-RF₂ and from 10-50mol% BaGeO₃ in BaGeO₃-Ca₂Al₂GeO₇ and BaGeO₃-CaTiGeO₅. The pseudobinary systems Ca₂Al₂GeO₇-RF₂ and CaTiGeO₅-RF₂ do not form glasses.

 

Studies of the effect of fluoride on glasses in BaGeO₃-RF₂(0.45CaF₂ ^. 0.55MgF₂)-Ca₂Al₂GeO₇(system I) and BaGeO₃-RF₂(0.45CaF₂ ^. 0.55MgF₂) - CaTiGeO₅(system II) were performed with constant rations of BaGeO₃/Ca₂Al₂GeO₇ = 1 and BaGeO₃/CaTiGeO₅ = 1 [20, 41]. Each system could be formed with no more than 30mol% fluoride. Table 5.4 shows some properties of system I and system II.

 

 

 

Table 5.4

Some Properties of Glasses: BaGeO₃-(0.45CaF₂ ^. 0.55MgF₂)-Ca₂Al₂GeO₇(I)

and BaGeO₃-(0.45CaF₂ ^. 0.55MgF₂)-CaTiGeO₅ (II) (from [20, 41])

 

System	RF2 mol%	Density g/cm3	Refractive Index nD	Molecular Refraction R(cm3)
I	5	4.697	1.7064	22.96
II	5	4.794	1.7511	20.42
I	10	4.615	1.7027	22.36
II	10	4.797	1.7360	19.56
I	15	4.568	1.6841	21.25
II	15	4.667	1.7178	18.76
I	20	4.511	1.6671	20.17
II	20	4.645	1.7032	17.79
I	25	4.432	1.6578	19.40
II	25	4.565	1.6962	17.22
I	30	4.424	1.6548	18.45
II	30	4.510	1.6753	16.26

 

The introduction of fluorides results in a reduction in the density of the glasses. This reduction can be ascribed to the lower average atomic weight of the fluorides compared to the weight of other components and of an expanded structure. Because fluorine is less capable of polarization than oxygen, the index of refraction is reduced. The introduction of fluorides leads to lower interatomic forces in the glass and, as a result, the vitrification temperature tg decreases and the coefficient of thermal expansion increases [41].

 

From the IR spectra of the systems BaGeO₃-RF₂(0.45CaF₂ ^. 0.55MgF₂)-Ca₂Al₂GeO₇ with BaGeO₃/CaAl₂GeO₇=1, it has been established that, for RF₂ content up to 30mol%, the character of the spectra remains unchanged but the location of the minimal shifts (Figure 5.18) in the domains of 900-700cm^-1 and 600-400cm^-1 [43]. Since the vibrational frequencies of the [GeO₄] and [AlO₄] tetrahedra and the [AlO₆] octahedra are close to each other, it is difficult to determine the role of each in forming the structure. However, the fluoride content effects the Ge-O valence bond in tetrahedral [GeO₄], a band at 770cm^-1 that broadens and splits slightly.

 

Figure 5.18: IR spectra of glasses, curves 1-3, and of the products of crystallization,

curves 1a-3a, in the system BaGeO₃ – RF₂ – Ca₂Al₂GeO₇. BaGeO₃/Ca₂Al₂GeO7=1 and RF₂(0.45CaF₂ ^. 0.55MgF₃) = 10 (1, 1a); 20(2, 2a); 30 (3, 3a) (from [43]).

 

The splitting is due to the more asymmetrical structure in the glass that is formed when the fluoride content reaches a given value [20, 43]. Taking this into account, the observed splitting of the 770cm^-1 line into two bands appears to be related to the ability of large fluorite concentrations to form [AlO₆] octahedra that can polarize the [GeO₄] tetrahedra. It can be assumed, based on studies of increasing Al₂O₃ concentration in glass, where the absorption increases in the region of 720-780cm^-1 [60], and 760cm^-1 [61], that a fraction of the aluminum atoms incorporates itself into the glass and creates [AlO₄] tetrahedra. The spectra of the glasses indicate mixed values of the coordination of aluminum to oxygen. At low fluoride concentrations, [AlO₆] octahedra can change into [AlO₄] tetrahedra. This is confirmed by studies performed by Tarte [62], which documented the shift of the maximum from 560cm^-1 to higher frequencies of up to 615cm^-1.

 

Similarity between the IR spectra of glasses (Figure 5.18, curves 1-3) and crystallized glasses (curves 1a-3a) implies that distribution and grouping in the microstructures of glasses and crystallized glasses are very similar [20, 43]. Analogous results can be observed in the IR spectra of glasses and crystallized glasses in the BaGeO₃-CaTiGeO₅-RF₂ systems [20, 43].

 

5.4.8 BaGeO₃ - BaAl₂Ge₂O₈ - RF₂ and BaGeO₃ - Pb₂GeO₄ - RF₂ Vitreous Systems

Authors of the [42] studied the physicochemical properties of BaGeO₃-RF₂(0.45CaF₂ ^. 0.55MgF₂)-BaAl₂Ge₂O₈ (system I) and BaGeO₃-RF₂(0.45CaF₂ ^. 0.55MgF₂)-Pb₂GeO₄ (system II). Effects of fluorides on glass formation and properties were studied with equimolecular rations of BaGeO₃/BaAl₂Ge₂O₈=1 and BaGeO₃/Pb₂GeO₄=1. The largest amount of fluorides introduced into system I was 40mol% and the largest amount introduced into system II was 35mol%.

 

Table 5.5 presents some optical properties. Introduction of fluorides increases the concentration of lighter components, which, in turn, reduces the mass density. The introduction of fluorides into germanate melts does not lead to destruction of the basic germanate structure [20]. The physicochemical properties of fluorogermanate systems change immediately when fluoride is introduced [42].

 

 

 

 

Table 5.5

Some Properties of Glasses: BaGeO₃-(0.45CaF₂ ^. 0.55MgF₂)-BaAl₂Ge₂O₈

( I ) and BaGeO₃-(0.45CaF₂ ^. 0.55MgF₂)-Pb₂GeO₄ ( II ) (from [42]).

 

In BaGeO₃-RF₂(0.45CaF₂ ^. 0.55MgF₂)-Pb₂GeO₄ glasses and products of their crystallization (Figure 5.19) the IR spectra are characterized by broad bands in the domains of 900-700cm^-1, 600-500cm^-1, and 400cm^-1 [43].

Figure 5.19: IR spectra of glasses, curves 1-3, and of the products of crystallization,

curves 1a-3a, in the system BaGeO₃ – RF₂ – Pb₂GeO₄. BaGeO₃/Pb₂GeO₄=1 and RF₂(0.45CaF₂ ^. 0.55MgF₃) = 0 (1, 1a); 10(2, 2a); 20 (3, 3a) (from [43]).

 

As the concentration of RF₂ is increased to 35mol%, holding the ratio BaGeO₃/Pb₂GeO₄=1 constant, the minima in the domain of 800-700cm^-1 are shifted and further depressed. The valence vibrations are responsible for the absorption band in the domain of 900-700cm^-1, while vibrational deformations of Ge-O in tetrahedra of [GeO₄] are responsible for the absorption band in the domain of 500-400cm^-1.

 

The absorption bands of the lead germanate glass system (PbO-GeO₂) are in the same domain as those of BaGeO₃-RF₂(0.45CaF₂ ^. 0.55MgF₂)-Pb₂GeO₄ glass system [20]. As Figure 5.19 illustrates, introduction of fluorides into the germanate melts does not change the general structure of the germanate glasses. Because the essential transmissivity minima of the glasses (curves 1-3) coincide with those of their crystallized state (curves 1a-3a), it can be assumed that the structure of the glass contains groups that are also present in the crystallized state.

 

When fluorides are introduced into glass compound, the center of the transmissivity spectral curve falls between 790-770cm^-1, which is the characteristic location of spectral bands of germanates containing a large amount of lead [20]. Fluorides do not cause significant structural changes in the bulk of the glass. The results of analyzing IR spectra (Figure 5.19) confirm the existence of fluoride groupings that possess a common germanate structure [20].

 

 

 

5.4.9 PbGeO₃-Al₂O₃-PbF₂ Vitreous Systems

The maximum PbF₂ content in the glasses is approximately 70-75mol% [46]. Figure 5.20 shows the IR spectra of glass.

 

Figure 5.20: IR spectra of glasses (curves 1-3) in the systems PbGeO₃ – PbF₂ and PbGeO₃ –

Al₂O₃ – PbF₂ (curves 4-8), containing 10mol% Al₂O₃. PbF₂ concentration, mol%: 0 (1, 4); 20 (2); 40 (3); 10 (5); 30 (6); 50 (7); 70 (8). (from [46]).

 

The initial spectra of the glasses lead metagermanate, curve 1, have absorption spectra at 755cm^-1 and 550cm^-1, due to valence and deformational vibrations of Ge-O in tetrahedral [GeO₄] of the polygermanate fragments of glass structure. The vibrations of Pb-O are represented by the absorption bands at lower frequencies.

 

The low frequency absorption bands of the pseudobinary PbGeO₃-PbF₂ system (Figure 5.20, curves 1-3) shift when PbF₂ concentration increases. Introducing up to 30mol% of PbF₂ in glasses with 10mol% of Al₂O₃ reduces the intensity of the principal absorption band and causes of shift in the absorption bands from those of pure glass (90PbGeO₃ ^. 10Al₂O₃) at the lower frequency range. The Ge-O bonds cause an increase of PbF₂ content above 30mol% to further shift the low frequency absorption bands. At the same time, the absorption band increases significantly (Figure 5.20, curves 6-8) in the region 600-500cm^-1, which shifts from 540 to 555cm^-1.

 

Increasing the concentration of PbF₂ beyond 30mol% increases formation of fluoride and oxyfluoride groups of the type [PbF₄]^2-, [AlF₄]^1-, and [Pb(O,F)₄]^2-. This increase is indicated by the shift of the absorption maximum in the region 500-400cm^-1 (Figure 5.20, curves 6-8). By promoting glass formation, the fluoride and oxyfluoride groups extend the range of glass formation for 70mol% concentration of PbF₂. These results correlate with the work of Shibata et al. [63], who studied the system of PbF₂-AlF₃ glasses and concluded that the presence of [AlF₄]^1- of AlF₃ of up to 20-30mol%. Thus, the presence of aluminum atoms allows lead fluoride to participate, along with aluminum fluoride groups, in the glass cell formation.

 

Vopilov et al. [47] studied the system of (0.9-X)PbGeO₃-0.1Al₂O₃-XPbF₂ glasses (X=0.2, 0.4, 0.5, 0.6 and 0.7) using continuous and pulsed ¹⁹F NMR for temperatures in the range 140-490°K. For low temperatures (173°K), the spectra of all compositions are typified by broad asymmetrical bands (Figure 5.21a, curves 1-5) and an assemblage that comprises different intensities and widths. Both the magnitude of the chemical shifts and the studies performed by Vopilov et al. [64] support the conclusion that the weak field band results from atoms of fluorine coordinating with cations of lead F(Pb). The band that shifts into the strong field can be ascribed to anions in positions F(Al) or to fluorine ions that incorporate into the structure of the glass in the form of [GeO₃F]^n-. It is more likely that the strong field component of the NMR spectra is related to ions in the positions F(Ge), since a strong dependence is absorbed in the intensity of lead fluoride concentration, at the same time that the aluminum oxide content, in all samples, remains at 10mol%.

Increasing PbF₂ concentration in the glass increases the number of resonating nuclei and, consequently, the intensity of the NMR signal. The relative intensity of the components at positions F(Pb) and F(Ge) changes in favor of the latter (Figure 5.21a).

 

 

Figure 5.21: (a) Behavior of NMR ¹⁹F spectra as function of concentration, at T=173°K, for

the system (0.9-x)PbGeO₃ – 0.1Al₂O₃ – XPbF₂. (1) x=0.2, (2) x=0.4, (3) x=0.5, (4) x=0.6, (5) x=0.7. (b) Behavior of NMR ¹⁹F spectra as a function of temperature for 10PbO – 10GeO₂ – 70PbF₂ – 10Al₂O₃mol%. (6) T=480°K, (7) T=341°K, (8) T=293°K, (9) T=200°K, (10) T=173°K. (From [47]).

 

Thus, for low PbF₂ concentrations, the fluorine atoms occupy positions of type F(Pb), while increasing the fluorine content raises the occupancy at F(Ge) positions. The basic causes for widening of the spectral lines are dipole interactions between the magnetic moments of ¹⁹F nuclei and increased concentration of PbF₂ [47].

 

The temperature dependent NMR spectra are affected by the lead fluoride concentration. For X=0.2-0.4, the NMR spectra do not change with temperature in the range studied, indicating that there is no diffusion of fluorine ions. When the concentration is in the range X=0.5-0.7 (50-70mol%PbF₂), heating causes changes (Figure 5.21b, curves 6-10). Above 400°K, the F(Pb) band narrows, which indicates the movement of F^- in the F(Pb) subsystem. The fluorine ions from the F(Ge) groups are less mobile and the band starts narrowing at higher temperatures. When the temperature is higher than 460°K, the glass spectrum has a single symmetrical line that is modulated in width, indicating the diffusive nature of the fluorine ion transfer. Increasing the PbF₂ concentration facilitates the diffusion of fluorine ions [20, 47].

 

The mechanism of fluorine diffusion is analogous to the diffusion observed in fluoroborate systems [64]; however, the poorer diffusivity of fluorine in germanate glasses is apparently connected to the incorporation of a large number of fluorine atoms in the oxide structure of the glass [47].