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SPECTROSCOPY AND STRUCTURE OF SOME NON-SILICATE VITREOUS MATERIALS
“Ligands and Modifiers in Vitreous Materials: Spectnoscopy of condensed Systems”
Publisher: World Scientific, Singapore, New Jersey, London, Honk Hong 1999
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  obtained the equation for the RDF.
Warren, Kruter and Morningstar  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 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 )
Figure 5.2: GeO2 radial distribution function (RDF) derived from the spectra shown in Figure 5.1 (after ).
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  shows, larger peaks can result from superpositioning several different distance separations. Moss and Price  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. 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 ).
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 GeO2.
Zarzycki  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 ).
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  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): (1) glass forming
GeO2, 107r dose; (2) hexagonal GeO2, 6x106r dose. Measurements at 78°K.
Figure 5.7: EPR spectra of tetragonal GeO2, irradiated by an electron beam dose of 1017 electrons (after ).
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  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% . 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 .
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 . 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. 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).
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).
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-RFx. Sun  synthesized glasses for optical components using the RF-TiO2-GeO2 system. The refractive index of these glasses is in the range nD=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-RFx 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].
Limits of Glass Formation (mol%) (after [20, 28, 29])
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.
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% ; when RF2=0.25MgF2 . 0.75YF3, it is a magnesium-yttrium fluoride eutectic from 5-65% . Transparent glasses are obtained in BaGeO3-MgF2 systems when the content of MgF2 is 15-65% .
In the pseudobinary system PbGeO3-PbF2, the range of glass formation is determined by the amount of fluoride introduced, which is 40% .
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.
Some Physicochemical Properties of Glass System
BaGeO3-(0.25MgF2 . 0.75YF3) [20, 45]
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 . 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 .
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:
(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,
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:
(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 .
(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)n1-. 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 . 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)n1-, [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 . Authors  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 .
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-RFx Vitreous Systems
The system BaGeO3-Ga2O3-RFx is the basis of a low crystallinity glass that is transparent in the near IR domain to 6 microns .
Table 5.3 shows some properties of the system of glasses BaGeO3 - (0.25MgF2 . 0.75YF3) - Ga2O3 as functions of composition.
Some Optical properties of Glasses of the
BaGeO3-(0.25MgF2 . 0.75YF3)-Ga2O3 system (from )
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 . 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] .
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 .
5.4.6 BaGeO3-Al2O3-RFx Vitreous Systems
In the systems BaGeO3-Al2O3-RFx the maximum Al2O3 content is approximately 20mol%. The physicochemical properties of these glasses, where
RFx=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 ). 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] . The quadruple coordinated atoms of aluminum begin to form their own structure, which consists mostly of aluminate groups .
5.4.7 BaGeO3 - Ca2Al2GeO7 - RF2 and BaGeO3 - CaTiGeO5 - RF2 Vitreous Systems.
Authors  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.
Some Properties of Glasses: BaGeO3-(0.45CaF2 . 0.55MgF2)-Ca2Al2GeO7(I)
and BaGeO3-(0.45CaF2 . 0.55MgF2)-CaTiGeO5 (II) (from [20, 41])
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 .
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 . 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 ).
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 , and 760cm-1 , 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 , 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  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 . The physicochemical properties of fluorogermanate systems change immediately when fluoride is introduced .
Some Properties of Glasses: BaGeO3-(0.45CaF2 . 0.55MgF2)-BaAl2Ge2O8
( I ) and BaGeO3-(0.45CaF2 . 0.55MgF2)-Pb2GeO4 ( II ) (from ).
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 .
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 ).
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 . 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 . 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 .
5.4.9 PbGeO3-Al2O3-PbF2 Vitreous Systems
The maximum PbF2 content in the glasses is approximately 70-75mol% . 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 ).
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. , 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.  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.  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 ).
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 .
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 ; 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 
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