Raman and Infrared Structural Investigation of xRb2O·(1 − x)GeO2

J. Phys. Chem. 1996, 100, 11755-11765

11755

Raman and Infrared Structural Investigation of xRb2O‚(1 - x)GeO2 Glasses E. I. Kamitsos,*,† Y. D. Yiannopoulos,† M. A. Karakassides,‡ G. D. Chryssikos,† and H. Jain§ Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou AVe., Athens 116 35, Greece, Institute of Materials Science, National Centre for Scientific Research “Demokritos”, 153 30 Agia ParaskeVi Attikis, Greece, and Department of Materials Science and Engineering, Lehigh UniVersity, Bethlehem, PennsylVania 18015 ReceiVed: February 12, 1996; In Final Form: April 18, 1996X

Infrared reflectance and Raman spectra of rubidium germanate glasses, xRb2O‚(1 - x)GeO2, have been measured and studied as a function of Rb2O mole fraction in the range 0 ex e 0.60. The spectra of corresponding crystalline germanate phases, GeO2-quartz, GeO2-rutile, Rb2Ge4O9, K4Ge9O20, K2Ge2O5, and K2GeO3, have been also measured for a better understanding of the glass spectra. At very low content (x e 0.01) Rb2O was found to induce a redistribution of GeO4 tetrahedra in rings smaller than those encountered in quartz- and glassy GeO2. The spectral manifestations of this trend are the frequency increase of the Raman active symmetric stretching vibration of Ge-O-Ge bridges and the parallel decrease of the infrared frequency of the asymmetric stretching of Ge-O-Ge bridges. Beyond the composition x ) 0.01 and up to x ≈ 0.150.20, the predominant modification mechanism involves the transformation of GeO4 tetrahedra into GeO6 octahedra, with characteristic Raman scattering at 315, 590, and 630 cm-1. At higher alkali content and up to the limits of glass formation, tetrahedral Qn species with nonbridging oxygens are formed ((4 - n) is the number of nonbridging oxygens per tetrahedron). Thus, Q3 appears beyond x ) 0.10, giving place to Q2 (x > 0.30), Q1 (x > 0.50), and Q0 (x ) 0.60). The characteristic Raman bands of these germanate tetrahedra were measured at 865, 765, 740, and 720 cm-1, respectively. Based on semiempirical models, the spectroscopic results were quantified to yield the average coordination number of Ge atoms, as well as the fraction of Ge atoms in 6-fold coordination. Both were found to exhibit a maximum value at approximately 15 mol % Rb2O.

1. Introduction Germanate glasses are of growing interest because of their unique physical properties and the potential use as optical fibers and infrared transmitting windows.1,2 Early studies of alkali germanate glasses have shown that the refractive index and the density exhibit distinct maxima at alkali content in the range 10-20 mol % M2O (M ) alkali), the exact content for the appearance of the maximum being dependent on the nature of alkali metal.3-6 More recent studies of alkali germanate glasses by Mundy and Jin7,8 have demonstrated the presence of a minimum at ∼10 mol % M2O in the composition dependence of the diffusion coefficient for ionic conductivity. The existence of extrema in physical properties is often referred as the “germanate anomaly” in analogy to the well-known “borate anomaly” effect in borate glasses.9 To explain the anomalous composition dependence of physical properties, it was proposed that addition of alkali oxide to GeO2 glass causes the partial conversion of the germaniumoxygen coordination sphere from four (tetrahedral) to six (octahedral) without breaking the Ge-O-Ge bridging bonds.3-6 This proposition was supported by the results of spectroscopic techniques including infrared absorption,10-13 Raman scattering,14,15 X-ray diffraction,16,17 neutron scattering,18 and extended X-ray absorption fine structure (EXAFS).19,20 The main conclusion of these studies was that addition of alkali oxide up to 15-20 mol % M2O causes the formation of GeO6 octahedra * Corresponding author. Telephone: +30-1 724 9483. Telefax: +30-1 724 9483. E-mail: [email protected]. † National Hellenic Research Foundation. ‡ National Centre for Scientific Research “Demokritos”. § Lehigh University. X Abstract published in AdVance ACS Abstracts, June 1, 1996.

S0022-3654(96)00434-0 CCC: $12.00

and that this leads to the densification of the germanate network. Higher M2O content results in breaking the Ge-O-Ge bridges through the creation of nonbridging oxygens (NBO’s). Therefore, GeO6 octahedra are converted to NBO-containing GeO4 tetrahedra with the consequent decrease in density and refractive index. The above structural model has been questioned by Henderson and Fleet21 on the basis of their Raman investigation of sodium germanate glasses. They interpreted the Raman results as indicating that the germanate anomaly does not result from the GeO4 f GeO6 conversion. Instead, they proposed the creation of three-membered rings of GeO4 tetrahedra upon addition of Na2O. In this approach, the extrema in physical properties are thought to occur when the concentration of small rings, like the three-membered ones, attains its maximum value. Recently, Huang et al.22 investigated the structure of xRb2O‚(1 - x)GeO2 glasses by combining EXAFS and X-ray photoelectron (XPS) spectroscopies to determine the coordination of germanium atoms and the relative concentration of bridging and NBO atoms, respectively. They found that with addition of Rb2O, both GeO6 octahedra and NBO’s are formed for 0 e x e 0.20, but the dominant structural transformation mechanism is the GeO4 f GeO6 conversion in this composition range. For x > 0.20 the concentration of NBO’s continues to increase at the expense of GeO6 octahedra. It is clear that there is a continuing controversy on how the structure of the network reorganizes upon addition of alkali oxide to germanate glasses. Previous studies from this laboratory on an analogous problem, namely the “borate anomaly” effect and related phenomena, have demonstrated the effectiveness of the combined appplication of infrared and Raman spectroscopies in shedding light on the changes of the coordina© 1996 American Chemical Society

11756 J. Phys. Chem., Vol. 100, No. 28, 1996 tion of the glass-forming cation and on the state of bonding of the oxygen atoms.23-25 In this work we present a vibrational investigation (infrared reflectance and Raman scattering) of xRb2O‚(1 - x)GeO2 glasses, with compositions spanning a very broad glass-forming range, 0 e x e 0.60. The choise of the rubidium germanate system for this study allows the direct comparison of structural results from infrared and Raman spectroscopies with those of the recent XPS and EXAFS investigation.22 2. Experimental Section 2.1. Sample Preparation. Twenty-one glass compositions were prepared in the series xRb2O‚(1 - x)GeO2 with 0 e x e 0.60. The appropriate amounts of polycrystalline GeO2 and Rb2CO3 were thoroughly mixed and preheated in platinum crucibles at 900-1000 °C for 1 h and then were melted for about 1/2 h at a temperature in the range 1100-1300 °C depending on composition. The homogenized bubble-free liquids with x e 0.33 were cast in a stainless steel mold and were annealed at 450 °C to remove internal stresses before furnace cooling to room temperature. Specimens appropriate for spectroscopic measurements were prepared by cutting and polishing glass samples to obtain disks of approximate dimensions 1 cm × 1 cm × 1 mm. Glasses with x > 0.33 are hygroscopic and were prepared by splat quenching the melt between two stainless steel blocks. Such samples were used for spectroscopic measurements immediately after preparation, without any heat- or mechanical treatment. For the purpose of spectral comparisons and assignments six crystalline germanate compounds were also investigated. These include the quartz and rutile forms of GeO2 and Rb2Ge4O9 (20 mol % Rb2O), as well as K4Ge9O20 (18 mol % K2O), K2Ge2O5 (33.3 mol % K2O), and K2GeO3 (50 mol % K2O). The last two K-germanate compounds were investigated instead of the corresponding Rb-germanates because the latter compounds are extremely hygroscopic and thus cannot be easily prepared and handled. K4Ge9O20 was studied, since it has a similar alkali content compared with Rb2Ge4O9 and yet quite different structure.16 Such structural differences are expected to manifest characteristic signatures in the Raman and infrared spectra, and these vibrational characteristics can be employed consequently to identify similar structural entities present in germanate glasses. For the preparation of quartz GeO2, commercial GeO2 powder was melted in a platinum crucible at 1500 °C for 5 h. The furnace was then turned off and allowed to cool slowly to 600 °C where it remained for 15 h. After this treatment the material was air quenched to room temperature. Rutile GeO2 was obtained by annealing the glass of composition 0.005Rb2O‚0.995GeO2 at 880 °C for 4 days. The Raman and infrared spectra of the two GeO2 polymorphs prepared in this work are in very good agreement with published spectra.14,15 Rb2Ge4O9 was prepared by slow cooling the corresponding melt from 1250 to 850 °C, where it was kept for 5 h. The Raman spectrum of the obtained polycrystalline material show scattering features analogous to those of the isostructural K2Ge4O9 compound.15 Following Furukawa and White,15 K4Ge9O20, K2Ge2O5, and K2GeO3 compounds were prepared by annealing the corresponding glasses at 600 °C for 24 h, 600 °C for 6 h, and 500 °C for 5 h, respectively. The Raman spectra of the compounds prepared in the context of this study are similar to those reported earlier by Furukawa and White.15 2.2. Spectroscopic Measurements and Data Analysis. Infrared measurements were performed only for glasses with Rb2O content in the range 0 e x e 0.33 because higher content levels resulted in hygroscopic glass samples of small sizes,

Kamitsos et al. which were not appropriate for infrared measurements. Infrared spectra were recorded in the reflectance mode on a Fouriertransform vacuum spectrometer (Bruker 113v) equipped with an 11° off-normal reflectance attachment. Appropriate sources (Hg arc and globar) and detectors (DTGS with polyethylene and KBr windows) were used to cover the entire infrared region. Five Mylar beam splitters of variable thickness (3.5-50 µm) were used in the far-infrared and a KBr one in the mid-infrared to measure the spectrum of each glass sample. Therefore, six spectral segments, each one corresponding to the optimum beam splitter throughput, were finally merged into one data file to give a continuous reflectance spectrum. The combination of this approach with the good quality of surfaces of the glass samples resulted in high-quality reflectance spectra in the range 20-4000 cm-1. The spectrum of a high reflectivity aluminum mirror was used as a reference. All spectra were measured with 2 cm-1 resolution at room temperature and represent the average of 200 scans. The reflectance data were extrapolated to frequencies ν f 0 and v f ∞, using the Bruker EXTPOLR program and then were analyzed by employing the Kramers-Kronig (KK) technique.24-26 The absorption coefficient spectra reported in this work were calculated from the expression

R ) 4πνk ) 2πν′′/n

(1)

where ν is the frequency in cm-1. The real (n) and imaginary (k) parts of the refractive index and the imaginary (′′) part of the complex dielectric permittivity were obtained by the KK analysis and the application of the Fresnel formulas.24,25 Raman spectra were measured at room temperature on a Ramanor HG 2S Jobin-Yvon spectrometer coupled to a personal computer for data acquisition, handling, and storage. Excitation was provided by the 488 nm line of a Spectra Physics 165 argon ion laser (500 mW) at a 90° scattering geometry and 5 cm-1 resolution. The Raman spectra reported in this work are in the temperature reduced form,

Ired) I(ν)/(1 + n(ν))

(2)

where I(ν) is the measured Raman intensity and n(ν) is the Bose distribution factor given by

n(ν) ) [exp(hcν/kBT) - 1]-1

(3)

where ν is the Raman shift in cm-1, c is the speed of light, T is the absolute temperature, and h and kB are the Planck and Boltzmann constants, respectively. The frequency maxima in both Raman and infrared spectra were determined with an accuracy of 1 cm-1 or better. To assist assigning complex profiles of the infrared and Raman spectra, deconvolution into component bands was performed whenever it was found to be necessary. For this purpose, we have employed a least-squares fitting program and a deconvolution procedure applied previously to the vibrational spectra of other glass systems.24,25 The minimum number of component bands that gives a reasonable agreement between experimental and calculated spectra has been used. 3. Results 3.1. Raman Scattering. Figure 1 shows Raman spectra of xRb2O‚(1 - x)GeO2 glasses with compositions in the range 0 e x e 0.25. The spectra of glasses with higher Rb2O content (0.30 e x e 0.60) are depicted in Figure 2. The spectrum of GeO2 glass (x ) 0) is similar to those published by other workers;15,27 besides the dominant band at 418 cm-1 it exhibits

xRb2O‚(1 - x)GeO2 Glasses

Figure 1. Reduced Raman spectra of xRb2O‚(1 - x)GeO2 glasses (0 e x e 0.25).

Figure 2. Reduced Raman spectra of xRb2O‚(1 - x)GeO2 glasses (0.30 e x e 0.60).

a component of medium intensity at 340 cm-1, shoulders at 520 and 690 cm-1, and two weak features at 855 and 975 cm-1. Addition of Rb2O to GeO2 glass causes progressive spectral changes, which appear quite similar to those in the spectra of xK2O‚(1 - x)GeO2 glasses (0 e x e 0.50) reported by Furukawa and White.15 In particular, the intensity of the strong band at 418 cm-1 and the one at 340 cm-1 decrease with increasing alkali content and almost vanish at ca. x ) 0.25. These changes are accompanied by the increasing intensity of the broad feature at ca. 525 cm-1 and the parallel appearance of bands at 315, 590, and 630 cm-1. The later bands exhibit their maximum relative intensity at approximately x ) 0.20 (Figure 1). The

J. Phys. Chem., Vol. 100, No. 28, 1996 11757

Figure 3. Comparison of the Raman spectrum of the 0.20Rb2O‚0.80GeO2 glass with those of crystalline germanate compounds: quartz-type GeO2, rutile-type GeO2, Rb2Ge4O9, and K4Ge9O20.

high-frequency doublet at 855 and 975 cm-1 (for x ) 0) shows a slight intensity increase and a frequency downshift as x increases. For x > 0.10, a new band develops at 865 cm-1 and becomes the main Raman band for x ) 0.33. The spectra of glasses with higher Rb2O content (Figure 2) show the progressive development of new high-frequency bands at 770 and 740 cm-1. For such compositions (x > 0.30) the band at ca. 520 cm-1 decreases in relative intensity and shifts to lower frequencies. In addition, new bands at ca. 250 and 370 cm-1 emerge from the low-frequency envelope as x increases above 30 mol % Rb2O. The Raman spectrum of a glass with medium Rb2O content (x ) 0.20) is compared in Figure 3 with those of two alkali germanate compounds of the same (Rb2Ge4O9) or similar (K4Ge9O20) alkali content and with the spectra of the two GeO2 polymorphs. It is observed that the Raman profile of the glass in the range 250-650 cm-1 represents to a good approximation the convolution of the corresponding profiles of Rb2Ge4O9 and K4Ge9O20 crystals. This suggests that the glass network contains structural arrangements encountered in these crystals. An analogous picture evolves from the consideration of Figure 4 where the spectrum of the x ) 0.40 glass is compared with those of the two high alkali content crystals: K2Ge2O5 (digermanate) and K2GeO3 (meta germanate). The high- (650-950 cm-1) and the medium-frequency (400-600 cm-1) profile of the glass spectrum envelopes quite satisfactorily the corresponding ranges of the crystal spectra. 3.2. Infrared Spectra. Infrared absorption spectra of Rbgermanate glasses are shown in Figures 5 and 6. Spectra of germanate glasses with Rb2O content x ) 0, 0.10, 0.20, 0.25, 0.30, and 0.40 have been reported by Murthy and Kirby.10 Although these earlier spectra are in general agreement with the ones in Figures 5 and 6, they show much broader absorption envelopes than those measured in this study. This effect is probably due to the different method used to prepare specimens appropriate for infrared spectroscopy. Because of sample hygroscopicity, Murthy and Kirby prepared their specimens in the form of mulls and recorded the transmission spectra, while

11758 J. Phys. Chem., Vol. 100, No. 28, 1996

Figure 4. Comparison of the Raman spectrum of the 0.40Rb2O‚0.60GeO2 glass with those of crystalline germanate compounds: K2Ge2O5 (digermanate) and K2GeO3 (meta germanate).

Kamitsos et al.

Figure 6. Infrared spectra of xRb2O‚(1 - x)GeO2 glasses (0.125 e x e 0.33).

Figure 7. Apparent frequency of absorption maximum and shoulder of the high-frequency envelope vs Rb2O mole fraction. Lines are drawn to guide the eye.

Figure 5. Infrared spectra of xRb2O‚(1 - x)GeO2 glasses (0 e x e 0.10).

we measured in vacuum the reflectance spectra of freshly cut and polished glass samples. The latter technique is known to result in band shapes and relative intensities as close to the real ones as possible.24-26 As the alkali content increases, the infrared spectra are being progressively affected. In particular, the high-frequency envelope (800-1100 cm-1) shifts to lower frequencies and becomes broader for 0 < x e 0.15, while for higher x values a shoulder develops on the high-frequency side of the main band. The frequencies of the apparent maximum of the main band and its shoulder are plotted in Figure 7 vs Rb2O mole fraction. Trends

similar to those depicted in this figure have been observed for alkali germanate glasses, for silver-, thallium-, and aluminogermanates,10-13 as well as Ti-containing Na-germanates,28 Bi-germanates and Pb-Bi-germanates.29 To assist the elucidation of the structural origin of the observed trends, we compare in Figure 8 the infrared spectrum of the x ) 0.20 Rb-germanate glass with those of relevant crystals. These include K2Ge2O5 (contains GeO4 tetrahedra with one NBO each), K4Ge9O20 (contains interlinked GeO4 tetrahedra and GeO6 octahedra), and Rb2Ge4O9. The latter crystal has the same stoichiometry with the x ) 0.20 glass and contains interlinked GeO4 and GeO6 polyhedra. In addition, the spectra of rutile-GeO2 (edge-sharing GeO6 octahedra) and quartz-GeO4 (corner-sharing GeO4 tetrahedra) are shown. It is observed in Figure 8 that the high-frequency absorption envelopes of the rutile and quartz forms of GeO2 bracket the absorption envelope

xRb2O‚(1 - x)GeO2 Glasses

Figure 8. Comparison of the infrared spectrum of the 0.20Rb2O‚0.80GeO2 glass with those of crystalline germanate compounds: quartz-type GeO2, rutile-type GeO2, Rb2Ge4O9, K4Ge9O20, and K2Ge2O5.

of the glass spectrum, which appears to contour those of the three crystalline alkali germanate compounds. Absorption in the medium-frequency range (400-650 cm-1), as well as in the far-infrared range (below 400 cm-1), is also affected by adding Rb2O (Figures 6 and 7). The far-infrared spectra of Rb-germanate glasses have been studied extensively elsewhere30 with the purpose of elucidating the type and distribution of sites occupied by Rb cations and their effect on ionic conduction. 4. Discussion 4.1. Raman Band Assignments. Before considering the spectra of Rb-germanate glasses, it is useful to review first the assignments of the Raman bands of GeO2 glass (x ) 0). These assignments have progressively developed on the basis of results of experimental techniques, including isotopic substitution,31 neutron bombardment,32 and SiO2 substitution.33,34 Calculations based on the continuous random network (CRN) model have also contributed to the assignments of the vibrational features of GeO2 glass.31,35-39 The dominant band of the Raman spectrum at 418 cm-1 has been assigned to the symmetric stretching vibration (νs) of Ge(4)-O-Ge(4) bridges in predominantly six-membered rings, i.e., rings made of six GeO4 tetrahedra.33,34,37 In the notation Ge(4)-O-Ge(4), the number in parentheses indicates the coordination number of germanium atom. This symmetric stretching mode involves primarily oxygen motion along the bisector of the Ge-O-Ge angle, with a small degree of Ge motion as shown by Raman studies of isotopically substituted vitreous GeO2.31,39,40 The corresponding mode in silicate glasses was shown to be delocalized and have a mixed stretchingbending character.41 The shoulder at ca. 520 cm-1 is known as a “defect” line, in analogy with the Raman band of g-SiO2 at 605 cm-1. Galeener and co-workers have attributed the 520 cm-1 shoulder to the symmetric breathing mode of regular rings, most probably four-

J. Phys. Chem., Vol. 100, No. 28, 1996 11759 and three-membered rings, which involves oxygen motion only.31,37-39 Isotopic substitution studies have indicated that the 340 cm-1 band arises from a mode involving mainly Ge motion and very little O motion.31 This is probably a deformation mode, but its exact structural origin is unknown. The ∼590 cm-1 shoulder can be attributed to the bending mode of Ge-O-Ge bridges involving motion of both O and Ge atoms.27,37-39 The two high-frequency features at ∼855 and 975 cm-1 arise from the asymmetric stretching vibration (νas) of Ge(4)-O-Ge(4) bridges involving both O and Ge motion.34,37-40 This mode is weakly Raman active but gives a strong infrared absorption in the same region as will be discussed below. On the basis of the above information, we focus now on the structural reorganization of the germanate network induced by the addition of Rb2O to GeO2. For 0 e x e 0.25, of main interest is the development of the broad band at ca. 525 cm-1 and the parallel appearance of the three peaks at 315, 590, and 630 cm-1. The search for the structural origin of the above features can benefit from the consideration of the Raman spectra of Rb2Ge4O9 and K4Ge9O20 crystals (Figure 3). Rb2Ge4O9 is isostructural to K2Ge4O9 and Na2Ge4O9 crystals.16,42,43 The basic structural unit consists of a GeO6 octahedron linked by corner oxygens to six three-membered rings (Ge3O6) of GeO4 tetrahedra, and the fraction of Ge atoms in 6-fold coordination is N6 ) 0.25. The Raman spectrum of Rb2Ge4O9 crystal is dominated by scattering centered at ca. 495 cm-1. With reference to the structure of Rb2Ge4O9 and the assignments for GeO2 glass, we attribute the ca. 495 cm-1 band to νs(Ge(4)-O-Ge(4)) in three-membered rings of GeO4 tetrahedra. This assignment is consistent with the theoretical work of Galeener and co-workers37-39 who demonstrated that νs(Ge(4)-O-Ge(4)) will increase upon decreasing the average value of the intertetrahedral Ge-O-Ge bond angle (θ). To confirm this assignment, we note that the formation of smaller rings in the germanate network will naturally result in smaller average values of θ. Diffraction studies of GeO2 glass by Leadbetter and Wright44 have shown that the average GeO-Ge angle is θ ) 133°, and Figure 1 gives νs ) 418 cm-1, while for quartz-like GeO2 θ ) 130.1° 45 and νs is measured at 440 cm-1 (Figure 3). For the Rb2Ge4O9 crystal an average value of θ ) 126.3° for the three-membered rings of GeO4 tetrahedra can be estimated from the reported crystallographic data,42 and thus, it is reasonable to assign the ca. 495 cm-1 band to νs(Ge(4)-O-Ge(4)). Besides the strong doublet at 495 cm-1, the Rb2Ge4O9 crystal exhibits in the same frequency range a weaker doublet at ca. 535 cm-1. We tentatively attribute this band to νs(Ge(4)-OGe(6)), i.e., to bridges connecting a germanium tetrahedron with a germanium octahedron. Such bridges are part of the structure of the Rb2Ge4O9 crystal.42 Along these lines, we ascribe the ca. 520 cm-1 band in the Raman spectra of Rb-germanate glasses to a convolution of contributions from the symmetric stretching vibration of Ge(4)-O-Ge(4) bridges, in the threemembered ring of GeO4 tetrahedra, and of mixed Ge(4)-OGe(6) bridges. Crystalline K4Ge9O20 is isostructural to Na4Ge9O20.16,46 Its network is built up of GeO6 octahedra and GeO4 tetrahedra linked together in a three-dimensional network. Specifically, four GeO6 octahedra share edges to form G4O16 groups, which are connected by GeO4 tetrahedra in forming chains of composition (Ge5O16)n.46 Four of these chains are joined by (GeO3)n spirals of corner-sharing GeO4 tetrahedra to form a three-dimensional structure with N6 ) 0.444. Therefore, the structure of K4Ge9O20 exhibits all three kinds of bridges

11760 J. Phys. Chem., Vol. 100, No. 28, 1996 connecting germanate polyhedra, namely Ge(4)-O-Ge(4), Ge(4)-O-Ge(6), and Ge(6)-O-Ge(6) bridges. In the frequency range of interest, the Raman spectrum of K4Ge9O20 shows six weak components in the range 400-550 cm-1, two strong doublets centered at ca. 320 and 600 cm-1, and a sharp band at 650 cm-1. On the basis of the above, we attribute Raman bands in the range 400-500 cm-1 to a combined contribution of νs(Ge(4)-O-Ge(4)) and νs(Ge(4)O-Ge(6)). The three bands at ca. 320, 600, and 650 cm-1 are probably arising from characteristic vibrational modes of interconnected GeO6 octahedra in G4O16 arrangements. It is of interest to note here that the Rb2Ge4O9 crystal, which contains GeO6 octahedra but no Ge(6)-O-Ge(6) bridges, exhibits no bands of considerable intensity at ca. 320 and 600 and 650 cm-1. Recently, Durben and Wolf47 studied the Raman spectrum of GeO2 glass as a function of pressure up to 56 GPa. They found that the transformation of corner-sharing GeO4 tetrahedra into edge-sharing GeO6 octahedra is manifested by the development of strong Raman bands near 330 and 610 cm-1 and a shoulder at ca. 850 cm-1. Verweij and Buster14 have studied the Raman spectra of Li-, Na-, and K-germanate crystals and glasses. Bands observed in the region 550-680 cm-1 were attributed to νs(Ge(6)-O-Ge(6)) and/or to the symmetric stretching vibration of GeO6 octahedra, in analogy to the A1 mode of the rutile-type GeO2 (strongest band at 702 cm-1; see Figure 3). On the basis of our results and the reported assignments, we propose that the development of Raman bands in the spectra of alkali-germanate glasses at ca. 315 and 590-630 cm-1 is a manifestation of the formation of connected GeO6 octahedral units. Comparison of the Raman spectra of Rb-germanates (Figure 1) with those reported for Li-, Na-, and K-germanate glasses14,15,21 shows that (a) these characteristic bands are exhibited by Rb, K, and Na spectra but not by Li spectra for alkali oxide content at least up to 33 mol %, (b) the relative intensity of these bands increases from Na to Rb, and (c) the relative abundance of the structural units giving rise to these bands shows a maximum at x ) 0.20 for Rb glasses, at x ) 0.25 for K glasses, and at x ) 0.33 for Na glasses. Therefore, there is a clear cation dependence of the structural units composed of connected GeO6 octahedra, like the G4O16 groups in the K4Ge9O20 crystal. The above discussion justifies the deconvolution of the ca. 200-700 cm-1 Raman envelope into component bands as shown in Figure 3. This frequency range could be well simulated by three bands at ca. 445, 520, and 590 cm-1. For 0.20 e x e 0.30, a fourth component at 630 cm-1 was also included. Clearly, the Ge-O-Ge bending mode at ca. 585 cm-1 (spectrum of GeO2 glass in Figure 1) is included within the band deconvoluted at 590 cm-1, where the characteristic scattering of connected GeO6 octahedra also occurs. The relative intensities of the deconvoluted bands at 418-445 cm-1 and 517-530 cm-1, and the sum of 590 and 630 cm-1 are shown in Figure 9. The results show that (a) the relative abundance of Ge(4)-O-Ge(4) bridges in predominantly sixmembered rings of interlinked GeO4 tetrahedra decreases with x and almost vanishes at ca. 40 mol % (band at 418-445 cm-1), (b) assuming that the scattering cross section of the Ge-OGe bending mode at 585 cm-1 does not depend strongly on x, the maximum of the sum of the relative intensities of the of 590 and 630 cm-1 bands observed at ca. 15-20 mol % Rb2O can be attributed to a maximum relative abundance of structural units composed of connected GeO6 octahedra, and (c) the relative intensity of the 517-530 cm-1 band increases slowly with addition of Rb2O up to ca. 5 mol %, shows a sharp increase

Kamitsos et al.

Figure 9. Composition dependence of the relative intensity of deconvoluted Raman bands at 418-445, 515-530, and 595 + 630 cm-1. Lines are drawn to guide the eye.

between x ) 0.05 and 0.1, and remains constant up to x ) 0.15. This behavior suggests a similar composition dependence of the sum of the relative populations of three-membered rings of GeO4 tetrahedra and Ge(4)-O-Ge(6) bridges. For x > 0.15, the relative intensity of the 517-530 cm-1 band increases rapidly. We recall that for x > 0.10, a new band develops at 870 cm-1. This feature can be attributed to the symmetric stretching vibration of Ge-O- bonds (O-dNBO) in germanate tetrahedra containing three bridging and one nonbridging oxygen atoms.14,15 This is a Q3-type germanate tetrahedron according to the NMR terminology. Therefore, a new type of bridge, Ge(4)-O-Ge(Q3) with at least one germanium center being of Q3-type, forms rapidly when x exceeds approximately 0.10. This is probably the main cause of the rapid increase of the 517530 cm-1 band intensity for x > 0.15. The symmetric stretching vibration of this new type of bridge is expected to be active in the same frequency range and thus to contribute to the intensity enhancement of the 517-530 cm-1 band. The depolymerization of the network due to creation of NBO’s is expected to relieve some of the network strain and to increase therefore the intertetrahedral angle (Ge-O-Ge). This will eventually lead to a reduction of the νs frequency of the new type of Ge-O-Ge bridges,37-39 and this is in fact observed in Figure 2 by the systematic frequency lowering of the ca. 520 cm-1 band as x increases. Increasing the amount of Rb2O (x g 0.30) results in increasing concentration of NBO’s. This effect is manifested by the appearance of new bands at 765 cm-1 (x > 0.30), 740 cm-1 (x > 0.50), and a shoulder at ca. 720 cm-1 (x ) 0.60). These features are attributed to the symmetric stretching vibration of Ge-O- bonds in Q2, Q1, and Q0 species, respectively,14,15 in analogy to the highly modified silicate glasses.41 This assignment is supported by the comparison of the spectrum of the x ) 0.40 glass with those of the crystalline compounds of digermanate and meta germanate stoichiometry (Figure 4). The band at ca. 515 cm-1 νs(Ge(4)-O-Ge(4)) appears to decrease in frequency and intensity for Rb2O content exceeding 30 mol %. As discussed above, the frequency lowering originates from the increase of the intertetrahedral angle with increasing number of NBO’s per GeO4 tetrahedron. The depolymerization of the germanate network upon increasing concentration of NBO’s obviously reduces the population of

xRb2O‚(1 - x)GeO2 Glasses Ge(4)-O-Ge(4) bridges, and this is manifested by the intensity reduction of the ca. 515 cm-1 band. The two bands developing at lower frequencies, at ca. 250 and 370 cm-1, can be attributed to the Raman active bending modes of Q2 and Q1 tetrahedra.48 These modes become localized as the number of NBO’s increases and the germanate tetrahedron starts acquiring molecular-like character. 4.2. Infrared Band Assignments. The infrared spectrum of the GeO2 glass exhibits its maximum absorption at 915 cm-1 (Figure 5). This band is assigned to the asymmetric stretching vibration (νas) of Ge(4)-O-Ge(4) bridges.34,37-39 The highly asymmetric profile of this band may reflect the broad distribution of intertetrahedral Ge-O-Ge angles. The weak feature at 585 cm-1 arises from the bending vibration of Ge(4)-O-Ge(4) bridges, while the band at 315 cm-1 has been attributed to the rocking motion of the bridging oxygen atom perpendicular to the Ge-O-Ge plane.40 The continuous variation of the germanate structure with addition of Rb2O is reflected by spectral changes throughout the infrared spectrum. Thus, the high-frequency envelope (800-1050 cm-1) shows initially a pronounced frequency decrease for 0 e x e 0.15, and then it splits into two components for 0.15 e x e 0.33. The two lower frequency bands at 585 and 315 cm-1 for x ) 0 change progressively with x and split eventually into components for x > 0.15. We focus here our attention on the changes of the most intense infrared band, i.e., the high-frequency envelope, and the structural information that can be extracted from it. A similar shift of νas(Si-O-Si) to lower values upon addition of metal oxide has been observed in alkali and Pb-silicate glasses10,11,40 and attributed to the weakening of the three-dimensional SiO-Si network because of its gradual depolymerization through formation of Si-O- bonds. It is noted though that the analogous shift of νas(Ge-O-Ge) is almost twice that of νas(Si-O-Si) observed in silicate glasses with the same metal oxide content.11 The larger shift of νas in germanate glasses has been attributed by Murthy and Kirby10 to the formation of GeO6 octahedra, the presence of which causes the effective weakening of Ge-O bonds. It was therefore proposed that in the composition range 0 < x e 0.20, the predominant mechanism of network modification in alkali germanate glasses is the change of the coordination number of germanium from four to six. This approach was followed subsequently by other workers in the field.11-13,28,29 The decrease of the infrared frequency of the stretching mode of a T-O bond with increasing coordination number of the glass-forming cation T has been well documented by Tarte and co-workers for numerous inorganic crystalline compounds and glasses.50,51 For crystalline and glassy borates this frequency shift is so large that distinct infrared peaks have been resolved for the B-O stretching mode in BO4 tetrahedra (at ca. 1000 cm-1) and BO3 triangles (at ca. 1300 cm-1).24,25 However, as shown in earlier studies52,53 and discussed by Henderson and Fleet,21 a weakening of the T-O bond can result also from the decrease of the intertetrahedral T-O-T angle (θ) without any change in the coordination number of cation T. Therefore, it is essential to quantify the effect of decreasing θ on the lowering of νas(Ge-O-Ge) frequency before considering the effect of increasing germanium coordination number in Rb-germanate glasses. Following the simplified force-constant model developed by Lucovsky40 to study vibrational modes of g-SiO2, we define two effective force constants for Ge-O-Ge bridges: a bondstretching force constant, kr, and a bond-bending force constant, kθ. These are effective force constants, since they will be

J. Phys. Chem., Vol. 100, No. 28, 1996 11761

Figure 10. Calculated frequencies for νs(Ge(4)-O-Ge(4)) and νas(Ge(4)-O-Ge(4)) vs the intertetrahedral Ge(4)-O-Ge(4) bond angle (θ). Data points refer to the experimental values obtained for GeO2 glass and quartz crystal. For details see text.

evaluated from the fit of two experimental frequencies, despite the fact that they are influenced by other forces as well. The two stretching frequencies of the Ge-O-Ge bridge are related to the effective force constants by

ν2s) (1/4π2c2)(2kr/mo)(cos2R + 2γ sin2R)

(4)

ν2as) (1/4π2c2)(2kr/mo)sin2R

(5)

where mo is the mass of oxygen atom, 2R ) θ is the average value of the Ge-O-Ge bond angle, γ ) kθ/kr, and the germanium motion is assumed to be negligible in the asymmetric stretching mode.40 For quartz GeO2 the measured frequencies are νs ) 440 cm-1 and νas ) 905 cm-1 (Figures 3 and 8) and the average intertetrahedral angle is θ ) 130.1°.45 With these experimental values the application of eqs 4 and 5 results in kr ) 4.7 × 105 dyn/cm and γ ) 1 × 10-2. The validity of the model is tested by using these derived force constants to calculate the stretching frequencies of Ge(4)-O-Ge(4) bridges in GeO2 glass, for which the average value of θ is 133°.44 The calculated frequencies are νs ) 419 cm-1 and νas ) 915 cm-1, in excellent agreement with the experimental values for g-GeO2 (Figures 1 and 5). Thus, the frequencies of the main vibrational features of GeO2 in the glassy and quartz forms can be well accounted for in terms of the Ge-O-Ge angle and the above simplified force-constant model. Assuming that the effective force constants are independent of θ, the frequencies νs and νas have been evaluated and are presented in Figure 10 vs the intertetrahedral angle. For the range of θ values shown in the figure it is observed that νas increases almost linearly with θ, while νs shows the opposite trend. The average value of θ in Rb-germanate glasses is not known. Nevertheless, we can employ the values obtained from the crystallographic study of the Rb2Ge4O9 crystal and evaluate the effect of θ on the vibrational characteristics of this compound. The three-membered rings of GeO4 tetrahedra in the crystal are characterized by two types of Ge(4)-O-Ge(4) bridges, one with θ ) 124.6° and the other with θ ) 128.0° .42 The average value of θ ) 126.3° and eqs 4 and 5 yield νs ) 468 cm-1 and νas ) 891 cm-1. The Raman spectrum (Figure 3) of Rb2Ge4O9 shows a strong doublet at 495 cm-1 attributed

11762 J. Phys. Chem., Vol. 100, No. 28, 1996 to νs, and the infrared spectrum (Figure 8) gives the main absorption centered at ca. 850 cm-1 (νas). The comparison between experimental and calculated frequencies is poor and suggests that the effect of θ alone is not enough to account for the main vibrational frequencies of this crystal, which has a relatively high Rb2O content, x ) 0.20. It is noted, however, that experimental values of νas on the order of 890 cm-1 have been obtained for Rb-germanate glasses with low Rb2O content, x ≈ 0.01 (Figure 7). It appears, therefore, that for glasses with x e 0.01, the progressive change of ring statistics in favor of small rings, such as the three-membered rings, is probably the main origin for the observed variation of νas and νs. For higher alkali content, 0.01 < x e 0.15, additional structural rearrangements should be taken into consideration in order to understand the observed structural changes. Henderson and Fleet21 proposed that NBO’s are formed in Na-germanate glasses even with very low Na2O additions. Inspection of the Raman spectra of Rb-germanate glasses in Figure 1 shows that the scattering at 865 cm-1 characteristic of Q3 tetrahedra appears for Rb2O content greater than approximately 10 mol %. Given the sensitivity of Raman spectroscopy, this suggests that the concentration of NBO’s in glasses of lower Rb content should be rather small. This consideration is supported by the results of the recent XPS study of the same glass series.22 It was shown there that even though NBO’s could be detected for x < 0.10, their concentration becomes appreciable only in glasses with x > 0.20. Thus, the results of Raman, infrared, and XPS spectroscopies support the assessment that the conversion GeO4 f GeO6 is the dominant modification senario in Rb-germanate glasses with compositions x < 0.20. In an attempt to quantify this transformation mechanism, we consider further the high-frequency infrared envelope. Because of its complex profile, it has been deconvoluted, as shown in Figure 8, for the x ) 0.20 glass. It was found that this highfrequency envelope can be well simulated by at least three components at ca. 775, 840, and 905 cm-1. The band at 775 cm-1 corresponds quite well to the absorption maximum of the K2Ge2O5 crystal and thus can be attributed to νas of Ge-Obonds in Q3 tetrahedra. This assignment is supported by the fact that the intensity of this band increases with x and becomes the main band of the x ) 0.33 glass spectrum. The corresponding symmetric stretching vibration, νs(Ge-O-), is measured at 865 cm-1 in the Raman spectrum (Figure 2). The two crystals K4Ge9O20 and Rb2Ge4O9, which have no NBO’s, give strong absorption centered at ca. 850 cm-1. It is recalled that in these crystals the GeO4 tetrahedra and GeO6 octahedra are interlinked. It is then reasonable to attribute the bands at 850 cm-1 to νas of Ge-O-Ge bridges, which are perturbed by the presence of GeO6 octahedra, in agreement with the proposition of Murthy and Kirby.10 This perturbation may be brought about by direct participation of a six-coordinated Ge atom in the bridge and/or by its second-neighbor influence on the Ge(4)-O-Ge(4) bridge. Figure 11 shows examples of high-frequency infrared envelopes for four glass compositions, which have been deconvoluted into three components. According to the above discussion, the lower-frequency component (780-793 cm-1) can be assigned to νas(Ge-O-) in the Q3 species and the middle-frequency one (850-877 cm-1) to νas(Ge-O-Ge). The high-frequency component at 905-958 cm-1 exhibits a frequency and intensity dependence on the Rb2O content, which parallels that of the middle-frequency band. This suggests that the high-frequency component originates also from vibration of Ge-O-Ge bridges. It can be attributed, for example, to the longitudinal optic (LO) component of νas(Ge-O-Ge), with its transverse optic (TO)

Kamitsos et al.

Figure 11. Examples of deconvoluted high-frequency envelopes of xRb2O‚(1 - x)GeO2 glasses.

Figure 12. Composition dependence of the νas frequency of Ge-OGe bridges perturbed by the presence of GeO6 octahedra. νas is the frequency of the middle component of the deconvoluted spectra shown in Figure 12. The line is drawn to guide the eye. For details see text.

component giving rise to the middle-frequency band.27 An alternative assignment for the high-frequency component can be proposed on the basis of the work of Kirk55 for g-SiO2. It was shown that the asymmetric stretching (AS) vibration of SiO-Si bridges gives rise to two vibrational modes. In one mode, AS1, adjacent oxygen atoms move in phase with each other, and in the other mode, AS2, they move 180° out of phase. According to this approach, the high-frequency component (905-958 cm-1) can be attributed to the AS2 mode and the middle-frequency one (850-877 cm-1) to the AS1 mode of the asymmetric stretching of Ge-O-Ge bridges. Regardless of the correct form of vibration, it is quite clear that both the high- and the middle-frequency components of the main infrared envelope arise from vibrations of Ge-OGe bridges. The frequency of the strongest middle-frequency component, designated by νas(Ge-O-Ge), is plotted in Figure 12 vs Rb2O content. It is observed that for 0 < x e 0.01, there is a fast decrease of νas, while at higher x values it reduces with

xRb2O‚(1 - x)GeO2 Glasses

J. Phys. Chem., Vol. 100, No. 28, 1996 11763

a smaller rate, reaches a minimum value at ca. x ) 0.15, and then increases smoothly. Consideration of the Raman spectra in Figure 1, the composition dependence of the relative intensities in Figure 9, and the results of the XPS study22 shows clearly that there are no pronounced structural changes in terms of NBO and/or GeO6 creation in the composition range 0 < x e 0.01. We therefore attribute the decrease of νas in the range 0 < x e 0.01 to a structural reorganization of GeO4 tetrahedra, leading to a change in ring statistics. This is probably in favor of the small three-membered rings, which results in the effective decrease of the intertetrahedral Ge-O-Ge angle. This is fully consistent with the predictions of the simplified force-constant model discussed above. At higher rubidium content, 0.01 < x e 0.33, the variation of νas can be taken as a manifestation of the composition dependence of the relative abundance of GeO6 octahedra. 4.3. Coordination of Germanium Atoms in Rb-Germanate Glasses. Figure 9 shows that the abundance of GeO6 octahedra, manifested by the sum of the relative intensities of the Raman components at 595 and 630 cm-1, exhibits a maximum at ca. 15-20 mol % Rb2O. It would be desirable to quantify these spectroscopic results in terms of the average coordination number of germanium atoms or equivalently the fraction of Ge atoms in 6-fold coordination, N6. This would require the knowledge of the corresponding Raman cross section. However, the Raman cross section of bands characteristic of germanate polyhedra is not known, and thus it is impossible at this point to quantify the Raman data. An alternative approach has been proposed by Dachille and Roy55 who related the wavelength, λ, of the main infrared absorption band of simple crystalline compounds with the cation coordination number, CN. In particular, they showed that the wavelength of the strongest infrared band due to the stretching of T-X bonds in compounds of the type TX2 (BeF2, SiO2, GeO2) is related to the coordination number CN of cation T by the empirical expression,

K ) [CN µ(AT+ AX)1/3]/[(ZTZX)λ2]

(6)

where µ is the reduced mass of vibration, AT and AX are the atomic numbers and ZT and ZX are the valences of atoms T and X, respectively, and K is a constant. When λ is expressed in µm and µ in atomic mass units, the experimental results of Dachille and Roy55 for a number of TX2 compounds gave the average value K ) 0.168. The validity of the Dachille and Roy relation was confirmed subsequently by Sclar et al.56 in their infrared study of SiO2 and GeO2 polymorphs. Using the infrared band due to the asymmetric stretching of T-O-T bridges (T ) Si, Ge) as a probe, they obtained the average value of K ) 0.170.56 Assuming that the Dachille and Roy relation (eq 6) can be applied for glassy materials as well, we employ the νas(GeO-Ge) frequency obtained by deconvolution (Figure 12) as a probe of the average coordination number of germanium atom in Rb-germanate glasses. By use of a simple diatomic approximation for the reduced mass, µ ) mGemO/(mGe + mO) ) 13.11 au, and AGe + AO ) 40, ZGeZO ) 8, eq 6 reduces to

K) 5.6 CN/λ2

(7)

where λ (in µm) is obtained from νas(Ge-O-Ge). We calculate next the value of the constant K using our infrared data. For rutile-GeO2 (CN ) 6) the frequency value νas ) 715 cm-1, which appears to be the center of the high-frequency infrared

Figure 13. Average coordination number of germanium atoms, CN, vs Rb2O mole fraction, calculated according to eq 7. The line is drawn to guide the eye.

absorption (Figure 8), gives K ) 0.172. For the reasons presented in the previous section, we use as a representative value of νas when all germanium atoms are 4-fold coordinated (CN ) 4) in Rb-germanate glass the corresponding frequency of the x ) 0.01 glass (νas ) 877 cm-1). This value leads also to K ) 0.172. Therefore, the Dachille and Roy relation holds with our data as well, with K being slightly different from the previously reported values.55,56 The νas(Ge-O-Ge) values for glasses with composition 0.01 < x e 0.33 were then utilized to calculate the average coordination number of Ge atoms by employing eq 7, and the results are shown in Figure 13. Clearly, addition of Rb2O to GeO2 glass results in a gradual change of germanium coordination from four to six, with a maximum of CN ) 4.3 obtained for glasses with compositions in the range 15-20 mol % Rb2O. An increase of germanium coordination number upon addition of alkali oxide to Li-, Na-, and K-germanate glasses was obtained by Sakka and Kamiya on the basis of EXAFS and X-ray diffraction studies, with a maximum value of CN ≈ 4.4 around the composition 20 mol % M2O.12 The analysis of the pulsed neutron total scattering data of Ueno et al.16 for Nagermanate glasses also gave a maximum value of CN ) 4.45 at 20 mol % Na2O. The fraction of Ge atoms in 6-fold coordination, N6, can be evaluated from the expression N6 ) (CN/2) - 2, where CN is the experimental value of Ge coordination (Figure 13). The so-obtained N6 values are given in Figure 14 vs Rb2O and compared with the theoretical values calculated from the formula N6 ) x/(1 - x). This expression is valid when NBO’s are not present, i.e., when addition of Rb2O to GeO2 leads only to formation of GeO6 octahedra. N6 values obtained from the analysis of the XPS22 on the same glasses data are also depicted in Figure 14. It is observed that there is a good agreement between infrared and XPS experimental results and that up to approximately x ) 0.15 the experimental results follow quite closely the theoretical curve. This shows clearly that with additions of Rb2O up to approximately 15 mol % the predominant mechanism of network modification is the change of germanium coordination from four to six. The maximum N6 values of ∼0.20 is obtained at x ≈ 0.15. The deviation of the experimental data from the theoretical curve observed for x > 0.15 marks the increasing importance of NBO formation. This result is fully consistent with the Raman spectra, which show that the characteristic band of the Q3 species at 870 cm-1 is observed for glasses with Rb2O exceeding 10 mol %.

11764 J. Phys. Chem., Vol. 100, No. 28, 1996

Figure 14. Fraction of six-coordinated germanium atoms, N6, vs Rb2O content in xRb2O‚(1 - x)GeO2 glasses. The solid line represents the theoretical case N6 ) x/(1 - x), which holds when addition of Rb2O to the germanate glass causes the formation of GeO6 octahedra only. Data points are obtained from the analysis of infrared (IR) and XPS22 spectroscopic results.

5. Conclusions Rubidium germanate glasses, xRb2O‚(1 - x)GeO2 (0 e x e 0.60), were synthesized, and their infrared reflectance and Raman spectra were measured to elucidate the effect of Rb2O addition on the glass structure. The spectra of crystalline germanate compounds, quartz and rutile forms of GeO2, Rb2Ge4O9, K4Ge9O20, K2Ge2O5, and K2GeO3, were also measured to assist band assignments of infrared and Raman glass spectra. The strong Raman band at 418 cm-1, due to the symmetric stretching vibration of Ge-O-Ge bridges in six-membered rings of GeO4 tetrahedra, was found to diminish in intensity with increasing Rb2O mole fraction x in the range 0 < x e 0.25. At the same time other features develop, with particular interest focused on the bands at 315, 590, and 630 cm-1, which gain their maximum relative intensity at 15-20 mol % Rb2O. Comparison of the glass spectra with those of alkali-germanate crystalline compounds and with the Raman spectrum of GeO2 glass measured at high pressures (∼30 GPa)47 showed that the three features at 315, 590, and 630 cm-1 should characterize the presence of interconnected GeO6 octahedra, probably in Ge4O16 arrangements. Increasing the amount of Rb2O was found to cause also the change of ring statistics in favor of small (e.g., three-membered) rings of GeO4 tetrahedra, which give strong scattering at 495 cm-1. The presence of mixed Ge(4)O-Ge(6) bridging bonds of GeO4 tetrahedra and GeO6 octahedra was also inferred from the Raman band at ∼500-520 cm-1. For x > 0.10, the Raman spectra manifest the formation of germanate tetrahedra containing one nonbridging oxygen (Q3), giving a sharp band at 865 cm-1. The formation of nonbridging oxygens, and thus the depolymerization of the germanate network, predominates for Rb2O content exceeding 20 mol %. Raman bands developing progressively at 765 cm-1 (x > 0.30), 740 cm-1 (x > 0.50), and 720 cm-1 (x ) 0.60) were attributed to Q2, Q1, and Q0 tetrahedra with two, three, and four nonbridging oxygens, respectively. The evolution of the infrared spectra was also considered, and the high-frequency absorption profile from 800 to 1050 cm-1 was deconvoluted into three bands to evaluate the effect of rubidium oxide addition on the glass structure. The middlefrequency band at 850-877 cm-1 was attributed to the asymmetric stretching vibration of Ge-O-Ge bridges, and its

Kamitsos et al. frequency, νas(Ge-O-Ge), was found to exhibit a strong dependence on the value of x. The initial rapid drop of νas(Ge-O-Ge) for x < 0.01 was explained in terms of the simplified force-constant model of Lucovsky.40 Thus, a change in ring statistics in favor of the three-membered rings causes the decrease of the intertetrahedral Ge-O-Ge bond angle (θ) and consequently the drop of νas(Ge-O-Ge). However, the frequency of the symmetric stretching vibration of bridges, νs(Ge-O-Ge), shows the opposite dependence on θ, as it was predicted by the model and shown by the Raman spectra. The further decrease of νas(Ge-O-Ge) with x for 0.01 < x 0.20 could not be attributed to variations of the intertetrahedral angle θ alone. Therefore, we have considered the formation of GeO6 octahedra and their effect on νas(Ge-O-Ge), as suggested originally by Murthy and Kirby10 and shown in this work from the study of the Raman spectra. An average coordination number of Ge atoms, CN, was then calculated for 0.01 < x < 0.33 by employing the νas(Ge-O-Ge) frequency and the empirical relation of Dachille and Roy.55 It was found that CN(Ge) attains a maximum value of 4.3 for glass compositions in the range 15-20 mol % Rb2O. The evaluation of CN(Ge) allowed the calculation of the fraction of Ge atoms in 6-fold coordination, N6, which was found to follow closely the theoretical value N6 ) x/(1 - x) up to ∼15 mol %. The deviation of the experimental value from the theoretical curve for x > 0.15 was attributed to the increasing formation of nonbridging oxygen atoms. Therefore, the results of this combined infrared and Raman study, in agreement with those of the recent XPS investigation,22 show that the coordination change GeO4 f GeO6 and the formation of GeO4 rings smaller than those encountered in quartz and glassy GeO2 are the most important modification effects in Rb-germanate glasses with 0 < x < 0.20. The breaking up of Ge-O-Ge bridges and thus the formation of nonbridging oxygens dominate in glasses with higher Rb2O content and lead to the progressive depolymerization of the network structure. Acknowledgment. This work was supported by NATO Collaborative Research Grants Program (CRG 931213), NHRF, and USDoE. References and Notes (1) Nassau, K.; Chadwick, D. L.; Miller, A. E. J. Non-Cryst. Solids 1987, 93, 114. (2) Lines, M. E. J. Non-Cryst. Solids 1988, 103, 279. (3) Ivanon, A. O.; Evstropiev, K. S. Dokl. Akad. Nauk. SSSR, 1962, 145, 797. (4) Ivanov, A. O. SoV. Phys. Solid State 1964, 5, 1933. (5) Murthy, M. K.; Ip, J. Nature 1964, 201, 285. (6) Murthy, M. K.; Aguayo, J. J. Am. Ceram. Soc. 1964, 47, 444. (7) Mundy, J. N.; Jin, G. Solid State Ionics 1986, 21, 305. (8) Mundy, J. N.; Jin, G. Solid State Ionics 1987, 23, 263. (9) Griscom, D. L. In Borate Glasses: Structure and Applications; Pye, L. D., Frechette V. D., Kreidl, N. K., Eds.; Plenum: NewYork, 1978; pp 11-149. (10) Murthy, M. K.; Kirby, E. M. Phys. Chem. Glasses 1964, 5, 144. (11) Riebling, E. F. J. Mater. Sci. 1972, 7, 40. (12) Sakka, S.; Kamiya, K. J. Non-Cryst. Solids 1982, 49, 103. (13) Kamiya, K.; Yoko, T.; Sakka, S. J. Mater. Sci. 1985, 20, 906. (14) Verweij, H.; Buster, J. H. J. Non-Cryst. Solids 1979, 34, 81. (15) Furukawa, T.; White, W. B. J. Mater. Sci. 1980, 15, 1648. (16) Kamiya, K.; Yoko, T.; Miki, Y.; Itoh, Y.; Sakka, S. J. Non-Cryst. Solids 1987, 91, 279. (17) Kamiya, K.; Sakka, S. Phys. Chem. Glasses 1979, 20, 60. (18) Ueno, M.; Misawa, M.; Susuki, K. Physica B 1983, 120, 347. (19) Cox, A. D.; McMillan, P. W. J. Non-Cryst. Solids 1981, 44, 257. (20) Lapeyre, C.; Petiau, J.; Calas, C.; Gauthier, F.; Gombert, J. Bull. Mineral. 1983, 106, 77. (21) Henderson, G. S.; Fleet, M. E. J. Non-Cryst. Solids 1991, 134, 259.

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