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In situ Raman spectroscopy of K2O-GeO2 melts
Journal of Non-Crystalline Solids 531 (2020) 119850
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In situ Raman spectroscopy of K2O-GeO2 melts Olga N. Koroleva a b
a,b,⁎
, Armenak A. Osipov
T
a
Institute of Mineralogy SU FRC MG UB RAS, Miass, 456317, Russia South-Ural State University, Miass, 456318, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Germanate melts Raman spectroscopy Structure
Raman spectra of potassium germanate melts K2O-GeO2, containing 10, 20, 30 and 40 mol.% K2O were measured. Based on a comparison of melts and glasses spectra we were able to determine the basic structural units formed in the system and to reveal the structural features of potassium-germanate melts during vitrification. It was shown that in melts, in contrast to glasses, an increase of the K2O content leads to a monotonic decrease of the fraction of germanium atoms in the octahedral coordination and the formation of [GeO4] tetrahedra with one and two non-bridging oxygen atoms. During cooling of germanate melts and their glass transition, an increase of the coordination number of germanium atoms and an increase of the degree of polymerisation of the network occur due to the formation of Ge–O–Ge bridging bonds.
1. Introduction
co-authors [35] with regard to the Na2O-GeO2 system. This work is devoted to the study of the melts structure of the system xK2O·(100x)GeO2, where x = 10, 20, 30, 40 (10 K, 20 K, 30 K and 40 K) by Raman spectroscopy.
Following an in situ high-temperature study of silicate systems by means of Raman spectroscopy significant progress has been made in understanding their structure [1–7]. The structure and physicochemical characteristics of the melt determine its rock-forming properties in geological processes and form the basis for understanding the mechanisms of crystallisation and glass transition in solving technological problems. The subject of our research is germanate melts, the study of which is of particular interest from the point of view of geochemistry, since they are considered as high-pressure analogues of silicate magmas [8–14]. The main feature of germanate systems is the change in the coordination number of germanium atoms. This in turn can explain the causes of "germanate anomaly". Its occurrence in germanate glasses containing up to 20 mol% of alkaline metal oxide is clearly related to the specific features of their structure [15–21]. The coordination of germanium atoms in alkali germanate glasses is believed to change from 4 to 6 up to those alkaline cation contents at which extremes on the curves of the physical properties dependence on the composition can be observed. Despite the fact that alkali germanate glasses have been studied by many scientists [17,22–28], information on the structure of germanate melts is difficult to find. The literature mainly presents the results of studies into the electrical properties of melts [29], phase equilibria [30], thermochemistry [31,32], Raman under pressure [33] and modelling [34]. Raman spectroscopy of alkali germanate melts at atmospheric pressure has been published only by Ivanova and
⁎
2. Methods Samples of 10К, 20К, 30К and 40К composition were synthesised from GeO2 and K2CO3 powders previously calcined in an oven at 100 °C for 4 h and weighed in the required proportions. The appropriate quantities of reagents were mixed together for each composition and melted in Pt crucibles at 1200–1380 0С until the complete homogenisation of the melt. For the registration of a Raman spectrum of a melt, a crucible with the glass sample was placed in a platinum compact oven equipped with a temperature controller providing for the stability of the set temperature with an accuracy of ± 1 °C. The non-polarised Raman spectra were registered by a powerful LTI-701 Nd laser (λ = 532 nm, 〈P〉 = 1 W) with a pulse frequency of 8.7 kHz and pulse duration based on acoustic-optical shutter of 2 µs. This was used in couple with a synchronised photon counter which opened only during a laser pulse. An uncooled FEU-79 photomultiplier was used to detect the Raman signal [7]. In order to compare the spectra obtained at various temperatures, we adjusted them for the thermal population of vibration levels [36,37]. To attain a detailed description of the melt structure, the Raman spectra were represented as a superposition of Gaussians in accordance with the technique developed for binary alkali germanate glasses [11,38]. The background was removed using a second-order
Corresponding author. E-mail address: [email protected] (O.N. Koroleva).
https://doi.org/10.1016/j.jnoncrysol.2019.119850 Received 27 September 2019; Received in revised form 1 December 2019; Accepted 5 December 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.
Journal of Non-Crystalline Solids 531 (2020) 119850
O.N. Koroleva and A.A. Osipov
Fig. 1. Raman spectra of 10К, 20К, 30К and 40К melts at 1100 °С.
polynomial fit in the 400–650 and 700–1000 cm−1 spectral region with its further extrapolation to the lower wave numbers and subtraction from the spectrum. 3. Results The normalised on peak-intensity Raman spectra of melts 10 K, 20 K, 30 K and 40 K, as presented in Fig. 1, can be divided into two regions: low frequency (300–700 cm−1) and high frequency (700–1000 cm−1). The maxima and intensities of the high frequency bands clearly change depending on the amount of potassium oxide in the melts. Namely, the intensity of the weak band about 860 cm−1 increases, and its maximum shifts towards low frequencies at 840 cm−1. In addition, there is a pronounced shoulder with a maximum of about 770 cm−1. In the low-frequency region, there is a wide band located near 520 cm−1, the intensity of which decreases with the increasing content of the cation modifier in the melt. A comparison of the normalised on peak-intensity Raman spectra of melts and glasses of the corresponding compositions (Fig. 2) allows us to study the changes of structure which occur during the vitrification of the potassium-germanate system. During the cooling of 10 K melt, the band at 860 cm−1 disappears in the high-frequency region of the spectrum. Instead of this band, two new lines about 820 and 920 cm−1 are observed in the glass spectrum. Despite the general similarity of the glass and melt spectra, the 20 K, 30 K and 40 K melt exhibits more broad bands, shifted to the lower frequency. Moreover, in addition to the main band of about 860 cm−1, the appearance of a new band of about 750 cm−1 is noticeable in the 20 K glass Raman spectrum. A similar situation is observed during cooling of the melt 30 K. In the low frequency region of the Raman spectra 10–40 K during the melt → glass transition, the new shoulders at 602 and 654 cm−1 become noticeable. These are most pronounced in the spectra of samples 20 K and 30 K.
Fig. 2. Comparison of Raman spectra of glassy GeO2, glasses (room temperature) and melts (1100 °С) of the К2O-GeO2 system.
transformation of tetrahedral [GeO4] into octahedral [GeO6] structures and that additions of alkali oxide beyond the maxima in the density (~10–20 mol.%) results in the six-coordinated germanium ions (Ge (IV)) reverting back to four-coordinated germanium ions (Ge(VI)). However, an alternative theory of the origin of the “germanate anomaly” is presented by G. S. Henderson et al. [40]. According to the authors’ theory, the germanium ions cannot be transformed into a sixcoordinated state. In their opinion, the increase in the density of germanate glasses is due to the formation of 3-member rings and the elongation of the tetrahedral anion-cation bond. Extremes on the curves of physical properties occur at the saturation point of the small ring. A further increase of the content of the cation modifier results in the destruction of the germanium-oxygen network and the formation of tetrahedra with one (Q3) and two (Q2) non-bridging oxygen atoms (NBO). This issue was discussed in great detail earlier [11] and we concluded that both points of view need to be taken into account. The bands caused by the TO/LO split of the anti-symmetric stretching vibrations of Ge-O-Ge bonds in a fully-polymerised glass network GeO2 are noticeable in the high-frequency range (700–1000 cm−1) in spectra of glasses and melts with low alkaline cation content. The contribution to the scattering intensity in this range increases with the concentration of the modifier oxide. This is due to vibrations of the non-bridge oxygen atoms (GeO4 – tetrahedrons with n bridging atoms of oxygen) [11,38]. Examples of the deconvolution of the low- and high-frequency regions of the melt spectra are given in Figs. 3 and 4, correspondingly. The parameters of the elementary lines determined by the curve-fitting results are presented in Table 2, for the low-frequency and high-frequency regions of the spectra, respectively. The lines positions of corresponding glasses included in the table demonstrate the shift of vibration frequencies with temperature. Fig. 5 shows the relative intensities of the Raman spectrum bands of melts depending on the composition. The Figure shows that a gradual increase in the potassium oxide content leads to a sharp decrease in the intensity of the L7 band. This indicates a decrease in the proportion of highly coordinated germanium atoms in the structure of melts. A
4. Discussion In order to interpret the results of spectroscopy, the low- and highfrequency regions of the Raman spectra were represented as a superposition of Gaussians. The previously established approach to curvefitting of Raman spectra of germanate glasses was used (see Table 1) [11,38]. According to a number of researchers, Raman scattering of germanate glasses and melts in the low-frequency region is due to vibrations of bridged connections Ge(4)-O-Ge(4) and Ge(4)-O-Ge(6) [17, 39]. The have suggested that increasing the amount of alkali causes the 2
Journal of Non-Crystalline Solids 531 (2020) 119850
O.N. Koroleva and A.A. Osipov
Table 1 Band positions and assignment in Raman spectra of glasses of the K2O-GeO2 system [11]. Band
Band position, cm−1
Band interpretation
L1 L2 L3 L4 L5 L6 L7 L8 L9 H1 H2 H3
435–445 470–475 490–525 540–565 570 590–595 600–605 625–630 640–655 800–820 900–930 870–880
Symmetric stretching vibrations of bonds Ge(IV)–O–Ge(IV) in four-cycled rings Vibrations of five-coordinated atoms Ge(V) Vibrations of bonds Ge(IV)–O–Ge(IV) in three-cycled rings Symmetric vibrations of bonds Ge(IV)−O− Ge(IV) Vibrations of Ge(IV)−O−Ge(VI) Symmetric stretching vibrations of two-cycled rings С2, formed from tetrahedral [GeO4] Vibrations species formed from tetrahedra [GeO4] and octahedra [GeO6]
H4 H5
775–785 840–850
Vibrations of six-coordinated atom Ge(VI) LO (longitudinal optical) split of the anti-symmetric stretching vibrations of Ge(4)–O–Ge(4) bonds in a fully polymerised glass TO (transverse optical) split of the anti-symmetric stretching vibrations of Ge(4)–O–Ge(4) bonds in a fully polymerised glass Stretching vibrations of NBO in Q3 -species (tetrahedron with three bridging atoms of oxygen, connected to the same Q3, that is the part of the sheet) Stretching vibrations of NBO in Q2-species (Tetrahedron with two bridging atoms of oxygen, connected to the same Q2, that is the part of the chain) Stretching vibrations of NBO in Q32-species (tetrahedron with 3 bridging atoms of oxygen, connected to Q2)
splitting of antisymmetric stretching vibrations of Ge(4)-O-Ge(4) bonds of the polymerised network. The addition of potassium oxide leads to the breakage of germanium-oxygen bonds and the formation of nonbridge oxygen atoms, as evidenced by the increase in the intensity of the H3 band. In this case, the intensity of the H1 and H2 bands remain nearly unchanged, while the intensity of the line L7 is reduced by almost half. This may mean that there is an increase in the number of tetrahedra with one non-bridging oxygen atom Q3 due to the destruction of octahedral GeO6 groupings with the addition of K2O. A reduction of the intensities of H1 and H2 bands in the Raman spectra and an increase of the intensities of the bands corresponding to the vibrations of Q2 tetrahedra (the tetrahedra GeO4 with two non-bridging oxygen atoms) and Q32 (Q3 tetrahedrons, united by bridging bonds with three Q2 units) are observed with a subsequent growth in the content of oxide modifier in the system. At the same time, there is a decrease in the intensity of the L4 band associated with the destruction of the Ge(4)-OGe(4) bridges in the low-frequency region. When studying potassium germanate glasses by means of Raman spectroscopy, we considered that the proportion of high-coordinate germanium atoms in these glasses does change non-monotonically with the changes in glass composition and reaches a maximum value at K2O content of 20 mol.% [11]. An increase of the concentration of 6-coordinated germanium atoms can be described by the reaction [41]:
GeØ4 + M2 O → GeØ62 − ·2M+
(1)
where Ø bridging oxygen atoms, connecting two germanium atoms; M potassium atoms, here. Reducing the number of 6-coordinated germanium atoms at a potassium oxide content above 20 mol.% is accompanied by the formation of germanate tetrahedra with different numbers of non-bridging oxygen atoms in the glass structure. This modification of the germanate network is generally described by the reaction [41]:
Fig. 3. Results of curve-fitting of the low-frequency region of Raman spectra of melts 10 K, 20 K, 30 K and 40 K at 1100 °C. (Designation of elementary lines corresponds to the designation adopted in [11]).
further increase of potassium cation content leads to a simultaneous decrease in the intensity of the L1 and L3 bands. The L1 band has already disappeared at a potassium oxide content of 20 mol.%, while the intensity of the L3 band gradually decreases in the spectra of 30 K and 40 K. This indicates the disappearance of four-membered rings in the melt and a decrease in the content of three-membered rings. Changes in the high-frequency region of the spectra are associated with the disappearance of the H1 and H2 bands, as well as with the appearance of the H3-H5 bands and an increase in their intensities. The first two bands are associated with the so-called TO/LO-splitting, which is characteristic of the germanium-oxygen net of pure GeO2. These two bands are present in Raman spectra of glasses and melts containing small amounts of cationic modifier (10 K and 20 K) [38]. The highfrequency region of the 10 K melt spectrum is characterised by an intense H3 band caused by vibrations of non-bridge oxygen atoms in Q3 tetrahedra. The shoulders on the low and high frequencies are described by H1 and H2 bands which are characteristic of the TO/LO
GeØ4 +
4−i M2 O → GeØi O4(4−−i i) −·(4 − i) M+ 2
(2)
where i can be varied from 0 to 4. By comparing the curve-fitting results of glasses [11] and melts Raman spectra, we were able to determine the changes of the relative intensities of bands during vitrification (Fig. 6) and to discuss the structural transformations occurring during the glass transition. The H3 band, typical for the high-frequency region of all the spectra considered here and related to vibrations of the Q3 structural units, is absent in the curve-fitting of the 10 K glass Raman spectrum [11]. At the same time, the intensity of the Н1 and Н2 bands characteristic of TO / LO splitting is significantly higher in the spectrum of 10 K glass compared to the melt spectrum of this composition Fig. 6b). Furthermore, it can be noted that in addition to the L7 band (Fig. 6a), the L8 and L9 bands, related to vibrations of a 6-coordinated germanium atom, 3
Journal of Non-Crystalline Solids 531 (2020) 119850
O.N. Koroleva and A.A. Osipov
Fig. 4. Results of curve-fitting of the high-frequency region of Raman spectra of melts 10 K, 20 K, 30 K and 40 K at 1100 °C. (Designation of elementary lines corresponds to the designation adopted in [11]).
K2O addition. The Н1 and Н2 bands are still necessary for curve-fitting of the Raman spectrum 20 K melt, while the high-frequency envelope of the 30 K and 40 K spectra can be reproduced if 3 lines (H3-H5), corresponding to the vibrations of the germanium-oxygen tetrahedra Q3, Q2, and Q32 are included (Fig. 4c, d). It can be noted that during the glass transition of the 20 K melt, the high-frequency shoulder shape of the Raman spectrum changes on the low-frequency side (Fig. 2). After comparing the results of the deconvolution of the 20 K melt (Fig. 4b) and glass [11] Raman spectra, it can be assumed that such changes were caused by the rising intensity of the H1 band. The intensity of the H3 line, indicating the existence of germanate
appear in the curve-fitting of the low-frequency envelope of the glass spectra [11]. The changes observed in the Raman spectra indicate an increase of the degree of polymerisation of the germanium-oxygen network due to a shift of the equilibrium of the disproportionation reaction ((3) to the left with glass transition.
GeØ62 − ·2M+ + GeØ4 ↔ 2(GeØ3 O−·M+)
(3)
Starting from sample 20 K, two additional lines appear in the highfrequency region of the Raman spectra — H4 and H5. These are caused by the vibrations of structural units Q2 and Q32, respectively (Fig. 4b). This indicates an increasing degree of depolymerisation of melts with
Table 2 Elementary lines parameters in Raman spectra of melts of the K2O-GeO2 system (* - lines positions in spectra of corresponding glasses from Koroleva et al. [11]). Band
Concentration of K2O, mol.% 10
20
30
40
−1
region 400–700 сm L1 L3 L4 L7 region 600–1000 сm−1
446/438* 492/521* 540/566* 594/603*
Н1 H2 Н3 Н4 H5
776/818* (23) 941/922* (7) 867/-* (70) – –
(17) (27) (37) (19)
444/-* (16) – 494/520* (31) 486/492* (39) 543/553* (42) 539/539* (52) 595/603* (11) 593/602* (9) -1 Vibration frequency, cm (Intensity)
– 496/524* (45) 534/-* (55) –
– – 862/877* (74) 780/778* (13) 831/850* (14)
– – 848/874* (54) 756/781* (21) 798/846* (25)
757/762* (9) 929/-* (3) 863/879* (66) 795/-* (13) 820/-* (9)
4
Journal of Non-Crystalline Solids 531 (2020) 119850
O.N. Koroleva and A.A. Osipov
sample with the highest K2O content, it follows that changes of the local structure can be described by the disproportionation reaction (4).
Q 4 + Q 2 ↔ 2Q3
(4)
The equilibrium of the reaction shifts to the left as the melt cools and glass forms. It is likely that the transformation of the structure of melts with a lower K2O content is associated with both a change in the equilibrium of the reaction (3) and a shift in the equilibrium of the reaction (4). A wide band in the low-frequency region of the spectra of potassium germanate melts narrows with an increase of the content of the modifier cation, and therefore can be curve-fitted by a smaller number of lines. The low-frequency region of the Raman spectrum of 20 K glass is characterised by the intense shoulders corresponding to vibrations of the 6-coordinated germanium atom (L7 and L9) [11]. An increase in the coordination number of germanium atoms with a decreasing melt temperature is clearly described by the Ge (IV) → Ge (VI) transition. A simultaneous decrease in the content of Q3 units during the glass transition of the 20 K melt allows us to conclude that the degree of polymerisation of the disordered network increases (Fig. 6c, d). A similar situation is observed with the glass transition of a 30 K melt (Fig. 6e, f). There are intense bands corresponding to vibrations of the 6-coordinated germanium atom (L7 and L9) in the low-frequency region of the Raman spectrum of this glass. However, the L9 band is not observed in the 30 K melt spectrum, and the L7 band intensity gradually decreases with an increasing content of the cation modifier in the melts, and completely disappears in the 40 K spectrum. The changes observed indicate the disappearance of six-coordinated germanium atoms in the melt of this composition. The number of four-membered rings in melts gradually decreases with an increase in K2O. These structures disappear as the composition of the melt approaches 30 K. Therefore, the lowfrequency region of the Raman spectrum of the 40 K melt is curve-fitted by just two lines L3 and L4 (Fig. 3d).
Fig. 5. Band intensities of curve-fitting of Raman spectra of melts as a function of K2O mol% at medium- (a) and high-frequency (b) ranges.
5. Conclusions Based on this study we were able to establish that both four-fold and six-fold germanium atoms are present in the K2O-GeO2 melts containing up to 40 mol% of potassium oxide. The Ge (VI) → Ge (IV) transition is observed with an increase in the content of modifier cation. As a result, the concentration of GeO6 units in the structure of melts decreases monotonically. The Q4 and Q3 germanate tetrahedra and GeO6 units coexist in the structure of low-alkaline melts of the potassium-germanate system. Whereas the structure of K2O-GeO2 melts with more than 20 mol% K2O is predominantly represented by Q3 and Q2 tetrahedra. Our studies showed that the structure of K2O-GeO2 melts differs from the structure of the corresponding glasses. The degree of polymerisation increases during the melt → glass transition and new Ge–O–Ge bonds are formed with the simultaneous disappearance of non-bridging oxygen atoms. In addition, it was found that the Raman bands characterised by vibrations of a six-coordinated germanium atom increase with decreasing temperature from 1100 to 25 °C. An increase in the fraction of highly coordinated germanium atoms with decreasing temperature can be described by the disproportionation reaction (3), while changes in the concentration of germanate tetrahedra of various types are associated with a shift in the equilibrium of the disproportionation reaction (4). Thus, cooling of the potassium-germanate melts results in the formation of a more polymerised germaniumoxygen network of glasses in which the anionic groups of [GeO4] tetrahedra are linked by [GeO6] octahedra.
Fig. 6. Band intensities of curve-fitting of Raman spectra of melts as a function of temperature at low- (a, c, e, g) and high-frequency (b, d, f, h) ranges.
tetrahedra with one non-bridging oxygen atom in the structure, increases in all spectra with temperature. However, the intensity increment of this line depends on the composition of the sample and decreases with increasing content of potassium oxide. The appearance of H4 and H5 bands in the spectra of glasses and melts with relatively high contents of potassium oxide (К2О ≥ 20 mol%), indicating the formation of germanate tetrahedra with two non-bridging oxygen atoms in the structure, and the nature of the dependences of their intensity on composition and temperature show (Fig. 6) that transformations of the structure of melts during their glass transition can no longer be described only by reaction (3). For example, based on the nature of the temperature dependence of the intensity of the H3-H5 bands for a
Author contribution Olga N. Koroleva: Interpretation of Raman spectra, discussion. Armenak A. Osipov: Registration of Raman spectra, methodology. 5
Journal of Non-Crystalline Solids 531 (2020) 119850
O.N. Koroleva and A.A. Osipov
Declaration of Competing Interest
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None. Acknowledgement This work was supported by state assignment project № АААА-А19119042590024-1 and by the Russian Foundation for Basic Research (project nos. 18-05-00079). We thank our colleague T.N. Ivanova for valuable support in data acquisition and sample preparation. We also thank the anonymous reviewers for providing useful comments, which improved the quality of the manuscript. References [1] J.F. Stebbins, Effects of temperature and composition on silicate glass structure and dynamics: Si-29 NMR results, J. Non-Cryst. Solids 106 (1988) 359–369. [2] D.R. Neuville, B.O. Mysen, Role of aluminium in the silicate network: in situ, hightemperature study of glasses and melts on the join SiO2-NaAlO2, Geochim. Cosmochim. Acta 60 (1996) 1727–1737. [3] B.O. Mysen, A. Lucier, G.D. Cody, The structural behavior of Al3+ in peralkaline melts and glasses in the system Na2O-Al2O3-SiO2, Am. Mineral. 88 (2003) 1668–1678. [4] W.J. Malfait, V.P. Zakaznova-Herzog, W.E. Halter, Quantitative Raman spectroscopy: high-temperature speciation of potassium silicate melts, J. Non-Cryst. Solids 353 (2007) 4029. [5] V.N. Bykov, O.N. Koroleva, A.A. Osipov, Structure of silicate melts: Raman spectroscopic data and thermodynamic simulation results, Geochem. Int. 47 (2009) 1067–1074. [6] O.N. Koroleva, The structure of lithium silicate melts revealed by high-temperature Raman spectroscopy, Spectrosc. Lett. 50 (2017) 257–264. [7] A.A. Osipov, High-temperature Raman spectroscopy, Pure Appl. Chem. 91 (2019) 1749–1756. [8] L.W. Finger, R.M. Hazen, Systematic of high-pressure silicate structures, Rev. Mineral.Geochem. 41 (2000) 123–155. [9] G.S. Henderson, G. Calas, J.F. Stebbins, The structure of silicate glasses and melts, Elements 2 (2006) 269–273. [10] O.N. Koroleva, T.N. Ivanova, Raman spectroscopy of the structures of Li2O-SiO2 and Li2O-GeO2 melts, Russ. Metall. (Metally) 2014 (2014) 140–146. [11] O.N. Koroleva, M.V. Shtenberg, T.N. Ivanova, The structure of potassium germanate glasses as revealed by Raman and IR spectroscopy, J. Non-Cryst. Solids 510 (2019) 143–150. [12] D.Y. Pushcharovsky, Structural principles of silicate, phosphates, sulphates and related structures, Modern Perspectives in Inorganic Crystal Chemistry. NATO ASI Series (Series C: Mathematical and Physical Sciences) 382 (1992) 203–227. [13] A.E. Ringwood, M. Seabrook, High-pressure phase transformations in germanate pyroxenes and related compounds, J. Geophys. Res. 68 (1963) 4601–4609. [14] M.D. Welch, M.A. Cooper, F.C. Hawthorne, The crystal structure of brunogeierite, Fe2GeO4 spinel, Mineral Mag. 65 (2001) 441–444. [15] H. Arima, T. Kawamata, K. Sugiyama, Local structure around Ge in lithium germanate glasses analyzed by AXS and EXAFS techniques, J. Mineral. Petrol. Sci. 110 (2015) 60–64. [16] M.M. Ashton-Patton, Properties of Mixed Alkali Germanate Glasses, Alfred University, New York, 2008, p. 240. [17] D. Di Martino, L.F. Santos, A.C. Marques, R.M. Almeida, Vibrational spectra and structure of alkali germanate glasses, J. Non-Cryst. Solids 293 (2001) 394.
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
In situ Raman spectroscopy of K2O-GeO2 melts Olga N. Koroleva a b
a,b,⁎
, Armenak A. Osipov
T
a
Institute of Mineralogy SU FRC MG UB RAS, Miass, 456317, Russia South-Ural State University, Miass, 456318, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Germanate melts Raman spectroscopy Structure
Raman spectra of potassium germanate melts K2O-GeO2, containing 10, 20, 30 and 40 mol.% K2O were measured. Based on a comparison of melts and glasses spectra we were able to determine the basic structural units formed in the system and to reveal the structural features of potassium-germanate melts during vitrification. It was shown that in melts, in contrast to glasses, an increase of the K2O content leads to a monotonic decrease of the fraction of germanium atoms in the octahedral coordination and the formation of [GeO4] tetrahedra with one and two non-bridging oxygen atoms. During cooling of germanate melts and their glass transition, an increase of the coordination number of germanium atoms and an increase of the degree of polymerisation of the network occur due to the formation of Ge–O–Ge bridging bonds.
1. Introduction
co-authors [35] with regard to the Na2O-GeO2 system. This work is devoted to the study of the melts structure of the system xK2O·(100x)GeO2, where x = 10, 20, 30, 40 (10 K, 20 K, 30 K and 40 K) by Raman spectroscopy.
Following an in situ high-temperature study of silicate systems by means of Raman spectroscopy significant progress has been made in understanding their structure [1–7]. The structure and physicochemical characteristics of the melt determine its rock-forming properties in geological processes and form the basis for understanding the mechanisms of crystallisation and glass transition in solving technological problems. The subject of our research is germanate melts, the study of which is of particular interest from the point of view of geochemistry, since they are considered as high-pressure analogues of silicate magmas [8–14]. The main feature of germanate systems is the change in the coordination number of germanium atoms. This in turn can explain the causes of "germanate anomaly". Its occurrence in germanate glasses containing up to 20 mol% of alkaline metal oxide is clearly related to the specific features of their structure [15–21]. The coordination of germanium atoms in alkali germanate glasses is believed to change from 4 to 6 up to those alkaline cation contents at which extremes on the curves of the physical properties dependence on the composition can be observed. Despite the fact that alkali germanate glasses have been studied by many scientists [17,22–28], information on the structure of germanate melts is difficult to find. The literature mainly presents the results of studies into the electrical properties of melts [29], phase equilibria [30], thermochemistry [31,32], Raman under pressure [33] and modelling [34]. Raman spectroscopy of alkali germanate melts at atmospheric pressure has been published only by Ivanova and
⁎
2. Methods Samples of 10К, 20К, 30К and 40К composition were synthesised from GeO2 and K2CO3 powders previously calcined in an oven at 100 °C for 4 h and weighed in the required proportions. The appropriate quantities of reagents were mixed together for each composition and melted in Pt crucibles at 1200–1380 0С until the complete homogenisation of the melt. For the registration of a Raman spectrum of a melt, a crucible with the glass sample was placed in a platinum compact oven equipped with a temperature controller providing for the stability of the set temperature with an accuracy of ± 1 °C. The non-polarised Raman spectra were registered by a powerful LTI-701 Nd laser (λ = 532 nm, 〈P〉 = 1 W) with a pulse frequency of 8.7 kHz and pulse duration based on acoustic-optical shutter of 2 µs. This was used in couple with a synchronised photon counter which opened only during a laser pulse. An uncooled FEU-79 photomultiplier was used to detect the Raman signal [7]. In order to compare the spectra obtained at various temperatures, we adjusted them for the thermal population of vibration levels [36,37]. To attain a detailed description of the melt structure, the Raman spectra were represented as a superposition of Gaussians in accordance with the technique developed for binary alkali germanate glasses [11,38]. The background was removed using a second-order
Corresponding author. E-mail address: [email protected] (O.N. Koroleva).
https://doi.org/10.1016/j.jnoncrysol.2019.119850 Received 27 September 2019; Received in revised form 1 December 2019; Accepted 5 December 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.
Journal of Non-Crystalline Solids 531 (2020) 119850
O.N. Koroleva and A.A. Osipov
Fig. 1. Raman spectra of 10К, 20К, 30К and 40К melts at 1100 °С.
polynomial fit in the 400–650 and 700–1000 cm−1 spectral region with its further extrapolation to the lower wave numbers and subtraction from the spectrum. 3. Results The normalised on peak-intensity Raman spectra of melts 10 K, 20 K, 30 K and 40 K, as presented in Fig. 1, can be divided into two regions: low frequency (300–700 cm−1) and high frequency (700–1000 cm−1). The maxima and intensities of the high frequency bands clearly change depending on the amount of potassium oxide in the melts. Namely, the intensity of the weak band about 860 cm−1 increases, and its maximum shifts towards low frequencies at 840 cm−1. In addition, there is a pronounced shoulder with a maximum of about 770 cm−1. In the low-frequency region, there is a wide band located near 520 cm−1, the intensity of which decreases with the increasing content of the cation modifier in the melt. A comparison of the normalised on peak-intensity Raman spectra of melts and glasses of the corresponding compositions (Fig. 2) allows us to study the changes of structure which occur during the vitrification of the potassium-germanate system. During the cooling of 10 K melt, the band at 860 cm−1 disappears in the high-frequency region of the spectrum. Instead of this band, two new lines about 820 and 920 cm−1 are observed in the glass spectrum. Despite the general similarity of the glass and melt spectra, the 20 K, 30 K and 40 K melt exhibits more broad bands, shifted to the lower frequency. Moreover, in addition to the main band of about 860 cm−1, the appearance of a new band of about 750 cm−1 is noticeable in the 20 K glass Raman spectrum. A similar situation is observed during cooling of the melt 30 K. In the low frequency region of the Raman spectra 10–40 K during the melt → glass transition, the new shoulders at 602 and 654 cm−1 become noticeable. These are most pronounced in the spectra of samples 20 K and 30 K.
Fig. 2. Comparison of Raman spectra of glassy GeO2, glasses (room temperature) and melts (1100 °С) of the К2O-GeO2 system.
transformation of tetrahedral [GeO4] into octahedral [GeO6] structures and that additions of alkali oxide beyond the maxima in the density (~10–20 mol.%) results in the six-coordinated germanium ions (Ge (IV)) reverting back to four-coordinated germanium ions (Ge(VI)). However, an alternative theory of the origin of the “germanate anomaly” is presented by G. S. Henderson et al. [40]. According to the authors’ theory, the germanium ions cannot be transformed into a sixcoordinated state. In their opinion, the increase in the density of germanate glasses is due to the formation of 3-member rings and the elongation of the tetrahedral anion-cation bond. Extremes on the curves of physical properties occur at the saturation point of the small ring. A further increase of the content of the cation modifier results in the destruction of the germanium-oxygen network and the formation of tetrahedra with one (Q3) and two (Q2) non-bridging oxygen atoms (NBO). This issue was discussed in great detail earlier [11] and we concluded that both points of view need to be taken into account. The bands caused by the TO/LO split of the anti-symmetric stretching vibrations of Ge-O-Ge bonds in a fully-polymerised glass network GeO2 are noticeable in the high-frequency range (700–1000 cm−1) in spectra of glasses and melts with low alkaline cation content. The contribution to the scattering intensity in this range increases with the concentration of the modifier oxide. This is due to vibrations of the non-bridge oxygen atoms (GeO4 – tetrahedrons with n bridging atoms of oxygen) [11,38]. Examples of the deconvolution of the low- and high-frequency regions of the melt spectra are given in Figs. 3 and 4, correspondingly. The parameters of the elementary lines determined by the curve-fitting results are presented in Table 2, for the low-frequency and high-frequency regions of the spectra, respectively. The lines positions of corresponding glasses included in the table demonstrate the shift of vibration frequencies with temperature. Fig. 5 shows the relative intensities of the Raman spectrum bands of melts depending on the composition. The Figure shows that a gradual increase in the potassium oxide content leads to a sharp decrease in the intensity of the L7 band. This indicates a decrease in the proportion of highly coordinated germanium atoms in the structure of melts. A
4. Discussion In order to interpret the results of spectroscopy, the low- and highfrequency regions of the Raman spectra were represented as a superposition of Gaussians. The previously established approach to curvefitting of Raman spectra of germanate glasses was used (see Table 1) [11,38]. According to a number of researchers, Raman scattering of germanate glasses and melts in the low-frequency region is due to vibrations of bridged connections Ge(4)-O-Ge(4) and Ge(4)-O-Ge(6) [17, 39]. The have suggested that increasing the amount of alkali causes the 2
Journal of Non-Crystalline Solids 531 (2020) 119850
O.N. Koroleva and A.A. Osipov
Table 1 Band positions and assignment in Raman spectra of glasses of the K2O-GeO2 system [11]. Band
Band position, cm−1
Band interpretation
L1 L2 L3 L4 L5 L6 L7 L8 L9 H1 H2 H3
435–445 470–475 490–525 540–565 570 590–595 600–605 625–630 640–655 800–820 900–930 870–880
Symmetric stretching vibrations of bonds Ge(IV)–O–Ge(IV) in four-cycled rings Vibrations of five-coordinated atoms Ge(V) Vibrations of bonds Ge(IV)–O–Ge(IV) in three-cycled rings Symmetric vibrations of bonds Ge(IV)−O− Ge(IV) Vibrations of Ge(IV)−O−Ge(VI) Symmetric stretching vibrations of two-cycled rings С2, formed from tetrahedral [GeO4] Vibrations species formed from tetrahedra [GeO4] and octahedra [GeO6]
H4 H5
775–785 840–850
Vibrations of six-coordinated atom Ge(VI) LO (longitudinal optical) split of the anti-symmetric stretching vibrations of Ge(4)–O–Ge(4) bonds in a fully polymerised glass TO (transverse optical) split of the anti-symmetric stretching vibrations of Ge(4)–O–Ge(4) bonds in a fully polymerised glass Stretching vibrations of NBO in Q3 -species (tetrahedron with three bridging atoms of oxygen, connected to the same Q3, that is the part of the sheet) Stretching vibrations of NBO in Q2-species (Tetrahedron with two bridging atoms of oxygen, connected to the same Q2, that is the part of the chain) Stretching vibrations of NBO in Q32-species (tetrahedron with 3 bridging atoms of oxygen, connected to Q2)
splitting of antisymmetric stretching vibrations of Ge(4)-O-Ge(4) bonds of the polymerised network. The addition of potassium oxide leads to the breakage of germanium-oxygen bonds and the formation of nonbridge oxygen atoms, as evidenced by the increase in the intensity of the H3 band. In this case, the intensity of the H1 and H2 bands remain nearly unchanged, while the intensity of the line L7 is reduced by almost half. This may mean that there is an increase in the number of tetrahedra with one non-bridging oxygen atom Q3 due to the destruction of octahedral GeO6 groupings with the addition of K2O. A reduction of the intensities of H1 and H2 bands in the Raman spectra and an increase of the intensities of the bands corresponding to the vibrations of Q2 tetrahedra (the tetrahedra GeO4 with two non-bridging oxygen atoms) and Q32 (Q3 tetrahedrons, united by bridging bonds with three Q2 units) are observed with a subsequent growth in the content of oxide modifier in the system. At the same time, there is a decrease in the intensity of the L4 band associated with the destruction of the Ge(4)-OGe(4) bridges in the low-frequency region. When studying potassium germanate glasses by means of Raman spectroscopy, we considered that the proportion of high-coordinate germanium atoms in these glasses does change non-monotonically with the changes in glass composition and reaches a maximum value at K2O content of 20 mol.% [11]. An increase of the concentration of 6-coordinated germanium atoms can be described by the reaction [41]:
GeØ4 + M2 O → GeØ62 − ·2M+
(1)
where Ø bridging oxygen atoms, connecting two germanium atoms; M potassium atoms, here. Reducing the number of 6-coordinated germanium atoms at a potassium oxide content above 20 mol.% is accompanied by the formation of germanate tetrahedra with different numbers of non-bridging oxygen atoms in the glass structure. This modification of the germanate network is generally described by the reaction [41]:
Fig. 3. Results of curve-fitting of the low-frequency region of Raman spectra of melts 10 K, 20 K, 30 K and 40 K at 1100 °C. (Designation of elementary lines corresponds to the designation adopted in [11]).
further increase of potassium cation content leads to a simultaneous decrease in the intensity of the L1 and L3 bands. The L1 band has already disappeared at a potassium oxide content of 20 mol.%, while the intensity of the L3 band gradually decreases in the spectra of 30 K and 40 K. This indicates the disappearance of four-membered rings in the melt and a decrease in the content of three-membered rings. Changes in the high-frequency region of the spectra are associated with the disappearance of the H1 and H2 bands, as well as with the appearance of the H3-H5 bands and an increase in their intensities. The first two bands are associated with the so-called TO/LO-splitting, which is characteristic of the germanium-oxygen net of pure GeO2. These two bands are present in Raman spectra of glasses and melts containing small amounts of cationic modifier (10 K and 20 K) [38]. The highfrequency region of the 10 K melt spectrum is characterised by an intense H3 band caused by vibrations of non-bridge oxygen atoms in Q3 tetrahedra. The shoulders on the low and high frequencies are described by H1 and H2 bands which are characteristic of the TO/LO
GeØ4 +
4−i M2 O → GeØi O4(4−−i i) −·(4 − i) M+ 2
(2)
where i can be varied from 0 to 4. By comparing the curve-fitting results of glasses [11] and melts Raman spectra, we were able to determine the changes of the relative intensities of bands during vitrification (Fig. 6) and to discuss the structural transformations occurring during the glass transition. The H3 band, typical for the high-frequency region of all the spectra considered here and related to vibrations of the Q3 structural units, is absent in the curve-fitting of the 10 K glass Raman spectrum [11]. At the same time, the intensity of the Н1 and Н2 bands characteristic of TO / LO splitting is significantly higher in the spectrum of 10 K glass compared to the melt spectrum of this composition Fig. 6b). Furthermore, it can be noted that in addition to the L7 band (Fig. 6a), the L8 and L9 bands, related to vibrations of a 6-coordinated germanium atom, 3
Journal of Non-Crystalline Solids 531 (2020) 119850
O.N. Koroleva and A.A. Osipov
Fig. 4. Results of curve-fitting of the high-frequency region of Raman spectra of melts 10 K, 20 K, 30 K and 40 K at 1100 °C. (Designation of elementary lines corresponds to the designation adopted in [11]).
K2O addition. The Н1 and Н2 bands are still necessary for curve-fitting of the Raman spectrum 20 K melt, while the high-frequency envelope of the 30 K and 40 K spectra can be reproduced if 3 lines (H3-H5), corresponding to the vibrations of the germanium-oxygen tetrahedra Q3, Q2, and Q32 are included (Fig. 4c, d). It can be noted that during the glass transition of the 20 K melt, the high-frequency shoulder shape of the Raman spectrum changes on the low-frequency side (Fig. 2). After comparing the results of the deconvolution of the 20 K melt (Fig. 4b) and glass [11] Raman spectra, it can be assumed that such changes were caused by the rising intensity of the H1 band. The intensity of the H3 line, indicating the existence of germanate
appear in the curve-fitting of the low-frequency envelope of the glass spectra [11]. The changes observed in the Raman spectra indicate an increase of the degree of polymerisation of the germanium-oxygen network due to a shift of the equilibrium of the disproportionation reaction ((3) to the left with glass transition.
GeØ62 − ·2M+ + GeØ4 ↔ 2(GeØ3 O−·M+)
(3)
Starting from sample 20 K, two additional lines appear in the highfrequency region of the Raman spectra — H4 and H5. These are caused by the vibrations of structural units Q2 and Q32, respectively (Fig. 4b). This indicates an increasing degree of depolymerisation of melts with
Table 2 Elementary lines parameters in Raman spectra of melts of the K2O-GeO2 system (* - lines positions in spectra of corresponding glasses from Koroleva et al. [11]). Band
Concentration of K2O, mol.% 10
20
30
40
−1
region 400–700 сm L1 L3 L4 L7 region 600–1000 сm−1
446/438* 492/521* 540/566* 594/603*
Н1 H2 Н3 Н4 H5
776/818* (23) 941/922* (7) 867/-* (70) – –
(17) (27) (37) (19)
444/-* (16) – 494/520* (31) 486/492* (39) 543/553* (42) 539/539* (52) 595/603* (11) 593/602* (9) -1 Vibration frequency, cm (Intensity)
– 496/524* (45) 534/-* (55) –
– – 862/877* (74) 780/778* (13) 831/850* (14)
– – 848/874* (54) 756/781* (21) 798/846* (25)
757/762* (9) 929/-* (3) 863/879* (66) 795/-* (13) 820/-* (9)
4
Journal of Non-Crystalline Solids 531 (2020) 119850
O.N. Koroleva and A.A. Osipov
sample with the highest K2O content, it follows that changes of the local structure can be described by the disproportionation reaction (4).
Q 4 + Q 2 ↔ 2Q3
(4)
The equilibrium of the reaction shifts to the left as the melt cools and glass forms. It is likely that the transformation of the structure of melts with a lower K2O content is associated with both a change in the equilibrium of the reaction (3) and a shift in the equilibrium of the reaction (4). A wide band in the low-frequency region of the spectra of potassium germanate melts narrows with an increase of the content of the modifier cation, and therefore can be curve-fitted by a smaller number of lines. The low-frequency region of the Raman spectrum of 20 K glass is characterised by the intense shoulders corresponding to vibrations of the 6-coordinated germanium atom (L7 and L9) [11]. An increase in the coordination number of germanium atoms with a decreasing melt temperature is clearly described by the Ge (IV) → Ge (VI) transition. A simultaneous decrease in the content of Q3 units during the glass transition of the 20 K melt allows us to conclude that the degree of polymerisation of the disordered network increases (Fig. 6c, d). A similar situation is observed with the glass transition of a 30 K melt (Fig. 6e, f). There are intense bands corresponding to vibrations of the 6-coordinated germanium atom (L7 and L9) in the low-frequency region of the Raman spectrum of this glass. However, the L9 band is not observed in the 30 K melt spectrum, and the L7 band intensity gradually decreases with an increasing content of the cation modifier in the melts, and completely disappears in the 40 K spectrum. The changes observed indicate the disappearance of six-coordinated germanium atoms in the melt of this composition. The number of four-membered rings in melts gradually decreases with an increase in K2O. These structures disappear as the composition of the melt approaches 30 K. Therefore, the lowfrequency region of the Raman spectrum of the 40 K melt is curve-fitted by just two lines L3 and L4 (Fig. 3d).
Fig. 5. Band intensities of curve-fitting of Raman spectra of melts as a function of K2O mol% at medium- (a) and high-frequency (b) ranges.
5. Conclusions Based on this study we were able to establish that both four-fold and six-fold germanium atoms are present in the K2O-GeO2 melts containing up to 40 mol% of potassium oxide. The Ge (VI) → Ge (IV) transition is observed with an increase in the content of modifier cation. As a result, the concentration of GeO6 units in the structure of melts decreases monotonically. The Q4 and Q3 germanate tetrahedra and GeO6 units coexist in the structure of low-alkaline melts of the potassium-germanate system. Whereas the structure of K2O-GeO2 melts with more than 20 mol% K2O is predominantly represented by Q3 and Q2 tetrahedra. Our studies showed that the structure of K2O-GeO2 melts differs from the structure of the corresponding glasses. The degree of polymerisation increases during the melt → glass transition and new Ge–O–Ge bonds are formed with the simultaneous disappearance of non-bridging oxygen atoms. In addition, it was found that the Raman bands characterised by vibrations of a six-coordinated germanium atom increase with decreasing temperature from 1100 to 25 °C. An increase in the fraction of highly coordinated germanium atoms with decreasing temperature can be described by the disproportionation reaction (3), while changes in the concentration of germanate tetrahedra of various types are associated with a shift in the equilibrium of the disproportionation reaction (4). Thus, cooling of the potassium-germanate melts results in the formation of a more polymerised germaniumoxygen network of glasses in which the anionic groups of [GeO4] tetrahedra are linked by [GeO6] octahedra.
Fig. 6. Band intensities of curve-fitting of Raman spectra of melts as a function of temperature at low- (a, c, e, g) and high-frequency (b, d, f, h) ranges.
tetrahedra with one non-bridging oxygen atom in the structure, increases in all spectra with temperature. However, the intensity increment of this line depends on the composition of the sample and decreases with increasing content of potassium oxide. The appearance of H4 and H5 bands in the spectra of glasses and melts with relatively high contents of potassium oxide (К2О ≥ 20 mol%), indicating the formation of germanate tetrahedra with two non-bridging oxygen atoms in the structure, and the nature of the dependences of their intensity on composition and temperature show (Fig. 6) that transformations of the structure of melts during their glass transition can no longer be described only by reaction (3). For example, based on the nature of the temperature dependence of the intensity of the H3-H5 bands for a
Author contribution Olga N. Koroleva: Interpretation of Raman spectra, discussion. Armenak A. Osipov: Registration of Raman spectra, methodology. 5
Journal of Non-Crystalline Solids 531 (2020) 119850
O.N. Koroleva and A.A. Osipov
Declaration of Competing Interest
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