Infrared and Raman spectroscopy study of Si1−xGexO2 solid solutions with α-quartz structures

Journal Pre-proof Infrared and Raman spectroscopy study of Si1−xGexO2 solid solutions with α-quartz structures

Dmitry G. Koshchug, Alina N. Koshlyakova, Vladimir S. Balitsky, Sergey V. Vyatkin PII:

S1386-1425(20)30146-3

DOI:

https://doi.org/10.1016/j.saa.2020.118168

Reference:

SAA 118168

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date:

24 September 2019

Revised date:

4 February 2020

Accepted date:

16 February 2020

Please cite this article as: D.G. Koshchug, A.N. Koshlyakova, V.S. Balitsky, et al., Infrared and Raman spectroscopy study of Si1−xGexO2 solid solutions with α-quartz structures, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy(2020), https://doi.org/10.1016/j.saa.2020.118168

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Journal Pre-proof Infrared and Raman spectroscopy study of Si1−xGexO2 solid solutions with α-quartz structures Dmitry G. Koshchug1, Alina N. Koshlyakova2,*, Vladimir S. Balitsky3, Sergey V. Vyatkin1 1

Faculty of Geology, M.V. Lomonosov Moscow State University, Moscow, Russia 119991

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Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences (RAS), Moscow, Russia 119991

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Institute of Experimental Mineralogy, RAS, Chernogolovka, Moscow District, Russia 142432

Abstract Single crystals of α-quartz-type Si1–xGexO2 (x<0.12), grown under hydrothermal conditions in NH4F solutions, were investigated using infrared (IR) and Raman spectroscopy. Compositional dependencies of the IR absorption spectra were

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found both for the fundamental and combination vibrations. With an increase in Ge content in the crystals, new absorption bands, corresponding to the vibrations of O-Ge-O in the GeO4 tetrahedra (670, 930, 1010, 2125 cm-1),

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appeared, and the intensities of the absorption bands of O-Si-O in the SiO4 tetrahedra (263, 695, 2137, doublet at 2326 and 2333, 2499, 2599, 2673 cm-1) decreased. The shifts in the absorption bands at 354, 511, 2499 and 2673 cm-1 to

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lower wavenumbers linearly depends on the Ge content and are characteristic of vibrations in the Si-O-Ge chains. The origin of some combinational vibrations is also clarified. A combination vibration that caused the band at 1395 cm-1 was

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formed by vibrations with wavenumbers of 263 (type E) and 1170 cm-1 (type E). The bands at 2499 and 2673 cm-1 were

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composed of the band at 354 cm-1 and two vibrations at 1083 cm-1 (type E) and by two vibrations at 1170 cm-1 (type E), respectively.

* Corresponding author.

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Keywords Quartz, Single crystal materials, Crystal defects, Infrared spectroscopy, Raman spectroscopy, Crystal growth

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E-mail address: [email protected] (A.N. Koshlyakova).

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Journal Pre-proof 1. Introduction α-quartz is one of the most common piezoelectric materials used in piezoelectric devices. It is possible to improve the piezoelectric parameters of quartz by incorporating other atoms into the α-quartz structure. In particular, the isomorphous substitution of Ge for Si in the α-quartz structure results in a decrease in the dynamic disorder at high temperatures and structural distortions, which in turn leads to improved thermal stability and better piezoelectric properties (Miclau et al. 2009; Ranieri et al. 2009; El-Kelany et al. 2014; Lignie et al. 2015; Clavier et al. 2016). Preliminary studies have indicated that Ge-containing quartz has potential as an industrially-exploitable material. The temperature of the α–β phase transition in quartz containing 5–30 wt% GeO2 is 780–940°C; that is, ~200–350°C higher than that for pure quartz. Its resonance frequency remains stable up to at least 450°C, whereas, in pure quartz, it is stable only up to 280–290°C (Dorogovin 2000; Haines et al. 2002).

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One of the most important parameters of piezoelectric resonators is a quality factor (Q-factor) that depends primarily on their design features and much less on the internal friction of the piezoelectric element. An improvement in

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the design of such resonators, however, has made it necessary to take into account the parameters of the piezoelectric material, which depend on the presence of grain boundaries, dislocations, impurity atoms, and also on the phonon

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spectrum of the crystal.

In the works of Ranieri et al. (2009) and Lignie et al. (2015), changes in the vibrations in Si1–xGexO2 crystals with

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α-quartz structure, where x ranges from 0 to 1, were studied using Raman spectroscopy. The authors noted that the substitution of Si atoms by heavier Ge atoms altered the band positions of those vibrations in which these atoms take

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part. In particular, the band at 126 cm-1, corresponding to the complex bending and twisting vibrations of (Si/Ge)O4 tetrahedra, and the bands between 440 and 465 cm-1, corresponding to O-(Si/Ge)-O bending vibrations, shifted to lower

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wavenumbers with the substitution. The bands in the high wavenumber region (above 700 cm-1), corresponding to the Si-O and Ge-O internal stretching modes of the SiO4 and GeO4 tetrahedra, were found to decrease and increase in amplitude, respectively.

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In the fundamental vibration region, the infrared (IR) absorption spectra of Ge-containing quartz are substantially similar to those of the α-quartz spectrum, but differ from the α-GeO2 spectrum in terms of number of bands and the

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positions of the band maxima. Balitsky et al. (1990, 2005) found two new absorption bands in the IR spectra of Gecontaining quartz, with maxima near 1010 and 930 cm-1. These maxima fall between the bands corresponding to the SiO-Si antisymmetric stretching vibrations in quartz (1086 cm-1) and the Ge-O-Ge antisymmetric stretching vibrations in GeO2 (874 cm-1). The appearance of these bands was linked to the vibrations of the antisymmetric bridge bond Si-O-Ge, forming as a result of the substitution of Si by Ge in the α-quartz structure (Balitsky et al. 1990, 2005). In addition, two intense bands in the quartz IR spectra, with maxima near 695 and 511 cm-1, almost disappear from the Ge-containing quartz spectra. The disappearance of these bands can also be explained by the change in the bridge bond symmetry when Ge is incorporated into the quartz structure (Balitsky et al. 1990, 2005). To date, the vibrational spectrum of Si1−xGexO2 has not been fully described and, in particular, the composition dependence of the IR absorption bands in the region of the fundamental and combination frequencies in Ge-containing quartz has not yet been studied in detail. In this work, Ge-containing quartz was studied using IR and Raman spectroscopy in order to investigate the combination frequencies and to refine the changes in the fundamental vibrations. 2. Samples and measurements 2

Journal Pre-proof Quartz is traditionally synthesized in highly alkaline solutions; however, to synthesize Ge-containing quartz, fluoride aqueous solutions have proved to be more suitable, with the Ge content being approximately twice as large with all other conditions being equal. Another advantage of using ammonium fluoride as a solvent is its ‘sterility’ to alkaline elements, which are among the most harmful impurities in quartz; the use of ammonium fluoride eliminates the possibility of incorporating these impurities from the solvent in the crystals. Single crystals of Si1–xGexO2 solid solution were grown at the Institute of Experimental Mineralogy RAS (Chernogolovka, Russia). The growth conditions are given in the work of Balitsky et al. (2005). The distribution of Ge in the grown samples was studied using a scanning electron microscope (Jeol JSM6480LV), equipped with an energy-dispersive spectrometer (INCA X-MAX-N50) and using an electron-probe microanalyzer (CAMEBAX SX 50). The standard used was Ge metal. The samples grown at low temperatures are generally homogeneous within one growth sector, and different

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growth sectors differ insignificantly. A sample grown at a low temperature (≈350°С) is presented in Fig. 1, which shows

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a seed composed of synthetic quartz without Ge, and different growth sectors in grown Ge-containing quartz.

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Fig. 1. Back-scattered electron (BSE) image indicating the distribution of Ge in sample 1227-2, grown at 350°C. The dark gray field is a quartz seed. On the right, the same sample is magnified to highlight the borderline between two growth sectors. The black dots are ink marks. The differences between growth sectors are much sharper in samples grown at higher temperatures (Fig. 2). The highest content of Ge in such a sample is in growth sector (about 7.6 at% Ge in sample 1068-1, shown in Fig. 2), with the lowest content being in <-x> (about 7.2 at% in sample 1068-1).

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Fig. 2. BSE image indicating the distribution of Ge in sample 1068-1, grown at 600°C. The highest Ge content is in sample 1094-6, which was grown in fluoride solutions at a temperature of about

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700°C. The Ge is distributed nonhomogeneously in this sample, with an ~12.3 at% average, and 27.7 at% in the thin layer near the surface; however, it has not been confirmed that these zones of local Ge enrichment have α-quartz

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structure. The Ge content in the samples grown in the alkaline solutions is generally low, at up to 1.6 at% Ge. The distribution of Ge in these samples is generally homogeneous.

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IR spectra were obtained using a Fourier transform IR spectrometer (FSM 1201) in the 400–4000 cm-1 spectral region, with a resolution of 2 cm-1. Spectral data of the fundamental vibrations (400–1200 cm-1) were collected at room temperature from powdered samples. For the 1200–2200 cm-1 range, 100-µm-thick polished plates were prepared. For

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the 2000–3000 cm-1 range, spectral data were obtained from samples prepared as 1–3.3-mm-thick polished plates. The IR-inactive vibration modes and those that fell into the 100–400 cm-1 spectral range were registered using

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Raman spectroscopy. The Raman spectra were obtained in the 100–1300 cm-1 range at room temperature, using a spectrometer (EnSpectr R532) based on an Olympus CX41 microscope. The laser had a wavelength of 532 nm and a power of 60 mW. 3. Results Due to macroscopic defects in the grown Si1–xGexO2 crystals, it was not practical to estimate their absorption coefficients or absorbency or to compare the band intensities of the different crystals. Only the positioning of the bands and their shifts, dependent on the Ge content, were measured. Only those bands whose positions or intensities (relative to the other bands in the spectrum) depend on the crystal composition are described. 3.1. Fundamental vibrations The IR transmittance spectra of the Si1–xGexO2 powders are presented in Fig. 3. The Ge content (at%) in the samples is given to the right of each spectrum in Fig. 3. In the region of the Si-O-Si bending vibrations, the absorption band centered at 511 cm-1 shifts to lower wavenumbers, by about 15 cm-1, with the increase in Ge content. When the Ge 4

Journal Pre-proof content exceeds 8.7 at%, it becomes difficult to determine the band position because it overlaps with an intense band at 459 cm-1, forming a diffuse shoulder. In the region of the Si-O-Si symmetric stretching vibrations, a new band appears in the samples at 670 cm-1, when the Ge content exceeds 1.8 at%. An increase in Ge content causes an increase in the linewidth of all the bands. This increase is mostly expressed for the bands at 779 and 797 cm-1. When the concentration of Ge reaches 12.3 at%, this doublet merges into one broader band, at 792 cm-1. Also in this region, the intensity of the band at 695 cm-1 gradually decreases, the band disappears when the Ge content reaches 6.3 at%. In the region of the Si-O-Si antisymmetric stretching vibrations, two new bands, at 930 and 1010 cm-1, appears

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when the Ge content exceeds approximately 4 and 1.8 at%, respectively.

Fig. 3. IR transmittance spectra of samples (from top to bottom): Qtz (without Ge), 1229B, 1147, 1122, 1089 and 1094, with Ge contents of 0.0, 1.8, 4.0, 6.3, 8.7 and 12.3 at%, respectively. 3.2. Overtone and combination vibrations 5

Journal Pre-proof Fig. 4 shows the unpolarized transmittance spectra in the 1200–2300 cm-1 range. There are no significant band shifts with an increase in Ge content. A reduction in the 1417, 1950 and 2025 cm-1 band intensities was noted. All the bands

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between 1417 and 1950 cm-1 (that are not mentioned here) are well known in regular quartz.

Fig. 4. IR transmittance spectra of samples (from top to bottom): Qtz (without Ge), 1229A, 1147, 1122 and 1089, with Ge contents of 0.0, 1.6, 4.0, 6.3 and 8.7 at%, respectively. Fig. 5 shows the spectra with different polarization orientations relative to the crystallographic axes of the crystals. The bands centered at 1417, 1472, 1685, 1950 and 2025 cm-1 are polarized predominantly parallel to the c-axis. The bands centered at 1492, 1512, 1997 and 2137 cm-1 are polarized predominantly perpendicularly to the c-axis. The band at 930 cm-1 is not polarized.

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Fig. 5. IR transmittance spectra of samples Qtz (without Ge, top), and 1230 (with 2.2 at% Ge, bottom), showing different polarization orientations. Fig. 6 shows the unpolarized transmittance spectra in the 2000–2800 cm-1 range, while Fig. 7 shows the spectra

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with different polarization orientations.

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Fig. 6. IR transmittance spectra in the region of the combination vibrations of samples (from top to bottom): Qtz (without Ge), 1230, 1147, 1122 and 1089, with Ge contents of 0.0, 2.2, 4.0, 6.3 and 8.7 at%, respectively.

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Fig. 7. IR transmittance spectra in the region of the combination vibrations of samples: Qtz (without Ge, top) and 1229A (with 1.6 at% Ge, bottom), showing different polarization orientations.

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With an increase in Ge content, the following changes in the spectra for the range 2000–2800 cm-1occur: The intensity of the band at 2137 cm-1, which is polarized predominantly perpendicularly to the c-axis, decreases, and a band at 2125 cm-1, polarized predominantly parallel to the c-axis, emerges; 

Three weakly-resolved bands (at around 2226, 2240 and 2256 cm-1) widen rapidly, merging into one band with a maximum at 2232 cm-1;

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A band at 2330 cm-1 disappeares when the Ge content exceeds 6 at%;

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Two closely-separated bands at 2361 and 2381 cm-1 widen, merging into a single, wide band with a maximum at 2379 cm-1;

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The intensities of three bands observed at 2499, 2599 and 2673 cm-1 decrease, almost disappearing when the Ge content rose to 8.7 at%. At the same time, the bands at 2499 and 2673 cm-1 shift by about 15 cm-1 to lower wavenumbers. The intensities of these bands are almost isotropic with the changing of the polarization orientation. 9

Journal Pre-proof The Raman spectra of the Ge-bearing quartz are shown in Fig. 8. The shift of the band at 128 cm-1, corresponding to the Si-O-Si bending vibration, previously described by Ranieri et al. (2009), was not possible to detect. In the sample with the maximum Ge content (12.3 at%), the band at 465 cm-1 shifts by only 3 cm-1, to 462 cm-1. With an increase in Ge content, the band at 263 cm-1 broadens until it becomes a shoulder of the 206 cm-1 band, which is in agreement with published data (Ranieri et al. 2009). The band at 354 cm-1, also corresponding to the bending vibrations of Si-O-Si in pure quartz, shifts by about 15 cm-1 to lower wavenumbers; this was not described by Ranieri et al. (2009) nor Lignie et al. (2015). The bands in the high wavenumber range, corresponding to the stretching vibrations of Si-O-Si, have lowintensity Raman spectra; however, it was observed that the weak band at 695 cm-1 disappears in the Ge-quartz, as already described for IR spectra (Balitsky et al., 2005). New absorption bands at 930 and 1010 cm-1, which are clearly visible in the IR spectra (Fig. 3), were not detected in the Raman spectra of the Ge-containing quartz, likely because of

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the low intensity of the stretching modes.

Fig. 8. Raman spectra of samples (from top to bottom): Qtz (without Ge), 1238, 1147, 1089 and 1094 with Ge contents of 0.0, 2.1, 4.0, 8.7, 12.3 at% Ge, respectively. 10

Journal Pre-proof 4. Discussion The absorption spectra of the quartz sample (x=0) agree relatively well with the results of previous studies (Kats 1961), in terms of both the fundamental vibrations (400–1200 cm-1 wavenumber range; Fig. 3) and the combination vibrations (1200–3000 cm-1 range; Figs 4–7). Isomorphous substitution in a quartz structure leads to changes in the phonon spectrum of the crystal. Local vibrations arise in the region of the replacement atom, the frequency of which can fall into the range of that of a pure crystal (Ziman 1974). In this case, the pure crystal absorption (scattering) bands broaden. This was noted, to different extents, in all the absorption lines (Fig. 3). It is most pronounced for the lines at 779 and 797 cm-1, which broaden and, when the concentration of Ge rise to 11.9 at%, this doublet merges into one broader band at 792 cm-1. Three bands, corresponding to the combination vibrations (central band at 2240 cm-1), and a doublet at 2361 and 2381 cm-1, also broaden, giving rise to broad bands at 2232 and 2379 cm-1, respectively.

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Where the frequency of the local vibrations does not fall into the range of the pure crystal phonon spectrum, a new absorption band arises. As the impurity content increases, the intensities of the pure crystal absorption bands

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decrease (at 263, 1417, 1950, 2025, 2137, poorly-resolved doublet 2326 and 2333, 2499, 2599, 2673 cm-1) and the intensities of the new bands increase (at 670, 930, 1010, 2125 cm-1) (Julliot et al. 1987; Balitsky et al. 2005; Schlüter et

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al. 2010).

The fundamental frequencies of quartz (Table 1) include four Raman active modes of type A1, four IR active

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modes of type A2 and eight modes of type E, which appear in both the Raman and IR spectra (Lazarev et al. 1975).

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Table 1 IR and Raman bands in quartz and their vibration types (Lazarev et al. 1975). Raman spectra (cm-1) 206 355 465 1083

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IR spectra (cm-1)

364 512 779 1083 128 263 402 459 695 797 1083 1170

128 263 402 459 698 809 1083 1160

Type of vibration A1 A1 A1 A1 A2 A2 А2 А2 E E E Е E E Е E

The IR absorption band at 364 cm-1 was not recorded here because this range was not measured by the spectrometer used. The band at 459 cm-1 (type E) in the Raman spectrum overlaps with the strong band at 465 cm-1 (type A1). A new absorption band at 930 cm-1 is similar to the pure quartz absorption band at 1083 cm-1. It was recorded in both a parallel and perpendicular polarization direction relative to the c-axis (Fig. 5). Thus, this band represents the 11

Journal Pre-proof superposition of two absorption bands, caused by the vibrations of E and A2 types. The absorption bands at 670 and 1010 cm-1 did not register in the polarized spectra of the oriented crystal sections because of the strong absorption of pure quartz. The origin of some of the combination vibrations can be clarified by taking into account the symmetry of the fundamental vibrations in quartz. The intensity of the very weak band at 1395 cm-1 decreases rapidly with an increase in Ge content, although the position of this band does not change. This band is not polarized. There are two fundamental vibrations at 263 and 695 cm-1, the intensity of which decreased and they could take part in the formation of 1395 cm-1 band. The overtone of the fundamental vibration at 695 cm-1 can cause absorption only at a lower (1390 cm-1) wavenumber than 1395 cm-1. It is possible that this combination band may be composed of the bands at 263 (type E) and 1170 cm-1 (type E). As a result, the type of combination vibration should be A1+A2+E (Figgis 1967); that is, the band should not be polarized, which is in agreement with the measured spectra (Fig. 5). The total wavenumber of the

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combination band is 1433 cm-1, which exceeds that of experimentally-measured value. This frequency decrease is due to

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the anharmonicity of the vibrations (Kolesov 2018).

The intensities of the weak bands at 1417, 1950 and 2025 cm-1 decreased with an increase in Ge content in the

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quartz structure. It is not possible to explain the formation of these combination vibrations, involving bands at 263 and 695 cm-1, since they are both of type E. The absorption bands corresponding to the resulting combination vibrations are

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either not polarized or are polarized perpendicular to the c-axis. The spectra illustrated in Fig. 5 show that they are polarized parallel to the c-axis.

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Combination vibrations with absorption bands polarized parallel to the c-axis can be composed of nondegenerate antisymmetric and symmetric vibrations. Balitsky et al. (2005) and Ranieri et al. (2009) demonstrated that the incorporation of Ge into quartz leads to a distortion of its structure. It is possible that, in this case, a local symmetry

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reduction took place, resulting in the removal of the degeneracy of some doubly-degenerate vibrations of type E, producing absorption bands polarized along the c-axis. Clarification of this phenomenon will require measurement of the

to prepare such samples.

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absorption spectra in the region of the fundamental vibrations on very thin quartz plates. In this work, it was not possible

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Note that the intensity of the 2137 cm-1 band decreases (Fig. 6), and the band is polarized perpendicular to the caxis (Fig. 7), which makes it type E. In the region of the fundamental vibrations, there are two type E bands, at 263 and 695 cm-1, the intensity of which also decreases with an increase in Ge content. The combination vibrations of the type E bands may comprise type E (263 or 695 cm-1) and type A1 vibrations that cannot be registered in the IR spectrum. Also, disregarding the anharmonicity, the wavenumbers of the combination vibrations should be equal to 1874 or 1442 cm-1. According to group theory, a combination vibration of type A1 can be an overtone of an A1 vibration or a combination of A1 vibrations. There are four A1 bands in the quartz spectrum (Table 1), but the total frequencies of the overtones or combination bands differ significantly from the required 1874 or 1442 cm-1. It is possible that a vibration that does not correspond to the center of the Brillouin zone took part in the formation of the 2137 cm-1 band. The bands centered at 2499, 2599 and 2673 cm-1 are not polarized (Fig. 7), and their intensities decrease with an increase in Ge content, the 2499 and 2673 cm-1 bands shifting to lower wavenumbers. The bands at 263 and 695 cm-1, whose intensities decrease and frequencies do not change, may be involved in the formation of the band at 2599 cm-1. A combination of vibrations with frequencies of 263 cm-1 and the first overtone of the 1170 cm-1 vibration of type E would lead to the formation of a band at 2603 cm-1, which, with a small correction for anharmonicity, could correspond to the 12

Journal Pre-proof band at 2599 cm-1. A combination of type E vibrations at 695, 797 and 1170 would be possible, but the total frequency of 2662 cm-1 significantly exceeds the experimentally-measured frequency. The combination bands at 2499 and 2673 cm-1 should be composed of vibrations at 354 (type A1) and 511 (type A2), the frequencies of which depend on Ge content. A combination of the 354 vibration (type A1) with two 1083 vibrations (type E) or with two 1170 vibrations (type E) would lead to the formation of unpolarized vibrations at 2521 and 2695 cm-1, which, with a correction for anharmonicity, could correspond to the bands at 2499 and 2673 cm-1. A combination of the band at 511 cm-1 (type A2) and the first overtone at 1083 cm-1 (type A1+E) would lead to an unpolarized vibration with a frequency of 2679 cm-1; however, the correction for the anharmonicity is very small. Those absorption bands caused by the vibrations of atom chains that include an impurity atom will have shifted (354, 465, 511, 2499 and 2673 cm-1). All the spectra have a strong, nonlinear background of unknown shape. The positioning of the bands in the spectrum, determined by modeling the spectrum using the Gauss, Lorentz and PsdVoigt

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functions, depends strongly on the background subtraction method (Chukanov 2014; Chukanov & Chervonnyi 2016). In

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this regard, the positions of the band maxima were defined by the first derivative of the spectrum. The half-widths and intensities of the bands were not determined. In Fig. 9, the dependencies of these band positions on the Ge content are

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shown. It was not possible to plot such a dependency for the band at 465 cm-1 because its shift is comparable to that of the spectral resolution of the spectrometer. Ranieri et al. (2009) obtained an empirical linear equation to describe the

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shift of the band at 465 cm-1 in the Raman spectrum, depending on Ge content. This equation was confirmed by Lignie et al. (2015); however, it seems that there is a typo in Eq. (1), where ν = 442.210 – 0.218(xSi) – the xSi variable ought to

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be added to, rather than subtracted from, 442.210.

The data shown in Fig. 9 can be described by the following linear equation:

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ν = ν0 – kxGe

, where ν0 is the wavenumber position of the band in pure quartz, ν is the wavenumber of that band in

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Si1−xGexO2 crystals, and xGe is the Ge content (at%) of the crystals. The ν0s and their coefficients (k) for the studied samples are given in Table 2.

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Table 2. Positions of shifting bands in pure quartz and their coefficients (k). ν0, cm-1

k

354.1 513.6 2498.9 2673.4

1.34 1.33 1.77 1.88

One band in the Raman spectrum (at 354 cm-1) and one band in the IR spectrum (at 511 cm-1) shifted almost simultaneously (k ≈ 1.3), whereas the 2499 and 2673 cm-1 bands in the IR spectrum shifted much faster (k ≈ 1.8–1.9), with increasing Ge concentration. All these bands shifted much faster than was projected in the works of Ranieri et al. (2009) or Lignie et al. (2015) for the 465 cm-1 band, for which k = 0.21. In our work, the band at 465 cm-1 shifts by only 3 cm-1 in the sample with maximum Ge content, which is close to the spectrum resolution of the spectrometer used. For this reason, obtaining the dependency on Ge content for this band’s position was not possible. It was only possible to estimate the value of k with a high degree of uncertainty

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Journal Pre-proof using two values – the bands’ positions in pure quartz and in a sample with maximum Ge content. The resulting value is

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~0.3, which is much closer to that from Ranieri et al. (2009) and Lignie et al. (2015).

Fig. 9. Wavenumber positions of bands vs. Ge content. The datapoints were fitted by regression lines for (a) the band at 354 cm-1 in the Raman spectra, (b) the band at 511 cm-1 in the IR spectra, (c) the combination band at 2499 cm-1 and (d) the combination band at 2673 cm-1. 5. Conclusions The shifts in the absorption bands in the IR and Raman spectra, along with the broadening of the IR absorption bands, with an increase in Ge content indicates that Ge substitutes for Si, in an isomorphous way, in the quartz structure. There is a continuous series of solid solution in the SiO2-GeO2 system, at least until the Ge content reaches 12.3 at%. At low Ge concentrations, only a broadening of the absorption bands is notable, the intensity of the new bands being too low. With an increase in Ge content, new absorption bands appear (670, 930, 1010, 2125 cm-1), corresponding to the O-Ge-O vibrations in the GeO2 tetrahedra. Simultaneously, the intensity of the O-Si-O absorption bands (263,

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Journal Pre-proof 695, 2137, weakly-resolved doublet 2326 and 2333, 2499, 2599, 2673 cm-1) decrease. The shift in the absorption bands at 354, 511, 2499 and 2673 cm-1 to lower wavenumbers is characteristic of the vibrations in Si-O-Ge chains. The absorption band at 930 cm-1 results from the superposition of the two bands caused by vibrations of types E and A2. The combination vibration at 1395 cm-1 forms from vibrations with wavenumbers of 263 (type E) and 1170 cm-1 (type E). The combination band at 2599 cm-1 most likely results from the vibrations at 263 cm-1 and the first overtone of vibration at 1170 cm-1, using a small correction for anharmonicity. The bands at 2499 and 2673 cm-1 are composed of the band at 354 cm-1 and two vibrations at 1083 cm-1 (type E) and two vibrations at 1170 cm-1 (type E), respectively. It is possible that the vibration at 2673 cm-1 consisted of the vibration at 511 cm-1 and the first overtone of the 1083 cm-1 vibration, but, in this case, the correction for anharmonicity is very small.

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Acknowledgments The authors would like to thank M. F. Vigasina for her help in collecting the Raman spectral data and in interpreting

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those and the IR spectra; N. Boldyrev for collecting some of the IR spectral data and for providing a polarizer; A. I. Efimova for her help with the polarization measurements; and N. N. Koshlyakova and D. A. Khanin for performing the

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electron probe analyses.

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Journal Pre-proof References Balitsky VS, Balitsky DV, Nekrasov AN, Balitskaya LV (2005) Growth and characterization of SixGe1−xO2 solid solution single crystals with quartz structure. J. Cryst. Growth 275:807-811 Balitsky VS, Sorokina SL, Chichagov AV, Bondarenko GV (1990) Synthesis and basic physicochemical characteristics of germanium-containing quartz. DAN USSR (in Russian) 314:1480-1483 Chukanov NV (2013) Infrared spectra of mineral species: extended library. Springer Science & Business Media Chukanov NV, Chervonnyi AD (2016) Infrared spectroscopy of minerals and related compounds. Springer Clavier D, Prakasam M, Largeteau A, Boy JJ, Hehlen B, Cambon M, Hermet P, Haines J, Cambon O (2016) Piezoelectric and non-linear optical properties of α-quartz type Si1−xGexO2 single crystals. Cryst. Eng. Comm. 18:2500-2508

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Dorogovin BA (2000) Synthesis of minerals. VNIISIMS Alexandrov (in Russian)

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Journal Pre-proof Declaration of competing interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Journal Pre-proof Author Statement Dmitry Koshchug: Conceptualization, Methodology, Formal analysis, Investigation, Writing - Review & Editing, Supervision Alina Koshlyakova: Investigation, Writing - Original Draft, Visualization Vladimir Balitsky: Resources, Conceptualization

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Sergey Vyatkin: Conceptualization, Writing - Review & Editing

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Journal Pre-proof Graphical abstract

Highlights

- Si(1-x)GexO2 crystals with α-quartz structures were grown, x reaches 12.3 at% - New absorption bands in the crystals corresponding to the O-Ge-O vibrations appear - The intensity of the O-Si-O absorption bands in Si(1-x)GexO2 crystals decrease

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- Bands corresponding to Si-O-Ge chains shift to lower wavenumbers with Ge increase

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