Bismuth-Germanate Glasses: Synthesis, Structure, Luminescence, and Crystallization

: Bismuth-germanate glasses, which are well known as a promising active medium for broadband near-infrared spectral range ﬁber lasers and as an initial matrix for nonlinear optical glass ceramics, have been synthesized in a 5–50 mol% Bi 2 O 3 wide concentration range. Their structural and physical characteristics were studied by Raman and FT-IR spectroscopy, differential scanning calorimetry, X-ray diffraction, optical, and luminescence methods. It has been found that the main structural units of glasses are [BiO 6 ] and [GeO 4 ]. The growth in bismuth oxide content resulted in an increase in density and refractive index. The spectral and luminescent properties of glasses strongly depended on the amount of bismuth active centers. The maximum intensity of IR luminescence has been achieved for the 5Bi 2 O 3 -95GeO 2 sample. The heat treatment of glasses resulted in the formation of several crystalline phases, the structure and amount of which depended on the initial glass composition. The main phases were non-linear Bi 2 GeO 5 and scintillating Bi 4 Ge 3 O 12 . Comparing with the previous papers dealing with bismuth and germanium oxide-based glasses, we enlarge the range of Bi 2 O 3 concentration up to 50 mol% and decrease the synthesis temperature from 1300 to 1100 ◦ C.


Introduction
The Bi 2 O 3 -GeO 2 system has a wide glass transition region up to 85.7 mol% Bi 2 O 3 [1,2]. The basis of the structural network of bismuth-germanate glasses is [GeO 4 ] 4− -tetrahedra [3]. Bismuth oxide, as a modifier, creates additional bonds in glass, strengthening it. It is generally accepted that Bi 3+ ions are predominantly in octahedral coordination in glass, but it can be varied from octahedral [BiO 6 ] 6− to pyramidal [BiO 3 ] 3− at Bi-concentration growth. Along with Bi 3+ , bismuth can exist in other charge states in glasses [4].
Bismuth-containing glasses have a high refractive index and high density; they are transparent in the visible and IR spectral ranges [5,6]. Increased researchers' attention to these glasses arose after the discovery of a unique luminescence in the 1100-1500 nm range, the source of which is bismuth active centers (BACs) [7]. The structure of these centers has been subjected to changes over the past 20 years [8][9][10]. Up to date, the prevailing opinion is that BAC has a complex active structure, which is a combination of bismuth ions in low oxidation states and oxygen vacancies [11]. Understanding the nature of these centers would make it possible to optimize laser active media for the near-IR range.
In addition to active optics, bismuth-germanate glasses are used to produce glassceramic materials since the glass formation region of the Bi 2 O 3 -GeO 2 system includes The sample with the lowest content of bismuth oxide (5 mol%) had inclusions of bubbles due to the high melt viscosity at the synthesis temperature, and some of its properties were not studied. As a result, the 5-95 sample density was lower than the density of pure GeO2. Glasses 50-50 were inclined to surface crystallization during melt casting, which contradicted the data of [2], in which 85.7Bi2O3-14.3GeO2 glasses were presented and similar synthesis conditions (temperature 1100-1200 °C, quenching on a metal substrate at room temperature) were reported for their production.
The results of the elemental analysis of the glasses showed that all samples contained an aluminum impurity (Table 1) due to the synthesis in corundum crucibles. A similar result was observed in [14]. With an increase in the bismuth oxide content, the amount of aluminum in the glass composition increased, which was explained by the chemical aggressiveness of the bismuth oxide melt towards the crucible material. Additionally, bismuth volatilized insignificantly during the synthesis, which was also described in the literature [15]. At the same time, the Bi/Ge ratio in our initial mixture and in the synthesized glass remained nearly unchanged.  The sample with the lowest content of bismuth oxide (5 mol%) had inclusions of bubbles due to the high melt viscosity at the synthesis temperature, and some of its properties were not studied. As a result, the 5-95 sample density was lower than the density of pure GeO 2 . Glasses 50-50 were inclined to surface crystallization during melt casting, which contradicted the data of [2], in which 85.7Bi 2 O 3 -14.3GeO 2 glasses were presented and similar synthesis conditions (temperature 1100-1200 • C, quenching on a metal substrate at room temperature) were reported for their production.
The results of the elemental analysis of the glasses showed that all samples contained an aluminum impurity (Table 1) due to the synthesis in corundum crucibles. A similar result was observed in [14]. With an increase in the bismuth oxide content, the amount of aluminum in the glass composition increased, which was explained by the chemical aggressiveness of the bismuth oxide melt towards the crucible material. Additionally, bismuth volatilized insignificantly during the synthesis, which was also described in the literature [15]. At the same time, the Bi/Ge ratio in our initial mixture and in the synthesized glass remained nearly unchanged.

Glass Structure Characterization
Structural units in the glass network were characterized using Raman and IR spectroscopy (Figures 2 and 3). In the low-frequency region (<700 cm −1 ) for the Raman spectra ( Figure 2), the bands in the region of 500 cm −1 characterized [GeO 4 ]-tetrahedra vibrations. Their intensity decreased with a reduction in germanium oxide concentration [2,16]. Additionally, in this region, there was a wide band at 600 cm −1 , related to vibrations of the Bi-O bond of [BiO 6 ]-octahedra [17]. It is interesting that for the 50-50 glass, the band at 395 cm −1 , associated with the bending of the O-Ge-O bridge bond [16], had the highest intensity in comparison with other glasses. It can be explained by the tendency of this glass to surface crystallize GeO 2 phases.

Glass Structure Characterization
Structural units in the glass network were characterized using Raman and IR spectroscopy (Figures 2 and 3). In the low-frequency region (<700 cm −1 ) for the Raman spectra ( Figure 2), the bands in the region of 500 cm −1 characterized [GeO4]-tetrahedra vibrations. Their intensity decreased with a reduction in germanium oxide concentration [2,16]. Additionally, in this region, there was a wide band at 600 cm −1 , related to vibrations of the Bi-O bond of [BiO6]-octahedra [17]. It is interesting that for the 50-50 glass, the band at 395 cm −1 , associated with the bending of the O-Ge-O bridge bond [16], had the highest intensity in comparison with other glasses. It can be explained by the tendency of this glass to surface crystallize GeO2 phases.  The bands in the high frequency region of the Raman spectra (>700 cm −1 ) were assigned to [GeO4]-tetrahedra vibrations with different numbers of non-bridging oxygen atoms, so-called Qn-units, where n is the number of bridging oxygen atoms [18,19]. The growth of bismuth oxide content ( Figure 2) resulted in the increasing intensity of the

Glass Structure Characterization
Structural units in the glass network were characterized using Raman and IR spectroscopy (Figures 2 and 3). In the low-frequency region (<700 cm −1 ) for the Raman spectra ( Figure 2), the bands in the region of 500 cm −1 characterized [GeO4]-tetrahedra vibrations. Their intensity decreased with a reduction in germanium oxide concentration [2,16]. Additionally, in this region, there was a wide band at 600 cm −1 , related to vibrations of the Bi-O bond of [BiO6]-octahedra [17]. It is interesting that for the 50-50 glass, the band at 395 cm −1 , associated with the bending of the O-Ge-O bridge bond [16], had the highest intensity in comparison with other glasses. It can be explained by the tendency of this glass to surface crystallize GeO2 phases.  The bands in the high frequency region of the Raman spectra (>700 cm −1 ) were assigned to [GeO4]-tetrahedra vibrations with different numbers of non-bridging oxygen atoms, so-called Qn-units, where n is the number of bridging oxygen atoms [18,19]. The growth of bismuth oxide content ( Figure 2) resulted in the increasing intensity of the The bands in the high frequency region of the Raman spectra (>700 cm −1 ) were assigned to [GeO 4 ]-tetrahedra vibrations with different numbers of non-bridging oxygen atoms, so-called Q n -units, where n is the number of bridging oxygen atoms [18,19]. The growth of bismuth oxide content ( Figure 2) resulted in the increasing intensity of the bands in the high-frequency region. This indicated an increase in the defectiveness of the glass structure.
The FT-IR spectra of the glasses ( Figure 3) contained the main bands at 580, 670, 850, and 1105 cm −1 . The band at 580 cm −1 referred to asymmetric stretching of the Ge-O-Ge bridge bond vibrations [19] and was observed for all glasses; its intensity decreased with increasing Bi 2 O 3 content. The band at 670 cm −1 was assigned to vibrations of Bi-O bonds in [BiO 6 ] structural units [20]. The band at 880 cm −1 was assigned to Ge-O-Ge stretching [21].
The band at 1105 cm −1 was assigned to vibrations of the Bi-O-Bi or Bi-O-Ge bond [20]. It should be noted that the bands at 880 and 1105 cm −1 shifted to the low-frequency region with an increase in the bismuth oxide content, which indicated a weakening of the Ge-O bonds due to the incorporation of bismuth ions into the glass network. The FT-IR transmission spectra (Figure 3) confirmed the assumptions about the glass structure and were in agreement with the Raman spectra presented above.

DSC Characterization and Physical Properties
The glass transition temperatures (T g ) and maximum crystallization temperatures (T x ) of all samples ( Table 2) were determined from DSC curves (Figures S1-S10). Table 2. Glass characteristic temperatures *. The presence of several crystallization temperatures was associated with the formation of various crystalline phases. The difference in the number of crystallization temperatures (2 or 3) for different compositions can be associated both with a change in the type of crystallizing phases and with a rather high heating rate of the samples during the DSC processing. The formation of the metastable Bi 2 GeO 5 phase could be observed in the 600-650 • C temperature range, according to [22]. The crystallization temperature in the region of 650-700 • C may correspond to the transition of the metastable Bi 2 GeO 5 phase to the stable Bi 4 Ge 3 O 12 with the eulytite structure [23]. The shift of the crystallization temperatures of the same phase towards high values for glasses with a Bi 2 O 3 content <30 mol% is explained by the lower tendency of these glasses to crystallize ( Figure 4). The density and refractive index of glasses (Table 3) expectedly increased with the growth in bismuth oxide content. The obtained results correlated with the data [5,6]. A slight decrease in the density and refractive index can be explained by the entry of aluminum oxide from the crucibles into the glasses.  The density and refractive index of glasses (Table 3) expectedly increased with the growth in bismuth oxide content. The obtained results correlated with the data [5,6]. A slight decrease in the density and refractive index can be explained by the entry of aluminum oxide from the crucibles into the glasses.

Spectral-Luminescent Properties
The absorption spectra of glasses ( Figure 5) exhibited a characteristic shoulder at 500 nm associated with BACs [7][8][9]. The absorption coefficient in this region increased with the growth of the bismuth oxide content.
Similarly, with an increase in the bismuth oxide content, the short-wavelength absorption edge shifted from 340 nm (Sample ID 5-95) to 425 nm (Sample ID 50-50). This shift was due to the fact that the optical band gap of bismuth (III) oxide is smaller than that of germanium oxide (5.63 eV) and ranges from 2.5 to 3.2 eV for various Bi 2 O 3 polymorphs [14]. To determine the width of the optical energy gap (Eg) of glasses, the Tauc method was used ( Figure 6, Table 4).   To determine the width of the optical energy gap (E g ) of glasses, the Tauc method was used ( Figure 6, Table 4). To determine the width of the optical energy gap (Eg) of glasses, the Tauc method was used ( Figure 6, Table 4).    Under excitation of photoluminescence (PL) at wavelengths of 405, 425, 525, 650, and 805 nm for 5-95 samples (Figure 7), it was found that 450 nm was the optimal excitation for BACs (see Figure S19). We observed that a green laser (525 nm) action led to strong heating of the glasses, which significantly decreased the PL intensity.
Ceramics 2023, 6, FOR PEER REVIEW 8 for BACs (see Figure S19). We observed that a green laser (525 nm) action led to strong heating of the glasses, which significantly decreased the PL intensity. The PL spectra of glasses at λ ex = 450 nm ( Figure 8) represented a wide band in the near IR region. As can be seen, the luminescence region corresponded to the data of [7][8][9][10], which additionally confirms the presence of BACs in glasses. For tested glasses, when the bismuth oxide content increased, the PL intensity became lower due to concentration quenching. Sample 5-95 demonstrated the highest PL intensity (Figure 9). The observed broadband luminescence was attributed to low-valence forms of bismuth (Bi n<2+ ) in the BACs [11,24,25]. The PL spectra of glasses at λ ex = 450 nm ( Figure 8) represented a wide band in the near IR region. As can be seen, the luminescence region corresponded to the data of [7][8][9][10], which additionally confirms the presence of BACs in glasses. For tested glasses, when the bismuth oxide content increased, the PL intensity became lower due to concentration quenching.
Ceramics 2023, 6, FOR PEER REVIEW 8 for BACs (see Figure S19). We observed that a green laser (525 nm) action led to strong heating of the glasses, which significantly decreased the PL intensity. The PL spectra of glasses at λ ex = 450 nm ( Figure 8) represented a wide band in the near IR region. As can be seen, the luminescence region corresponded to the data of [7][8][9][10], which additionally confirms the presence of BACs in glasses. For tested glasses, when the bismuth oxide content increased, the PL intensity became lower due to concentration quenching. Sample 5-95 demonstrated the highest PL intensity (Figure 9). The observed broadband luminescence was attributed to low-valence forms of bismuth (Bi n<2+ ) in the BACs [11,24,25]. Sample 5-95 demonstrated the highest PL intensity (Figure 9). The observed broadband luminescence was attributed to low-valence forms of bismuth (Bi n<2+ ) in the BACs [11,24,25].

Discussion
Analysis of the optical absorption and luminescence spectra of the synthesized glasses (λ ex = 450 nm) showed the presence of BACs, the number of which increased with the bismuth oxide total concentration growth. The contour of the PL spectrum in the IR region was represented by a superposition of several bands, whose maxima, determined from the Gaussian components, were located at wavelengths ~1125, 1310, 1615, and 1885 nm ( Figure 9). According to [26,27], the bands at 1125 and 1310 nm corresponded to the 3 P1 → 3 P0 transitions for the Bi + ion and the 2 D3/2 → 4 S3/2 transitions for Bi 0 , respectively. At the same time, it was shown in [11] that BACs were not individual low-valence bismuth ions, but a complex system of cations and an oxygen vacancy. In this case, both bands at 1125 nm (Peak 4 in Figure 9) and 1310 nm (Peak 3 in Figure 9) belonged to oxygen-deficient centers =Bi···Ge≡. Thus, the difference in the band position was caused by the presence or absence of aluminum ions in the second coordination sphere of the BACs, respectively [11]. Previously, for the samples with a high content of Bi2O3 (>20 mol%), the luminescence was observed in the longer wavelength part of the spectrum (1800-3000 nm). It was supposed that this luminescence could be attributed to the formation of Bi5 3+ cluster centers [28] or oxygen-deficient centers = Bi···Bi = [11]. We assume that in our glasses two types of luminescent BACs were formed: namely, = Bi···Ge ≡ (~1125 and ~1310 nm) (Peaks 4 and 3 in Figure 9) and = Bi···Bi = (1615 and 1885 nm) (Peaks 2 and 1 in Figure  9) in a smaller amount. Bi2O3 content growth led to an increase in the amount of = Bi···Bi = type centers and to PL decreasing in the ~1300 nm region. This BACs transformation was in good agreement with the structural analysis data. The shift of the vibration bands towards low frequencies at the bismuth oxide content growth indicated an increase in the Ge-O and Bi-O bond lengths, which in turn resulted in the formation of =Bi···Bi = centers having shorter bond lengths than the = Bi···Ge ≡ centers [11].
The glass transition temperatures of bismuth-containing glasses were lower compared to the temperature of pure GeO2 glass (519 °C [29]), probably due to a decrease in melt viscosity upon the introduction of Bi2O3. The resulting range of Tg values (440-480 °C) was in good agreement with the data previously reported [30][31][32][33].
The DSC data showed the possibility of crystallization of several phases in glasses, and the set of crystalline phases varied for different glass compositions. The heat treatment of samples at 600 °C showed (Figures 10 and S11-S17) that predominantly α-GeO2 and β-GeO2 phases crystallized, accompanied by a certain amount of the Bi4Ge3O12 phase

Discussion
Analysis of the optical absorption and luminescence spectra of the synthesized glasses (λ ex = 450 nm) showed the presence of BACs, the number of which increased with the bismuth oxide total concentration growth. The contour of the PL spectrum in the IR region was represented by a superposition of several bands, whose maxima, determined from the Gaussian components, were located at wavelengths~1125, 1310, 1615, and 1885 nm ( Figure 9). According to [26,27], the bands at 1125 and 1310 nm corresponded to the 3 P 1 → 3 P 0 transitions for the Bi + ion and the 2 D 3/2 → 4 S 3/2 transitions for Bi 0 , respectively. At the same time, it was shown in [11] that BACs were not individual low-valence bismuth ions, but a complex system of cations and an oxygen vacancy. In this case, both bands at 1125 nm (Peak 4 in Figure 9) and 1310 nm (Peak 3 in Figure 9) belonged to oxygendeficient centers =Bi· · · Ge≡. Thus, the difference in the band position was caused by the presence or absence of aluminum ions in the second coordination sphere of the BACs, respectively [11]. Previously, for the samples with a high content of Bi 2 O 3 (>20 mol%), the luminescence was observed in the longer wavelength part of the spectrum (1800-3000 nm). It was supposed that this luminescence could be attributed to the formation of Bi 5 3+ cluster centers [28] or oxygen-deficient centers =Bi· · · Bi= [11]. We assume that in our glasses two types of luminescent BACs were formed: namely, =Bi· · · Ge≡ (~1125 and~1310 nm) (Peaks 4 and 3 in Figure 9) and =Bi· · · Bi= (1615 and 1885 nm) (Peaks 2 and 1 in Figure 9) in a smaller amount. Bi 2 O 3 content growth led to an increase in the amount of =Bi· · · Bi= type centers and to PL decreasing in the~1300 nm region. This BACs transformation was in good agreement with the structural analysis data. The shift of the vibration bands towards low frequencies at the bismuth oxide content growth indicated an increase in the Ge-O and Bi-O bond lengths, which in turn resulted in the formation of =Bi· · · Bi= centers having shorter bond lengths than the =Bi· · · Ge≡ centers [11].
The glass transition temperatures of bismuth-containing glasses were lower compared to the temperature of pure GeO 2 glass (519 • C [29]), probably due to a decrease in melt viscosity upon the introduction of Bi 2 O 3 . The resulting range of T g values (440-480 • C) was in good agreement with the data previously reported [30][31][32][33].
The DSC data showed the possibility of crystallization of several phases in glasses, and the set of crystalline phases varied for different glass compositions. The heat treatment of samples at 600 • C showed (Figure 10 and Figures S11-S17) that predominantly α-GeO 2 and β-GeO 2 phases crystallized, accompanied by a certain amount of the Bi 4 Ge 3 O 12 phase in samples containing up to 20 mol% Bi 2 O 3 . The crystallization peaks of all phases for these compositions were weakly separated ( Figures S1-S3), which indicated the almost simultaneous beginning of their crystallization process in glass. The simultaneous existence of both modifications of crystalline GeO 2 correlated well with the metastable phase diagram [3], in which~600 • C served as the transition temperature between α-GeO 2 and β-GeO 2 polymorphs. The Bi 2 GeO 5 phase, noted in the same phase diagram, was unstable in this concentration range, as shown in [33], and appeared only in trace amounts in the 5-95 sample according to XRD patterns (see Figure 10).
Ceramics 2023, 6, FOR PEER REVIEW 10 in samples containing up to 20 mol% Bi2O3. The crystallization peaks of all phases for these compositions were weakly separated ( Figures S1-S3), which indicated the almost simultaneous beginning of their crystallization process in glass. The simultaneous existence of both modifications of crystalline GeO2 correlated well with the metastable phase diagram [3], in which ~600 °C served as the transition temperature between α-GeO2 and β-GeO2 polymorphs. The Bi2GeO5 phase, noted in the same phase diagram, was unstable in this concentration range, as shown in [33], and appeared only in trace amounts in the 5-95 sample according to XRD patterns (see Figure 10). Crystallization in the 550-640 °C temperature range leads to the formation of the Bi4Ge3O12 phase [30][31][32], alone or together with other phases for glass compositions containing 10-40 mol% Bi2O3. Therefore, the crystallization peaks belonging to the 650-663 °C range can be associated with the maximum crystallization temperature of the Bi4Ge3O12 Crystallization in the 550-640 • C temperature range leads to the formation of the Bi 4 Ge 3 O 12 phase [30][31][32], alone or together with other phases for glass compositions containing 10-40 mol% Bi 2 O 3 . Therefore, the crystallization peaks belonging to the 650-663 • C range can be associated with the maximum crystallization temperature of the Bi 4 Ge 3 O 12 phase ( Figure 11). The crystallization peaks in the 690-744 • C range can be associated with the crystallization temperature of the β-GeO 2 phase for samples with a high content of GeO 2 or other phases for samples with a high content of Bi 2 O 3 . phase ( Figure 11). The crystallization peaks in the 690-744 °C range can be associated wi the crystallization temperature of the β-GeO2 phase for samples with a high content GeO2 or other phases for samples with a high content of Bi2O3. It is known that the set of crystalline phases in crystallized glasses changes as the content of Bi2O3 increases (≥20 mol%). Bi4Ge3O12 and Bi2GeO5 become the main phases [34,35]. Therefore, despite the XRD amorphous halo for our glasses containing 20-35 mol% Bi2O3 and heat-treated at 600 °C ( Figure S14), it can be assumed that the crystallization proceeded similarly in our samples. Consequently, for the 624-647 °C range, the crystallization peaks belonged to the Bi2GeO5 phase. This assumption was also supported by the fact that the Bi2GeO5 phase disappearance was noted in [36] when the hea treatment temperature increased above 640 °C. Heat-treatment of 25-75 glass ( Figure  S18) at 690 °C led to the Bi4Ge3O12 and Bi2Ge3O9 phases' formation in agreement with [33,34]. An increase of the heat treatment temperature to 720 °C for the 25-75 glass led t an insignificant decrease in the amount of the Bi2Ge3O9 phase, whose composition corre sponded to that of glass; therefore, the crystallization maximum at 690 °C on the DSC curve corresponded to the Bi2Ge3O9 phase formation. The formation of phases' mixture during 25-75 glass crystallization corresponded to the cross sections of the Bi-Ge-O phase diagram [37], where in the region of 25 mol% Bi2O3 we observed <!--Math-Type@Translator@5@5@MathML2 (no namespace).tdl@MathML 2.0 (no namespace)@ --<math><mrow><msub><mrow><mi>S</mi></mrow><mrow><msub><mrow><mi>B </mi><mi>i</mi></mrow><mrow><mn>4</mn></mrow></msub><msub><mrow><mi>G /mi><mi>e</mi></mrow><mrow><mn>3</mn></mrow></msub><msub><mrow><mi>O mi></mrow><mrow><mn>12</mn></mrow></msub></mrow></msub><mo>&#x2212;</m o><msub><mrow><mi>S</mi></mrow><mrow><msub><mrow><mi>B</mi><mi>i</mi> mrow><mrow><mn>2</mn></mrow></msub><mi>G</mi><msub><mrow><mi>e</mi>< mrow><mrow><mn>3</mn></mrow></msub><msub><mrow><mi>O</mi></mrow><mr w><mn>9</mn></mrow></msub></mrow></msub><mo>&#x2212;</mo><mi>V</mi></m row></math> <!--MathType@End@5@5@ --> bivariant equilibrium. The same phase equilibrium explains the absence of the β-GeO2 phase in the crystallized glasses, which demonstrated a weak crystallization peak at 721 °C on the DSC curve. It is known that the set of crystalline phases in crystallized glasses changes as the content of Bi 2 O 3 increases (≥20 mol%). Bi 4 Ge 3 O 12 and Bi 2 GeO 5 become the main phases [34,35]. Therefore, despite the XRD amorphous halo for our glasses containing 20-35 mol% Bi 2 O 3 and heat-treated at 600 • C ( Figure S14), it can be assumed that the crystallization proceeded similarly in our samples. Consequently, for the 624-647 • C range, the crystallization peaks belonged to the Bi 2 GeO 5 phase. This assumption was also supported by the fact that the Bi 2 GeO 5 phase disappearance was noted in [36] when the heat treatment temperature increased above 640 • C. Heat-treatment of 25-75 glass ( Figure S18) at 690 • C led to the Bi 4 Ge 3 O 12 and Bi 2 Ge 3 O 9 phases' formation in agreement with [33,34]. An increase of the heat treatment temperature to 720 • C for the 25-75 glass led to an insignificant decrease in the amount of the Bi 2 Ge 3 O 9 phase, whose composition corresponded to that of glass; therefore, the crystallization maximum at 690 • C on the DSC curve corresponded to the Bi 2 Ge 3 O 9 phase formation. The formation of phases' mixtures during 25-75 glass crystallization corresponded to the cross sections of the Bi-Ge-O phase diagram [37], where in the region of 25 mol% Bi 2 O 3 we observed S Bi 4 Ge 3 O 12 − S Bi 2 Ge 3 O 9 − V bivariant equilibrium. The same phase equilibrium explains the absence of the β-GeO 2 phase in the crystallized glasses, which demonstrated a weak crystallization peak at 721 • C on the DSC curve.
The crystallization of 45-55 and 50-50 glasses should be discussed in detail. These glasses were inclined to crystallization in the glass casting process already. Therefore, there was a possibility of the spontaneous nuclei of crystalline phases' existence in glasses that were not determined by XRD. The crystallization maximum at temperatures of 575-598 • C belonged probably to the Bi 2 GeO 5 phase since heat treatment at 600 • C led to the formation of this particular phase as the main one in 45-55 glass and the only one in 50-50 glass. To confirm these conclusions and to identify the crystallization peaks in the 518-548 • C range, which were not observed for the rest of the glass compositions, additional annealing of 45-55 and 50-50 glasses was carried out.
For the 45-55 sample (Figure 12), annealing at 520 • C led to the formation of the Bi 12 GeO 20 phase together with the Bi 2 GeO 5 and Bi 4 Ge 3 O 12 phases. The composition of the Bi 12 GeO 20 phase corresponds to the molar composition of 85.7Bi 2 O 3 -14.3GeO 2 , which is quite far from the original 45-55 glass composition. However, if we consider the Bi-Ge-O phase diagram cross sections [37] at temperatures close to the heat-treatment temperature (517 • C, 596 • C), it becomes clear that the 45-55 composition is in the range of monovariant equilibrium S Bi 12 GeO 20 − S Bi 4 Ge 3 O 12 − S Bi 2 GeO 5 − V. Annealing of 45-55 glass at 580 • C led to the crystallization of only two phases: Bi 2 GeO 5 (basic) and Bi 4 Ge 3 O 12 ( Figure 12). The area of S Bi 12 GeO 20 − S Bi 4 Ge 3 O 12 − S Bi 2 GeO 5 − V monovariant equilibrium narrowed at temperature increases from 799 K to 850 K in the Bi-Ge-O diagram cross section [37]. As a result, the 45-55 glass composition moved to the region of S Bi 4 Ge 3 O 12 − S Bi 2 GeO 5 − V bivariant equilibrium. Thus, for 45-55 glass, the exothermic peak at 518 • C referred to the maximum crystallization temperature of the Bi 12 GeO 20 phase, while the peak at 575 • C referred to the Bi 2 GeO 5 phase.
Ceramics 2023, 6, FOR PEER REVIEW 13 mi>O</mi></mrow><mrow><mn>5</mn></mrow></msub></mrow></msub><mo>&#x22 12;</mo><mi>V</mi></mrow></math> <!--MathType@End@5@5@ --> bivariant equilibrium. Thus, for 45-55 glass, the exothermic peak at 518 °C referred to the maximum crystallization temperature of the Bi12GeO20 phase, while the peak at 575 °C referred to the Bi2GeO5 phase. Heat treatment of 50-50 glass at 550 °C led to the single Bi2GeO5 phase formation corresponding to the glass composition ( Figure 13). The increase in heat-treatment temperature to 600 °C led to the appearance of the mixture of Bi2GeO5 and β-GeO2 phases. The further temperature rise to 750 °C caused the formation of the mixture of Bi2GeO5, β-GeO2, and β-Bi2O3 phases. Thus, the maximum crystallization temperatures of 548, 598, and 657 °C corresponded to the formation of Bi2GeO5, β-GeO2, and β-Bi2O3 phases, respectively. The formation of the β-GeO2 crystalline phase at high bismuth concentrations in the 50-50 sample could be caused by composition fluctuations in the initial glass. Heat treatment of 50-50 glass at 550 • C led to the single Bi 2 GeO 5 phase formation corresponding to the glass composition ( Figure 13). The increase in heat-treatment temperature to 600 • C led to the appearance of the mixture of Bi 2 GeO 5 and β-GeO 2 phases. The further temperature rise to 750 • C caused the formation of the mixture of Bi 2 GeO 5 , β-GeO 2 , and β-Bi 2 O 3 phases. Thus, the maximum crystallization temperatures of 548, 598, and 657 • C corresponded to the formation of Bi 2 GeO 5 , β-GeO 2 , and β-Bi 2 O 3 phases, respectively. The formation of the β-GeO 2 crystalline phase at high bismuth concentrations in the 50-50 sample could be caused by composition fluctuations in the initial glass. Summarizing the crystallization data, we can say that the crystallization temperatures of bismuth-germanate phases correlate well with the amount of bismuth in their composition. The decrease in Bi2O3 content in the row of individual compounds Bi12GeO20 → Bi2GeO5 → Bi4Ge3O12 → Bi2Ge3O9 (85.7-50-40-25 mol%) results in an increase in the maximum crystallization temperatures of the corresponding phases.

Conclusions
To fill the gaps in fundamental data for the first time, we investigated bismuth and germanium oxide-based glasses in a wide concentration range, with special emphasis on high Bi2O3 concentrations up to 50 mol%. We succeeded in decreasing the synthesis temperature from 1300 to 1100 °C. Glasses based on bismuth oxide and germanium oxide demonstrated a strong dependence of their structure and properties on the Bi2O3/GeO2 ratio. An increase in the bismuth oxide concentration led to an increase in the number of non-bridging oxygen ions and a weakening of the Ge-O bonds. Such a rearrangement of the glass structure contributed to the destruction of =Bi···Ge≡ bismuth luminescent centers and the formation of =Bi···Bi= luminescent centers, which led to a weakening of the PL Summarizing the crystallization data, we can say that the crystallization temperatures of bismuth-germanate phases correlate well with the amount of bismuth in their composition. The decrease in Bi 2 O 3 content in the row of individual compounds Bi 12 GeO 20 → Bi 2 GeO 5 → Bi 4 Ge 3 O 12 → Bi 2 Ge 3 O 9 (85.7-50-40-25 mol%) results in an increase in the maximum crystallization temperatures of the corresponding phases.

Conclusions
To fill the gaps in fundamental data for the first time, we investigated bismuth and germanium oxide-based glasses in a wide concentration range, with special emphasis on high Bi 2 O 3 concentrations up to 50 mol%. We succeeded in decreasing the synthesis temperature from 1300 to 1100 • C. Glasses based on bismuth oxide and germanium oxide demonstrated a strong dependence of their structure and properties on the Bi 2 O 3 /GeO 2 ratio. An increase in the bismuth oxide concentration led to an increase in the number of non-bridging oxygen ions and a weakening of the Ge-O bonds. Such a rearrangement of the glass structure contributed to the destruction of =Bi· · · Ge≡ bismuth luminescent centers and the formation of =Bi· · · Bi= luminescent centers, which led to a weakening of the PL intensity in the region of~1300 nm. The results of glass crystallization depended on the Bi 2 O 3 oxide content: the higher the Bi 2 O 3 concentration in a crystalline phase, the lower the temperature of its formation.