# Colorless Bi2O3-containing borophosphate glasses with high refractivity

K. Hayashi, Grégory Tricot, A. Saitoh

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K. Hayashi, Grégory Tricot, A. Saitoh. Colorless Bi2O3-containing borophosphate glasses with high refractivity. Journal of the Ceramic Society of Japan, 2023, Journal of the Ceramic Society of Japan, 131, 10.2109/jcersj2.23138. hal-04500069

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# Colorless Bi₂O₃-containing borophosphate glasses with high refractivity

Katsuki Hayashi<fnref n="1" />, Grégory Tricot<fnref n="2" /> and Akira Saitoh<fnref n="1" />,†

The effects of P₂O₅ substitution on the coordination structure of a Bi³⁺ in Bi₂O₃–B₂O₃ glasses were examined by applying ¹¹B and ³¹P MAS-NMR spectroscopy. Observation of the wide optical bandgap after the substitution revealed a substantial difference in the solvation shell structure between the P₂O₅ substituted and the P₂O₅-free Bi₂O₃-containing glasses, i.e., the P₂O₅ substitution replaced by the B₂O₃ is effective for forming the solvation shell, which leads to weak ligand field around a Bi³⁺. We report the optical properties of the Bi₂O₃–P₂O₅–B₂O₃ glass providing a wide optical bandgap (~3.6 eV) and high refractive index (~2.0).

Key-words: Borophosphate glass, Refractive index, Nuclear magnetic resonance, Structure

[Received August 18, 2023; Accepted September 22, 2023]

## 1. Introduction

Bi₂O₃-containing oxide glasses are widely employed in applications for optical devices.<fnref n="1" />) Although Bi₂O₃–B₂O₃ glasses that have many excellent physical properties are a highly desired host for optically active ions,<fnref n="2" />)⁻⁴) their fatal drawback is coloration.⁵),⁶) This issue, ascribed to a strong ligand field around a cation, was resolved by a co-doping technique.⁷),⁸) For instance, Saitoh et al. demonstrated the “solvation shell” model based on directly observing the ligand structures of Ce³⁺ and Er³⁺ in SiO₂ glasses to understand the striking doping effects of P₂O₅. The concept of solvation shell of ions in the SiO₂ glass, i.e., preferential coordination of doped P₂O₅ around the rare-earth ions, is favorable for designing highly efficient luminescence as an optical material. As a result, the photoluminescence properties of Ce³⁺ and Er³⁺ in those samples showed a slight Stokes shift and high emission intensity. Hence, we can consider the substitution of B₂O₃ by P₂O₅ in the Bi₂O₃–B₂O₃ glasses as effective in realizing colorless property, keeping a high refractive index, which will weaken the ligand field of a Bi³⁺.

Little qualitative structure by the effect of P₂O₅ has been reported in the binary Bi₂O₃–P₂O₅ glasses because of the narrow glass formation region up to 40 mol % of Bi₂O₃.⁵),⁹),¹⁰) It is considered that the borophosphate glasses consisting of tetrahedrally coordinated PO₄–BO₄–PO₄ linkage can widen the glass formation region⁴),¹¹) and thus offer the possibility of providing high refractive index materials.

The purpose of this study is to present Bi₂O₃–P₂O₅–

B₂O₃ glasses with high concentrations of Bi₂O₃, wide optical bandgap, and high refractive index. In addition, the glass network was characterized by ¹¹B and ³¹P MAS-NMR spectroscopies to determine the substitution effect of P₂O₅ on the coordination structure around a Bi³⁺.

## 2. Experimental

Glass samples were prepared using the conventional melt-quenching method. Reagent grades of Bi₂O₃ (99.99 %), B₂O₃ (99.9 %), and BiPO₄ (99.9 %) were used as raw materials. The nominal molar composition is xBi₂O₃–yP₂O₅–(100 – x – y)B₂O₃ with 40 ≤ x ≤ 60, 0 ≤ y ≤ 20. Batches that yielded 12–15 g were mixed and then melted for 30 min at 1000 °C in an alumina crucible in an electric furnace in the air. The dissolution of Al₂O₃ into the sample was 1–2 mol % by SEM-EDS. The melts were poured into a preheated stainless-steel mold and then annealed to remove residual thermal stress in the air for 24 h at 360–470 °C, depending on glass transition temperatures. The amorphous nature of the resulting bulk glasses was confirmed by X-ray diffraction analysis.

The density of the bulk sample was determined using the Archimedes method with kerosene as an immersion liquid for the glasses at room temperature. The estimated uncertainties due to composition fluctuations were ±0.01 g/cm³.

The refractive index was measured using the refractometer for a 3 × 3 × 10 mm³ rectangular glass rod at 587.6 nm wavelengths. The estimated uncertainties due to composition fluctuations were ±0.01. Optical transmission spectra were measured using a 1 mm-thick plate and a ~2 µm thick film. The film was obtained from a piece of glass balloon blown from each glass melt using an alumina pipe.

<footnote n="1">¹Graduate School of Science and Engineering, Ehime University, Matsuyama, Ehime 790-8577, Japan</footnote>

<footnote n="2">²Laboratoire de Spectroscopie pour les Interactions, la Réactivité & l'Environnement (LASIRE)-UMR CNRS 8516,
Université de Lille, F-59000 Lille, France</footnote>

<footnote>† Corresponding author: A. Saitoh; E-mail: asaito@ehime-u.ac.jp</footnote>
The ¹¹B and ³¹P MAS-NMR experiments were performed at 256.7 and 323.9 MHz on an 18.8 T spectrometer (AVANCE III, Bruker) using a 3.2-mm probe head operating at a spinning frequency of 20 kHz. The ¹¹B MAS-NMR spectra were obtained with a π/10 flip angle of 1.0 µs, recycle delay (rd) of 5 s, and 1024 transients. The ³¹P MAS-NMR spectra were obtained with a π/6 flip angle of 1.8 µs, rd of 120 s, and 32 transients. The ¹¹B and ³¹P chemical shifts are respective to NaBH₄ and H₃PO₄ as -42.06 and 0 ppm, respectively. The NMR spectra were decomposed using the dmfit software¹²) to extract the chemical shift and relative proportions parameters.

### 3. Results and discussion

**Figures 1(a) and 1(b) show the compositional dependences of the density and refractive index measured at 587.6 nm of wavelength in the xBi₂O₃-yP₂O₅-(100-x-y)B₂O₃ glasses. In each Bi₂O₃ concentration sample, the density and refractive index decrease with increasing P₂O₅, which replaces B₂O₃. Note that the x = 60, y = 10 sample particularly presents a high refractive index (~2.1) measured at 587.6 nm and a small photoelastic constant (+0.5 × 10⁻¹² Pa⁻¹) measured at 632.8 nm.**

**Figures 2(a)–2(c) show the compositional dependences of optical transmission spectra of the xBi₂O₃-yP₂O₅-(100-x-y)B₂O₃ glasses. The optical absorption edges of the Bi₂O₃–P₂O₅–B₂O₃ glasses blueshift, substituting B₂O₃ with P₂O₅. While the optical absorption edges of the Bi₂O₃–P₂O₅–B₂O₃ glasses redshift with increasing Bi₂O₃ concentration. The samples of large concentrations of Bi₂O₃ (x) and small amount of P₂O₅ (y) are likely to show pale yellow. Figure 2(d) compares the transmission spectra of the bulk and thin film samples. Notably, the x = 50, y = 20 sample presents the absorption edge being near a deep ultraviolet region (<350 nm).¹³)**

**Figures 3(a) and 3(b) show the compositional dependences of the absorption coefficient spectra and Tauc plots determined from the absorption coefficients of the 50Bi₂O₃–50B₂O₃ (x = 50, y = 0) and the 50Bi₂O₃–20P₂O₅–30B₂O₃ (x = 50, y = 20) glasses. Note that the optical bandgaps**

are determined from the Tauc plot ($\alpha h\nu)^{1/2}$ versus photon energy $h\nu$, where $\alpha$ denotes the optical absorption coefficient. Here, the photon energy dependence of the optical absorption of glass can be expressed as $(h\nu - E)^2/h\nu$, where E denotes the optical band gap.¹⁴) The optical bandgap of the 50Bi₂O₃–20P₂O₅–30B₂O₃ glass increases by ~0.2 eV with the incorporation of P₂O₅.

**Figures 4(a)–4(c) show ¹¹B MAS-NMR spectra, including the proportions of three- and four-coordinated borate units of the xBi₂O₃-yP₂O₅-(100-x-y)B₂O₃**

<figure><img src="image_2.png" /><figcaption>**Fig. 1. Compositional dependences of the (a) density and (b) refractive index of the xBi₂O₃-yP₂O₅-(100-x-y)B₂O₃ (40 ≤ x ≤ 60, 0 ≤ y ≤ 20) glasses.**</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>**Fig. 2. Compositional dependences of optical transmission spectra of the xBi₂O₃-yP₂O₅-(100-x-y)B₂O₃ (40 ≤ x ≤ 60, 0 ≤ y ≤ 20) glasses in the wavelength range 300–700 nm for samples with 1 mm and ~2 µm thicknesses. The dashed lines in (d) present the spectra for the film samples of x = 50, y = 0, 20.**</figcaption></figure>
glasses. Altogether, five different species can be observed with four B<sup>IV</sup> and one B<sup>III</sup> units. The assignments of each borate unit<sup>4),15)</sup> are listed in Table 1, and are derived from a 1D/2D NMR protocol applied to a similar system (SnO–B<sub>2</sub>O<sub>3</sub>–P<sub>2</sub>O<sub>5</sub>) in a previous study.<sup>4)</sup> As the concentrations of Bi<sub>2</sub>O<sub>3</sub> (x) and substituted P<sub>2</sub>O<sub>5</sub> (y) increase, the apparent ratio of B<sup>III</sup> appears to increase. However, there is no clear regularity in the ratio of B<sup>III</sup> to B<sup>IV</sup>, and the ratio tends to increase within the error of the deconvolution of <sup>11</sup>B NMR spectrum. On the other hand, the proportions of each unit of B<sup>IV</sup>(<sub>1</sub>)–B<sup>IV</sup>(<sub>4</sub>) do not essentially depend on the changes in Bi<sub>2</sub>O<sub>3</sub> and P<sub>2</sub>O<sub>5</sub> concentrations. However, the x = 50 and

y = 20 sample shows a large proportion of B<sup>IV</sup>; among them, the proportion of B<sup>IV</sup>(<sub>2</sub>) and B<sup>IV</sup>(<sub>3</sub>) is substantial. The experiments show that our glasses mainly contain three coordinated boron species (>65 %). The low proportion of B<sup>IV</sup>(<sub>1</sub>) (<5 %) indicates a strong B<sup>IV</sup>/B<sup>III</sup> mixing. Moreover, the four-coordinated B is also adjacent to a Bi<sup>3+</sup> as revealed by the significant presence of B<sup>IV</sup>(<sub>3</sub>) unit.

**Figures 5(a)–5(c)** show the <sup>31</sup>P MAS-NMR spectra with the proportions of phosphate units in the xBi<sub>2</sub>O<sub>3</sub>–yP<sub>2</sub>O<sub>5</sub>–(100 – x – y)B<sub>2</sub>O<sub>3</sub> glasses. Three species around –18, –10, and 3 ppm can be observed with various proportions depending on the glass composition. The assignments of these three species are reported in Table 1 and are derived from the 1D/2D NMR protocol applied to the tin borophosphate glass containing a high amount of SnO.<sup>4)</sup> As the concentration of Bi<sub>2</sub>O<sub>3</sub> (x) increases, the proportion of Q<sup>1</sup> decreases. On the other hand, as the concentration of P<sub>2</sub>O<sub>5</sub> (y) increases, Q<sup>0</sup> and Q<sup>0</sup><sub>B</sub> decrease, and Q<sup>1</sup> increases. The x = 50 and y = 20 sample shows that Q<sup>0</sup><sub>B</sub> and Q<sup>1</sup> units are major species of the phosphate units.

According to the <sup>31</sup>P NMR spectra, the four-coordinated P is bound to the three- and four-coordinated B from the assignment of Q<sup>0</sup><sub>B</sub>. The four coordinated P is terminated to a Bi<sup>3+</sup> as a Q<sup>1</sup> or Q<sup>0</sup> unit. Hence, the coordination of the Bi<sup>3+</sup> in the 50Bi<sub>2</sub>O<sub>3</sub>–20P<sub>2</sub>O<sub>5</sub>–30B<sub>2</sub>O<sub>3</sub> glass consists of PO<sub>4</sub> or BO<sub>4</sub> units. When the amount of P<sub>2</sub>O<sub>5</sub> substitution is large, PO<sub>4</sub> is the primary ligand, which can be similar to the solvation shell structure by P<sub>2</sub>O<sub>5</sub>.<sup>7),8)</sup>

**Figure 6** illustrates the coordination structure model of the borate and phosphate units in the vicinity of a Bi<sup>3+</sup> in the 50Bi<sub>2</sub>O<sub>3</sub>–20P<sub>2</sub>O<sub>5</sub>–30B<sub>2</sub>O<sub>3</sub> (x = 50, y = 20) glass, which has an optical bandgap of 3.6 eV and a refractive index of 2.0. A possibility of the formal charges on the non-bridging oxygens (NBOs) belonging to the phosphate units is –1/2 of the Q<sup>1</sup> and Q<sup>0</sup> units. Because in a middle phosphate group, an electron is delocalized over two NBOs via pπ-dπ bonding. In other words, in the phosphate units, the formal charge on the NBO in the P–O bond is –0.5 due to the delocalization of electrons over the two NBOs via the Opπ–P3dπ bond.<sup>7),16),17)</sup> Then, ligands around a Bi<sup>3+</sup> belong to this type of oxygens with the formal negative charge of –0.5, which is an intermediate value between conventional NBO (–1) and bridging oxygen (0). Therefore, the ligand field is considered weaker because the Bi<sup>3+</sup> surroundings are not composed of large formal charged oxygens (–1). On the other hand, in the

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** (a) Absorption coefficient spectra and (b) Tauc plots of the 50Bi<sub>2</sub>O<sub>3</sub>–50B<sub>2</sub>O<sub>3</sub> and 50Bi<sub>2</sub>O<sub>3</sub>–20P<sub>2</sub>O<sub>5</sub>–30B<sub>2</sub>O<sub>3</sub> glasses with 1 mm and ~2 µm thickness. In (b), the dashed lines present the estimated optical bandgap of the x = 50, y = 0 and x = 50, y = 20 samples.</figcaption></figure>

<table><caption>**Table 1.** Assignments of each borate and phosphate unit were analyzed by the <sup>11</sup>B and <sup>31</sup>P MAS-NMR spectra of the xBi<sub>2</sub>O<sub>3</sub>–yP<sub>2</sub>O<sub>5</sub>–(100 – x – y)B<sub>2</sub>O<sub>3</sub> glasses</caption><thead><tr><th>Units and notation</th><th>Assignments</th><th>Refs.</th></tr></thead><tbody><tr><td rowspan="4">Borate</td><td>B<sup>IV</sup>(<sub>1</sub>) Connected to tetrahedral species [B(OX<sup>IV</sup>)<sub>4</sub> with X = P or B]</td><td rowspan="4">4, 15</td></tr><tr><td>B<sup>IV</sup>(<sub>2</sub>) Connected to at least one B<sup>III</sup> units [B(OX<sup>IV</sup>)<sub>m</sub>(OB<sup>III</sup>)<sub>k</sub> (0 ≤ m ≤ 3, 1 ≤ k ≤ 4)]</td></tr><tr><td>B<sup>IV</sup>(<sub>3</sub>) Direct coordination to at least one Bi<sup>3+</sup></td></tr><tr><td>B<sup>IV</sup>(<sub>4</sub>) Not assigned</td></tr><tr><td rowspan="3">Phosphate</td><td>Q<sup>1</sup> Pyrophosphate unit with three non-bridging oxygen ions per phosphorus and one connected P atom</td><td rowspan="3">4</td></tr><tr><td>Q<sup>0</sup><sub>B</sub> PO<sub>4</sub> tetrahedral unit with one bridging oxygen with to B<sup>III</sup> or B<sup>IV</sup>(<sub>1</sub>) and no connected P atom</td></tr><tr><td>Q<sup>0</sup> Orthophosphate unit with four non-bridging oxygen ions per phosphorus</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>Fig. 4. ¹¹B MAS-NMR spectra of the xBi₂O₃–yP₂O₅–(100–x–y)B₂O₃ glasses with (a) x = 40, y = 0, 5, and 10, (b) x = 50, y = 0, 5, 10, 15, and 20, and (c) x = 60, y = 0, 5, 10, 15, and 20. The B³³ and B^IV (B^IV(1)–B^IV(4)) proportions are denoted, respectively. The notations (B³³ and B^IV(1)–B^IV(4)) indicate the coordination number of oxygens in superscript and type in parenthesis.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 5. ³¹P MAS-NMR spectra of the xBi₂O₃–yP₂O₅–(100–x–y)B₂O₃ glasses with (a) x = 40, y = 5 and 10, (b) x = 50, y = 5, 10, 15, and 20, and (c) x = 60, y = 5, 10, 15, and 20. The Q⁰, Q⁰B, and Q¹ proportions are denoted, respectively. The notations (Q⁰, Q⁰B, and Q¹) indicate the number of non-bridging oxygens in superscript.</figcaption></figure>

BO₃ and BO₄ units, the charge of –1 is allocated to each oxygen if it is a three-coordinated oxygen, and the charge of –3/4 if it is a four-coordinated oxygen to compensate for the +3 charge of B. If BO₄ units were involved in the

coordination in part, they would contribute to forming a weak ligand field since the net charge would be –3/4.¹⁸) The fact that the coordination structure around a Bi³⁺ in the 40Bi₂O₃–60P₂O₅ glass resembles a BiPO₄ crystal⁵,¹⁹)
<figure><img src="image_8.png" /><figcaption>Fig. 6. The coordination structure model of a Bi³⁺ in the colorless and high refractive 50Bi₂O₃–20P₂O₅–30B₂O₃ (x = 50, y = 20) glass, deduced from the analyses of ¹¹B and ³¹P MAS-NMR spectroscopy.</figcaption></figure>

is supportive evidence of a strong affinity for phosphate units.

Finally, it is worth noting the relationship between optical properties and coordination structure. Lone pair electrons of Bi³⁺ occupy the top of the valence band in crystal-line Bi₂O₃,¹⁹) which may compete with the O 2p electron level. Hence, the optical absorption derived from the Bi³⁺ 6s²–6s¹p¹ transition⁵,¹⁹) strongly correlates to the coordination structure around a Bi³⁺. Therefore, the bandgap energy increase by decreasing the negative charge on the ligand oxygen. This idea agrees with the observed widened optical bandgap by substitution of P₂O₅.

## 4. Conclusions

The colorless, high refractive index Bi₂O₃–P₂O₅–B₂O₃ glass is realized by substation B₂O₃ by P₂O₅. The results are summarized as follows:

(1) The 50Bi₂O₃–20P₂O₅–30B₂O₃ glass has a larger optical bandgap (~3.6 eV) and high refractive index (~2.0).

(2) The contrast in optical transmission spectra of the 50Bi₂O₃–20P₂O₅–30B₂O₃ glass to that of the 50Bi₂O₃–50B₂O₃ glass is understood because the Bi³⁺ in former glass has a solvation shell structure regulated by the phosphate anions.

(3) A weak ligand field is attributed to the less net charge belonging to NBOs of O⁻¹/² or O⁻³/⁴ around a Bi³⁺.

(4) The Bi₂O₃–P₂O₅–B₂O₃ glasses with a wide optical bandgap, high refractive index, and small photoelastic constant are preferred to optical lenses and filters in the visible region.

## 5. Declaration of competing interests

The authors declare to have no interest in the present paper.

**Acknowledgments** This work was partially supported by JSPS KAKENHI Grant Number JP25820333. K.H. was partially supported by the JSPS Overseas Challenge Program for Young Researchers.

## References

1) T. Maeder, *Int. Mater. Rev.*, **58**, 3–40 (2013).

2) C. Hwang, S. Fujino and K. Morinaga, *J. Am. Ceram. Soc.*, **87**, 1677–1682 (2004).

3) C. P. E. Varsamis, N. Makris, C. Valvi and E. I. Kamitsos, *Phys. Chem. Chem. Phys.*, **23**, 10006–10020 (2021).

4) A. Saitoh, G. Tricot, P. Rajbhandari, S. Anan and H. Takebe, *Mater. Chem. Phys.*, **149**, 150–648 (2015).

5) A. Saitoh, K. Hayashi, K. Hanzawa, S. Ueda, S. Kawachi, J. Yamaura, K. Ide, J. Kim, G. Tricot, S. Matsuishi, K. Mitsui, T. Shimizu, M. Mori, H. Hosono and H. Hiramatsu, *J. Non-Cryst. Solids*, **560**, 12072001–12072014 (2021).

6) K. Hayashi, T. Shimizu, S. Matsuishi, H. Hiramatsu and A. Saitoh, *J. Mater. Sci.-Mater. El.*, **33**, 2242–2256 (2022).

7) A. Saitoh, S. Matsuishi, M. Oto, T. Miura, M. Hirano and H. Hosono, *Phys. Rev. B*, **72**, 1–4 (2005).

8) A. Saitoh, S. Matsuishi, C. Se-Weon, J. Nishii, M. Oto, M. Hirano and H. Hosono, *J. Phys. Chem. B*, **110**, 7617–7620 (2006).

9) P. Roychoudhury, A. Paul, S. Mukherjee and K. Goswami, *Phys. Chem. Glasses*, **42**, 126–132 (2001).

10) T. Hashimoto, Y. Shimoda, H. Nasu and A. Ishihara, *J. Am. Ceram. Soc.*, **94**, 2061–2066 (2011).

11) G. Tricot, A. Saitoh and H. Takebe, *Phys. Chem. Chem. Phys.*, **17**, 29531–29540 (2015).

12) D. Massiot, F. Fayon, M. Capron, I. King, S. Le Calve, B. Alonso, J. O. Durand, B. Bujoli, Z. Gan and G. Hoatson, *Magn. Reson. Chem.*, **40**, 70–76 (2002).

13) M. Oto, S. Kikugawa, T. Miura, M. Hirano and H. Hosono, *J. Non-Cryst. Solids*, **349**, 133–138 (2004).

14) G. A. N. Connel, in "Amorphous Semiconductors", Ed. by M. H. Brodsky, Springer-Verlag, New York (1974).

15) B. Raguenet, G. Tricot, G. Silly, M. Ribes and A. Pradel, *J. Mater. Chem.*, **21**, 17693–17704 (2011).

16) T. Uchino and Y. Ogata, *J. Non-Cryst. Solids*, **181**, 175–188 (1995).

17) B. Gamoke, D. Neff and J. Simons, *J. Phys. Chem. A*, **113**, 5677–5684 (2009).

18) K. Mitsui, K. Suzuki and A. Saitoh, *Jpn. J. Appl. Phys.*, **61**, 0955071–0955077 (2022).

19) A. Walsh, G. W. Watson, D. J. Payne, R. G. Edgell, J. Guo, P.-A. Glans, T. Learmonth and K. E. Smith, *Phys. Rev. B*, **73**, 2351041–23510413 (2006).