Structure and dissolution of silicophosphate glass

Abstract

P₂O₅–SiO₂–Na₂O–CaO glasses are promising therapeutic ion-releasing materials. Herein, we investigated the state of silicon (Si) in P₂O₅–SiO₂–Na₂O–CaO glass using a model with a composition of 55.0P₂O₅–21.3SiO₂–23.7Na₂O (mol%), incorporating a six-fold-coordinated silicon structure (^[6]Si). The model was constructed using a classical molecular dynamics method and relaxed using the first-principles method. Further, we experimentally prepared glasses, substituting Na₂O for CaO, to investigate the dissolution of glass with varying ^[6]Si and PO₄ tetrahedra (Q_Pⁿ) distributions (n = number of bridging oxygens (BOs) to neighboring tetrahedra). ^[6]Si in the glass model preferentially coordinated with Q_P³. When Si was surrounded by phosphate groups, phosphorus (P) induced the formation of ^[6]Si by elongating the Si–O distance, and ^[6]Si acted like a glass network former (NWF). Na⁺ coordinated with ^[6]Si–O–P bonds via electrostatic interactions with BO. ³¹P and ²⁹Si magic-angle-spinning-nuclear-magnetic-resonance spectra of three experimental glass samples with the compositions of 55.0P₂O₅–21.3SiO₂–xCaO–(23.7 − x)Na₂O (mol%, x = 0, 12.4, and 23.7) showed that Q_P³ and ^[6]Si increased with increasing Na₂O. When each glass powder was immersed in a tris-HCl buffer solution at 37 °C, the dissolution of NWF ions and network modifier (NWM) ions increased almost monotonically with time for all samples, indicating that the solubility of the samples was suppressed by the coexistence of CaO and Na₂O, attributed to the delocalization of the electron distribution of P in the ^[6]Si-coordinated Q_P³ units compared to that in the P- or ^[4]Si-coordinated Q_P³ units, which reduces hydrolysis.

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1 Introduction

Water-soluble phosphate glasses can promote bone regeneration by releasing inorganic ions as bone-formation-promoting factors. Calcium, phosphate, and silicate ions are involved in bone formation and promote the proliferation, differentiation, and mineralization of osteoblast-like cells at appropriate concentrations. The dissolution of these ions from the glass must be reasonably controlled.

The diffusion of glass network modifier (NWM) ions, such as Na⁺ and Ca²⁺ ions, in phosphate glass determines the solubility of the glass.¹ In the dissolution of phosphate glasses, ion-exchange reactions (hydration reactions) between NWM ions and protons dominate, and NWM ions diffuse to the surface.^2–5 To understand the relationship between the phosphate tetrahedral morphology (Q_Pⁿ unit: n is the number of bridging oxygen (BO)) and ion diffusion, we employed ab initio molecular dynamics (MD) simulations to study the dynamics of Na⁺ ions and protons in phosphate glass.⁶ As an example of a phosphate glass containing both Q_P² and Q_P³ units, 55.0P₂O₅–21.3SiO₂–23.7Na₂O (mol%) glass was simulated using a model in which Na⁺ ions are replaced by protons, assuming a progressing state of its dissolution. When a proton was adsorbed on the nonbridging oxygen (NBO) of the Q_P³ unit, it desorbed in a short time within 10 fs and readsorbed on the NBO on which protons had been adsorbed. On the other hand, when a proton was adsorbed on the NBO of the Q_P² unit, another proton coordinated before the adsorption could desorb sequentially, resulting in proton diffusion. When Na⁺ ions were present in the vicinity, proton adsorption on the Q_P² unit reduced the electrostatic interaction between Na⁺ and O²⁻ ions and induced the detachment of Na⁺ ions. This result explains the much early stages of the glass reaction with water. The solubility of the phosphate glass could be engineered by tuning their Q_Pⁿ distribution.

Dupree et al. reported that a six-fold-coordinated silicon (Si) structure (^[6]Si) is readily formed in silicate glasses containing more than 40 mol% P₂O₅.⁷ In our previous study, we found that in P₂O₅–SiO₂–Na₂O–CaO glasses with high P₂O₅ content, ion dissolution is improved in an ultraphosphate glass with high ^[6]Si content.⁸ We reported the ions release behavior for two types of ^[6]Si-containing glasses with the P₂O₅ content of 45 and 50 mol%, fixed SiO₂/Na₂O/CaO ratios. The glass with high P₂O₅ content controlled the ion release amounts effectively. Note that the amounts of ions release decreased in silicophosphate glasses containing ^[6]Si, whereas usually the amount of ions dissolved from SiO₂-free P₂O₅–Na₂O–CaO glasses increases with increasing P₂O₅ content. We predicted that the hydrolysis might be controlled by the formation of Si–O–P bonds between ^[6]Si and Q_P³. These results and prediction motivate us to investigate the relation between glass structure and solubility on the basis of the importance of the ^[6]Si and Q_Pⁿ distribution. By clarifying this, it may be possible to design phosphate glasses so that their solubility can be controlled freely.

In this study, first we attempted visualizing the electron density distribution around ^[6]Si–Q_P³ using our earlier 55.0P₂O₅–21.3SiO₂–23.7Na₂O glass model and discussed the possible effect of this structure on its solubility. Then, we experimentally verified whether the solubility of P₂O₅–SiO₂–(Na₂O, CaO) glasses can be improved by varying ^[6]Si and Q_Pⁿ distributions. It has been reported that the amount of ^[6]Si formation in phosphate glass varies not only with P₂O₅ content but also with the types of NWMs (alkali and/or alkaline earth ions).^7,9 In this study, we tried to form some ^[6]Si and Q_Pⁿ distribution following this phenomenon to investigate the effect of the glass structure on the durability; 55.0P₂O₅–21.3SiO₂–xCaO–(23.7 − x)Na₂O glasses (x = 0, 12.4, and 23.1) with fixed P₂O₅ and SiO₂ contents were primarily focused upon.

P₂O₅–Na₂O–CaO glass has been widely studied, especially its structure and chemical durability.^1,10–13 However, only a few attempts have been made to modulate its solubility by introducing ^[6]Si. Silicon plays an effective role in bone formation. In this study, we studied the surroundings of ^[6]Si in P₂O₅–SiO₂–Na₂O glass MD simulations and experimentally examined the solubility of glasses with different ^[6]Si and Q_Pⁿ distributions by varying the amounts of Na₂O and CaO.

2 Experimental section

2.1 Modeling of P₂O₅–SiO₂–Na₂O glass by classical MD simulation

In our previous report, we developed a 55.0P₂O₅–21.3SiO₂–23.7Na₂O (mol%) glass model using a classical MD program (DL_POLY).¹⁴ Details of the simulation can be found in our previous report.⁶ A system consisting of 510 atoms was melted at 1900 K for 100 ps and then cooled rapidly to 300 K at a rate of 2.0 K ps⁻¹. An NVT ensemble with a Nosé–Hoover thermostat¹⁵ was used. The bond angles of O–Si–O and O–P–O bonds were controlled using a three-body screened harmonic potential.^16,17 To reproduce a four-coordinated Si structure (^[4]Si) and ^[6]Si, the potential of θ₀ = 109.47° and k_b = 250 eV rad⁻¹ was assigned to the number of Si atoms corresponding to ^[4]Si with reference to the coordination number distribution revealed by ²⁹Si magic-angle-spinning-nuclear-magnetic-resonance (MAS-NMR) spectroscopy. The classical MD model was relaxed by density functional theory (DFT) calculations.^18,19 We employed the projector augmented-wave (PAW) method^20,21 with the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof functional²² for the exchange–correlation energy functional, as implemented in the Viena Ab initio Simulation Package code (VASP).^21,23 We used only a single k-point (Gamma point) and plane waves with energies of up to 400 eV. Full structural optimization was performed using a conjugate-gradient method²⁴ until the forces became smaller than 10 meV Å⁻¹. No large structural changes, such as bond recombination, were observed. For the electronic structure analysis, we used a model consisting of 198 atoms prepared under similar conditions.

2.2 Glass sample preparation

55.0P₂O₅–21.3SiO₂–xCaO–(23.7 − x)Na₂O glasses (mol%; x = 0, 12.4, and 23.7 for samples PSi–Na, PSi–NaCa, and PSi–Ca, respectively) were prepared. H₃PO₄ (85.0%, solution), SiO₂ (99.0%), NaH₂PO₄ (99.0%) were purchased from Kishida Chemical, Osaka, and CaHPO₄·2H₂O (98.0%) was purchased from Fujifilm Wako Pure Chemical, Osaka. The reagents were poured mixed with distilled water (DW) in a Pyrex® beaker to form a slurry. The slurry was stirred and then dried overnight under an infrared lamp to obtain the batch mixtures. Thereafter, the products were melted in a platinum crucible in an electric furnace at 1200 °C for 30 min under atmospheric conditions, after which they were cast onto a stainless-steel plate and subjected to iron-press quenching to obtain the glass samples. The compositions of the samples were analyzed by energy-dispersive spectrometry (EDX, JED-2300, JEOL) (Table 1). The compositions of the resulting glass samples were comparable to their nominal compositions.

Table 1 Nominal and analyzed glass compositions. The analyzed compositions are shown in parentheses with their standard deviations

Glass code	Composition (mol%)
	P2O5	SiO2	Na2O	CaO
PSi–Ca	55.0 (54.0 ± 1.1)	21.3 (19.3 ± 1.6)	—	23.7 (26.7 ± 0.7)
PSi–NaCa	55.0 (54.1 ± 1.2)	21.3 (21.2 ± 2.2)	11.3 (11.2 ± 0.7)	12.4 (13.5 ± 1.0)
PSi–Na	55.0 (55.5 ± 0.9)	21.3 (20.4 ± 1.1)	23.7 (24.1 ± 1.5)	—

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2.3 Spectroscopic analysis of the glasses

Raman spectroscopy (NRS-3300, JEOL, Tokyo) was performed using an Nd:YAG laser to examine the chemical bonding in the glass samples.

MAS-NMR (HNM-ECA A600II, JEOL, Tokyo) analysis was performed to examine the structure around P and Si atoms in the glass samples. ³¹P MAS-NMR was performed using a 3.2 mm probe with a Larmor frequency of 242.95 MHz, spinning at 20 kHz, under the conditions of a single-pulse experiment with a 1.1 μs width, 5.0 s recycle delay, and cumulated number of 256. Ammonium dihydrogen phosphate (NH₄H₂PO₄, 99%; Kishida Chemical, Osaka) was used as a reference at 1 ppm. ²⁹Si MAS-NMR was performed using an 8.0 mm probe with a Larmor frequency of 119.24 MHz, spinning at 6 kHz, a 5.0 μs pulse width, and a 120.0 s recycle delay. The accumulation was performed 120–360 times depending on the signal-to-noise ratio of the peak. The chemical shift was adjusted to 1.534 ppm for 4-dimethyl-4-silacyclopentane-1-sulfonic acid sodium salt (C₆H₁₃NaO₃SSi). For the ²⁹Si MAS-NMR measurement, glass samples containing 0.1 wt% MnCO₃ were prepared and used to shorten the relaxation time.

2.4 Number of ions dissolved in a tris-HCl buffer solution

A tris-HCl buffer solution (TBS) at pH 7.40 was prepared by dissolving 6.118 g of tris-hydroxymethylaminomethane [NH₂C(CH₂OH)₃, Kishida Chemical, Osaka] in 1 L of DW at 37 °C and adding 1 M HCl (Kishida Chemical, Osaka). Each glass sample was crushed using an alumina mortar and sieved to particle sizes of 125–250 μm. Next, 20 mg of the glass particles were immersed in 20 mL of TBS and stirred using an incubator shaker (KS4000i, IKA, Osaka) at 37 °C and a speed of 125 rpm for 3 days (n = 3). The concentrations of P⁵⁺, Si⁴⁺, Ca²⁺, and Na⁺ ions in the TBS after immersion were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES, ICPS-7000, Shimadzu, Kyoto). Although P and Si existed as phosphate and silicate ions in the solution, they were measured as P⁵⁺ and Si⁴⁺ ions, respectively, for the convenience of standard reagents.

3 Results

3.1 Structural analysis with a PSi–Na glass model

The Q_Pⁿ and Si-coordination number distributions of the simulated model are listed in Table 2. The Q_Pⁿ distribution was estimated assuming that Si acts as an NWF regardless of the coordination number. The network connectivity (NC),²⁵ calculated as the weighted average of the corresponding Q_Pⁿ distributions, was 2.86, which is close to the experimental value (2.80), indicating that ^[6]Si in the glass acts as an NWF. Although 8.3% of Q_P⁴ units, which were not observed in the experiment, were observed in the model, the presence of Q_P⁴ units has been reported in several glass systems,^26,27 and they could be present. However, considering that the amount of Q_P⁴ was small, the simulated and experimental values are comparable. The Si–O coordination number distribution showed 23.8% ^[4]Si, 9.5% ^[5]Si, and 70.4% ^[6]Si. These values are consistent with the experimental results, except for ^[5]Si, which was not quantitatively evaluated by ²⁹Si MAS-NMR.

Table 2 Q_Pⁿ distribution (%), network connectivity (NC), and Si–O coordination number distribution (%) from simulated and experimental results of PSi–Na

	QP^n distribution (%) and NC						Si–O coordination number distribution (%)
	QP^0	QP^1	QP^2	QP^3	QP^4	NC	^[4]Si	^[5]Si	^[6]Si
Sim.	0.0	0.0	23.1	68.5	8.3	2.86	23.8	9.5	66.7
Exp.	—	—	19.8	80.3	—	2.80	29.4	—	70.9

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The P–O and Si–O radial distribution functions (RDFs, g(r)) are shown in Fig. 1. The RDF of P–O shows two peaks at 1.47 and 1.58 Å, which are attributed to the P–NBO and P–BO bonds, respectively, and are consistent with X-ray diffraction results for 50P₂O₅–50Na₂O (mol%) glass.²⁸ The RDF of Si–O shows two peaks at 1.64 and 1.77 Å, respectively, which are attributed to the ^[4]Si–O and ^[6]Si–O bonds, respectively, and are consistent with X-ray absorption fine structure (XAFS) analysis results for R₂O–SiO₂–P₂O₅ (R = Li, Na, and K) glass.⁹

Fig. 1 (a) Phosphorus (P)–oxygen (O) and (b) silicon (Si)–O radial distribution functions (RDFs: g(r)) for the PSi–Na model.

The fractions of ^[4]Si–O–X and ^[6]Si–O–X (X = P, Si, and Na) bonds are shown in Fig. 2(a). The fractions of ^[4]Si–O–P and ^[4]Si–O–Si bonds were 0.85 and 0.15, respectively, which are consistent with the fractions of P and Si in the glass (P : Si = (55.0 × 2) : 21.3), indicating that the phosphate and silicate groups are randomly coordinated to ^[4]Si. On the other hand, the fractions of ^[6]Si–O–P and ^[6]Si–O–Si bonds were 0.96 and 0.04, respectively, indicating that the phosphate group is preferentially coordinated to ^[6]Si. Fig. 2(b) shows the results divided by the Q_Pⁿ units bound to Si in Fig. 2(a). For the ^[4]Si–O–P bond, Q_P² : Q_P³ : Q_P⁴ = 17.6 : 76.5 : 5.9, which is consistent with the ratios estimated from the Q_Pⁿ distribution in the model (Table 2). On the other hand, for the ^[6]Si–O–P bond, Q_P² : Q_P³ : Q_P⁴ = 4.9 : 79.0 : 16.0, with Q_P³ showing higher values, indicating that Q_P³ units are preferentially coordinated to ^[6]Si.

Fig. 2 Bridging-type distribution probability of Q_Pⁿ species estimated from a PSi–Na glass model. (a) Si–O–X (X = P, Si, and Na) and (b) Si–O–P bonds.

Fig. 3 shows an example of a coordination model around the SiO₆ octahedron. The Na⁺ is located 4.2 Å away from ^[6]Si and interacts electrostatically with BO in the ^[6]Si–O–P bond at an interatomic distance of 2.8 Å.

Fig. 3 Coordination environment around ^[6]Si. The numerical values indicate interatomic distances. (Cluster views are provided for a better understanding.) Color legend: P (purple), Si (ivory), Na (yellow), and O (red).

Fig. 4 shows the electron densities between the O–Si and O–P atoms in the Si–O–P bond. The horizontal axis represents the distance from BO in the bond direction. Electrons between the O–^[4]Si atoms in the ^[4]Si–O–P bond are strongly attracted toward O. However, since the electronegativities of Si and P are close, their distributions are similar. A similar trend was observed between the O–^[6]Si atoms forming ^[6]Si–O–P bonds. However, the distribution differs from that between O–P atoms, showing a strong distribution toward the O side. These results indicate that the ^[6]Si–O bond is more ionic than the ^[4]Si–O bond. To confirm this, we evaluated the Born effective charges in SiO₂ crystals via first-principles force calculations under an electric field,²⁹ similar to our previous study.³⁰ Born effective charges for ^[4]Si and O in alpha-quartz SiO₂ are 3.46 and −1.73, and those for ^[6]Si and O in stishovite SiO₂ are 4.04 and −2.02.

Fig. 4 Electron densities between O–P and O–Si atoms in (a) ^[4]Si–O–P and (b) ^[6]Si–O–P bonds. (Top): Schematics of the isosurface of the total electron density; (bottom): electron density profiles as a function of distance from oxygen.

3.2 Spectroscopic analysis of the glass structures

Fig. 5 shows the Raman spectra of the glass samples. They show peaks corresponding to P–O–P symmetric vibration around 700 cm⁻¹ ((POP)_sym.),^{31,32 [4]}Si–O–^[4]Si asymmetric vibration around 1100 cm⁻¹ ((SiOSi)_asym.),^{33 [4]}Si–O–^[4]Si symmetric vibration around 1170 cm⁻¹, O–P–O symmetric vibrations of NBO ((PO₂)_sym.),^31,32 O–P–O asymmetric vibrations with NBO ((PO₂)_asym.),³⁴ and P=O symmetric vibrations with NBO ((P=O)_sym.)^31,32 around 1280 cm⁻¹. Peaks related to ^[6]Si are observed around 740, 1200, and 1350 cm⁻¹.^35,36 The peaks originating from NBO vibrations ((PO₂)_sym., (PO₂)_asym., and (P=O)_sym.) blue-shifted with the addition of CaO, attributed to the weakening of the P–NBO interaction due to the replacement of Na⁺ ion coordinating to NBO with Ca²⁺ ion, which has a higher electronegativity. The peaks related to ^[6]Si (1200 and 1350 cm⁻¹) red-shifted with the addition of Na₂O.

Fig. 5 Laser Raman spectra of the glass samples.

Fig. 6 shows the ³¹P and ²⁹Si MAS-NMR spectra of the glass samples. The ³¹P MAS-NMR spectra, which originate from the Q_P² and Q_P³ units, were well fitted by two Gaussian functions. The ²⁹Si MAS-NMR also showed peaks originating from ^[4]Si at approximately −120 ppm and ^[6]Si at approximately −210 ppm. The peak near −160 ppm is ascribed to five-fold-coordinated Si,^37–39 and it was excluded from the quantitative evaluation since it overlaps with the spinning sideband. Table 3 lists the percentages of the structures estimated from the peaks. The Q_P³ unit and ^[6]Si increased with an increase in the amount of Na₂O.

Fig. 6 (a) ³¹P and (b) ²⁹Si MAS-NMR spectra of the glass samples.

Table 3 Q_Pⁿ and Si–O coordination number distributions (%) estimated from the ³¹P and ²⁹Si MAS-NMR spectra

Glass code	QP^n distribution (%)		Si–O coordination number distribution (%)
	QP^2	QP^3	^[4]Si	^[6]Si
PSi–Ca	65.4	34.6	62.1	37.9
PSi–NaCa	27.6	72.4	43.7	56.3
PSi–Na	19.7	80.3	29.1	70.9

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3.3 Dissolution of the glasses in a TBS

Fig. 7 shows the percentage of ions dissolved in TBS, which is the ratio of the dissolved ions from the glass to the ion amount in the glass before immersion. The dissolution of the NWF components (P⁵⁺ and Si⁴⁺) and the NWM components (Na⁺ and Ca²⁺) increased almost monotonically with time for all glasses. The changes were larger for PSi–Na, PSi–Ca, and PSi–NaCa, in that order. In all samples, the ionic dissolution behavior of the NWF and NWM components were similar and judged to be congruent: almost all of PSi–Na was dissolved after 48 h of immersion. Uo et al. reported that binary P₂O₅–(100 − a)Na₂O (mol%, a = 50–80) glass samples dissolved rapidly within 5 h of immersion in DW; thus, the dissolution of the glass in this study was considerably controlled.¹ Furthermore, the solubility of PSi–NaCa is suppressed more effectively than that of PSi–Ca, whereas that of P₂O₅–CaO glasses without SiO₂ (ref. 3 and 23) is suppressed with an increase in the CaO content.

Fig. 7 Percentages of ions released in TBS relative to the total amount in the glass samples. Error bar shows the standard deviation.

4 Discussion

4.1 Characteristic structures of glass samples containing six-fold-coordinated silicon

³¹P and ²⁹Si MAS-NMR analyses revealed increases in the formation of Q_P³ units and ^[6]Si with the addition of Na₂O. Analysis of the PSi–Na glass model also suggested that ^[6]Si acts almost exclusively as NWF. These results may indicate that the increase in BO due to the six-fold coordination of Si significantly influences the formation of Q_P³ units.

The ^[6]Si in the glass model preferentially coordinates to the Q_P³ unit, and the sharpness of ^[6]Si peaks in the ²⁹Si MAS-NMR spectra could be attributed to this coordination environment, which is consistent with the experimental structural analysis reported by Ren et al.³⁷ As shown by the RDF of the Si–O bond (Fig. 1(b)), the average of the ^[6]Si–O interatomic distance is larger than that of the ^[4]Si–O interatomic distance. Since the electronegativity of P is higher than that of Si, the Si–O interatomic distance would likely be elongated in an environment surrounded by phosphate groups, which favors the formation of ^[6]Si.

As shown in Fig. 4, the BO in the P–O–^[6]Si bond generates strong electrostatic interaction with Na⁺ ions due to the high electron density around the bond. In the case of the Na₂O-containing glass samples, the redshifts of the Raman peaks related to ^[6]Si (1200 and 1350 cm⁻¹) are most likely due to the influence of NWM ions present near the SiO₆ octahedron.

Considering the Na⁺ ions around ^[6]Si (within 4.5 Å), as shown in an example in Fig. 3, only one Na⁺ ion was identified in this model. Miyabe et al.⁴⁰ proposed a structure with two Na⁺ ions within 3.5–3.8 Å around ^[6]Si as charge compensation based on molecular orbital cluster simulation. However, their simulation was based on the assumption that the phosphate groups coordinating to ^[6]Si are terminated with protons without considering the atomic interactions in the range of medium to long distances, which is different in thus study. Zeng et al.^36,41 reported that the Q_Pⁿ distribution in glasses containing ^[6]Si strongly depends on the number of NWM ions, and the fraction of ^[6]Si changes after aging slightly below their glass transition temperature (T_g). Thus, they rejected the structure of SiO₆ octahedra with two Na⁺ ions as charge compensation. ^[6]Si forms in binary P₂O₅–SiO₂ glasses containing no NWM ions.^42,43 Based on these results, we conclude that, although Na⁺ ions have a strong tendency to coordinate around BO in ^[6]Si–O–P bonds via electrostatic interactions, its number is not limited to two.

4.2 Relationship between glass structure and the ionic dissolution behavior

As shown in Fig. 7, the solubility of the samples decreased in the order of PSi–Na, PSi–Ca, and PSi–NaCa. In PSi–Na, all Ca²⁺ ions in PSi–Ca and PSi–NaCa were replaced by Na⁺ ions with lower field strength. Thus, the PSi–Na structure is more open than those of PSi–Ca and PSi–NaCa, resulting in the higher solubility of PSi–Na. The solubility of PSi–NaCa was reduced compared to that of PSi–Ca. Ahmed et al.¹² reported that the solubility of P₂O₅–Na₂O–CaO glasses (45 ≤ P₂O₅ ≤ 60 (mol%)) increases with increasing Na₂O content. The coordination state of Si could influence the difference in the dissolution behavior of the glass samples herein from that reported in the previous study.

In the PSi–Na glass model, ^[6]Si coordinates preferentially to the Q_P³ unit. Wazer et al.⁴⁴ reported that the Q_P³ unit is easily hydrolyzed by H₂O since the electron distribution around P is localized between P–NBO bonds. In this study, not only the P–O–P bond but also P–O–^[4]Si and P–O–^[6]Si bonds are present in the glass samples, and the electron density distribution (Fig. 4) shows that the ^[6]Si–O bond is highly ionic. Since the electron distribution of P in the ^[6]Si-coordinated Q_P³ unit would be more delocalized compared to that in the P- or ^[4]Si-coordinated Q_P³ units, hydrolysis is reduced. As described in section 4.1, the presence of Na⁺ ions facilitates the ^[6]Si formation; in PSi–NaCa, the incorporation of Na⁺ ions is considered to increase the ^[6]Si content, resulting in the controlled solubility.

5 Conclusions

We investigated the structure of P₂O₅–SiO₂–Na₂O–CaO glass via theoretical simulation and spectroscopy. We found that ^[6]Si contributes to the formation of the glass network, producing Q_P³ units with a stable electronic configuration, and ^[6]Si–O bonds are more ionic than ^[4]Si–O and P–O bonds. Q_P³ units are preferentially coordinated to ^[6]Si, and Na⁺ ions easily coordinate around ^[6]Si–O–P bonds by interacting with O in the ^[6]Si–O–P bond. These results show that incorporating ^[6]Si into P₂O₅–SiO₂–Na₂O–CaO can alter the electronic state of the phosphate groups and the coordination state of the NWM ions. The solubility of the glass samples in a TBS varied nonlinearly with the Na₂O content, indicating that the formation of ^[6]Si could suppress the hydrolysis of the Q_P³ units and Na⁺ ion diffusion. Controlling phosphate glass structure using ^[6]Si is, therefore, an effective technique for tuning the chemical dissolution of the glass. In P₂O₅–SiO₂–Na₂O–CaO glass, the ^[6]Si content and Q_Pⁿ distribution can be controlled by balancing Na₂O and CaO contents.

Author contributions

All authors contributed to the writing of this manuscript and have approved its final version. K. T.: data curation, formal analysis, visualization, and writing – original draft; T. T.: methodology and writing – review and editing; T. K.: conceptualization, methodology, and writing – review and editing.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This work was partly supported by JSPS KAKENHI grants, #20H00304 and #22H01808. The authors would like to thank Enago (https://www.enago.jp) for the English language review.

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