IntroductionThe chemical stability of phosphate glass is vital for its applications in various fields, i.e., biomedicine, laser gain media, sealing, and high-level nuclear waste solidification. Alkali metals affect the chemical stability of phosphate glass. Some studies indicate that alkali metals with a higher ionic field strength can enhance the chemical stability of glass via reinforcing the ionic crosslinking between alkali ions and non-bridging oxygen atoms. However, a few studies indicate that certain phosphate glasses containing low-field strength alkali metals exhibit a better chemical stability, compared to the glasses containing high-field strength alkali metals. These results cannot be explained based on the simple field strength. It is thus necessary to further investigate the influence of alkali metals on the chemical stability of phosphate glass. In this study, we utilized various advanced solid-state nuclear magnetic resonance (SSNMR) techniques to analyze the network structure of the aluminum phosphate glasses containing different alkali metals in an atomic scale. The intrinsic correlation among alkali metals, glass structure, and chemical stability was discussed.MethodsThe glasses with molar compositions of [xNa₂O-yK₂O-(41.6-x-y)Cs₂O] -16.7Al₂O₃-41.7P₂O₅ were synthesized by melt quenching methods. The values of (x, y) used were (41.6, 0), (0, 41.6), (20.8, 0), and (0, 20.8), respectively. The raw materials required for glass preparation were Al(OH)₃ (purity>99%), Al(PO₃)₃ (purity>99%), NaPO₃ (purity>95.0%), KPO₃ (purity>99%,), Cs₂CO₃ (purity>99%), and NH₄H₂PO₄ (purity>99%). Each batch, consisting of 20 g of mixed raw materials, was loaded into a platinum crucible and subjected to a high-temperature furnace at 1250-1300 ℃ for 30 min to melt raw materials. Thereafter, the melt was poured onto a steel plate and quickly pressed to obtain a bulk glass.The chemical stability of these glasses was assessed by the Product Consistency Test B (PCT-B) method. 1 g of dried glass powder with particle sizes of 75-150 μm was mixed with 10 mL of neutral deionized water in a sealed polytetrafluoroethylene (PTFE) container. The container was then placed in a constant temperature environment at 90 ℃ for 7 d. After the corrosion experiments, the concentrations of leached elements in the leachates were measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), thus calculating the normalized mass loss (NL) of each element within the different glasses.All the SSNMR measurements were performed on a model Bruker Avance III HD 500 MHz spectrometer in a magnetic field of 11.7 T at room temperature. The measurements included ²⁷Al magic-angel spinning (MAS), ³¹P MAS, ²⁷Al{³¹P} rotational echo double resonance (REDOR), ³¹P{²⁷Al} rotational echo adiabatic passage double resonance (REAPDOR), and ³¹P 1D refocused INADEQUATE.The Raman spectra were determined by a model Renishaw inVia spectrometer, with a laser wavelength of 488 nm.Results and discussionThe NL of phosphorus (P) in the glasses containing alkali metals Na, K, and Na/Cs are 0.7, 15.2, and 21.1 g/m², respectively. The glass containing K/Cs exhibits a complete corrosion without detectable leachate obtained. These findings indicate that the chemical stability of the aluminum phosphate glasses with varying alkali metal compositions follows a decreased order of Na, K, Na/Cs, K/Cs. The P units within the different glasses are predominantly Q¹ units (Qⁿ, “n” represents the number of P-O-P bonds per P unit), indicating a consistent proportion of P—O—P bonds in these glasses. The variation in alkali metals does not affect the P—O—P bonds within the glasses. However, the variation in alkali metals significantly affects the Al species within the glasses. Specifically, the glasses containing alkali metals with a high ionic field strength exhibit a higher proportion of high-coordinated Al. In these glasses, the Al species with different coordinations are exclusively surrounded by phosphorus oxygen tetrahedra (PO₄), indicating that all Al species exist solely in the form of Al—O—P bonds. The compactness of the glass network increases with the proportion of Al—O—P bonds. The compact glass network can impede the dissolution and leaching of glass elements, as well as the diffusion of hydrogen species from solution into glass, thereby enhancing the chemical stability of the glass. The compactness of the glass network can be characterized by the reciprocal of the molar volume (1/V_mol). A higher value of 1/V_mol indicates a greater number of atoms per unit volume, corresponding to a higher compactness of the glass network. The values of 1/V_mol (×10⁴ mol/L) in these glasses containing alkali metals Na, K, Na/Cs, and K/Cs are 2.6, 2.2, 2.1, and 2.0, respectively. Correspondingly, the proportions of Al—O—P bonds are 54.5%, 52.6%, 50.7%, and 50.5%, respectively. A decreasing order of Na, K, Na/Cs, K/Cs is consistent with a decreasing order of the chemical stability. These results provide an evidence that the proportion of Al—O—P bonds increases as the ionic field strength of alkali metal increased, resulting in an enhanced compactness of the glass network and improved chemical stability. Furthermore, it is noteworthy that the proportion of Al-O-P bonds in the glass containing Na/Cs is comparable to that in the glass containing K/Cs. The former demonstrates a denser glass network. This can be attributed to the smaller ionic radius and higher ionic field strength of Na, compared to K. The smaller ionic radius results in narrower structural gaps within the glass network, while the higher ionic field strength enhances its electric field attraction to non-bridging oxygen.ConclusionsThe chemical stability of the aluminum phosphate glasses containing different alkali metals decreased in an order of Na, K, Na/Cs, K/Cs. The chemical stability increased as the ionic field strength of alkali metals increased. The change in alkali metals affected the compactness of the glass network via modulating the electric field attraction to non-bridging oxygen, and substantially modified the network structure of the glass. In these glasses, the Al species with varying coordination numbers were exclusively surrounded by phosphorus oxygen tetrahedra (PO₄). The proportion of high-coordinated Al increased with increasing the ionic field strength of alkali metals, resulting in an augmentation of the chemical bond connections between Al and PO₄. This structural adjustment further enhanced the compactness of the glass network, ultimately improving its chemical stability.