Quantitative determination of the phosphorus environment in lithium aluminosilicate glasses using solid-state NMR techniques

Abstract

We investigated using solid-state NMR spectroscopy the short-range structural features in lithium aluminosilicate glasses with the addition of P₂O₅ and considering various Al₂O₃/Li₂O ratios. The phosphorus environment is determined quantitatively using ³¹P Magic Angle Spinning NMR constrained by results obtained from ³¹P–²⁷Al Multiple-Quantum Coherence-based NMR techniques. Phosphorus is mainly located as orthophosphate and pyrophosphate species in glasses with a low amount of Al₂O₃. These depolymerized units disappear with increasing Al₂O₃ content and a strong affinity of PO₄ tetrahedra for aluminum is revealed, which reduces phase separation. The local environments of framework (Si and Al) and charge-balancing (Li) cations are also studied through NMR experiments to assess the influence of P₂O₅ addition. The Si environment is mostly modified by the presence of P₂O₅ in glasses containing a low amount of Al₂O₃, with an increase of Q⁴_Si species in relation to phase separation phenomena observed in these compositions. Conversely, P₂O₅ addition does not have a significant influence on the ²⁷Al NMR response. ⁷Li NMR spectra reflect a change in the structural role of Li when P₂O₅ or Al₂O₃ is added. The observed structural changes can be rationalized to improve our knowledge of the structure–property relationships, focusing, in particular, on phase separation and nucleation/crystallization processes that are strongly affected by the presence of P and the evolution of its local environment with composition.