RESUMO
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 Q4Si 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.
