Doping of low-Tg phosphate glass with Al2O3, B2O3 and SiO2: Part II- insertion mechanism of Al2O3 and B2O3 in phosphate network characterized by 1D/2D solid-state NMR

In this work, the structural modifications induced by introducing small amounts (1-4 mol%) of Al2O3 and B2O3 in low-Tg pyrophosphate glass composition have been investigated using advanced solid-state 1D/2D MAS-NMR techniques. The 31P{X} D-INEPT sequence was used to establish the heteronuclear correlations between the phosphate moieties (31P) and doping elements (X = 27Al or 11B). The X{31P} REDOR and X/X DQ-SQ NMR techniques were implemented to find out the exact nature and chemical environment adopted by the doping elements within the glass matrix. The NMR analysis confirmed that majority of Al3+ ions entered into the phosphate network as six-coordinated species and all the B3+ ions entered as four-coordinated units. The combined NMR analyses revealed different insertion pathways of Al3+ ion and B3+ ion into the phosphate network. The findings of the study suggest that Al3+ ions attach purely with non-bridging oxygen of the phosphate network without breaking additional P-O-P bonds whilst B3+ ions interact with the network by breaking P-O-P linkages highlighting different chemical natures of these doping elements towards the interaction with phosphate units.