The structure of aluminophosphate glasses revisited: Application of modern solid state NMR strategies to determine structural motifs on intermediate length scales

In this paper we report a reinvestigation of the network organization in aluminophosphate glasses. Samples were prepared along the 50K2O–xAl2O3–(50 − x)P2O5 composition line with 0 < x < 20%. The evolution of the glass structure upon alumina incorporation into the phosphate network is monitored employing a combination of advanced solid state NMR strategies including 31P{27Al}-CP-HETCOR NMR, 31P{27Al}-REAPDOR NMR, 27Al{31P}-REDOR NMR and 2D 31P-J-RESolved NMR spectroscopy. Details of the employed NMR protocol are given elsewhere. The network organization on intermediate length scales as a function of the alumina content is then described employing a modified Q notation, Qm,AlOxn, where n denotes the number of connected tetrahedral phosphate, m gives the number of aluminate species connected to a central phosphate unit and x specifies the nature of the bonded aluminate species (i.e., AlO4-, AlO5- or AlO6-species).