Structure/property correlations in ion-conducting mixed-network former glasses:: Solid-state NMR studies of the system Na2O-B2O3-P2O5

The structural organization of sodium borophosphate glasses with composition (Na2O)(0.4)[(B2O3)(x)(P2O5)(1-x)](0.6) (0.0 <= x <= 1.0) has been investigated by B-11 and P-31 magic-angle spinnning (MAS) NMR spectroscopy. Spectral deconvolutions and established chemical shift trends yield a detailed quantitative account of the local structural units present in these glasses. These units can be described in terms of coordination polyhedra P-mB((n)) and B-mP((n)), where n reflects the number of bridging oxygen atoms and mB <= n and mP <= n are the number of connected boron and phosphorus species, respectively. The favorable interaction between the two network formers boron oxide and phosphorus oxide results in the dominant formation of B-O-P linkages. Compared to binary sodium phosphate and borate glasses, the extent of network polymerization is significantly increased, particularly within the region 0 <= x <= 0.4, caused by the preferential formation of four-coordinate boron species linked to phosphorus. Glasses with higher boron contents also contain three-coordinated BO3/2 units, which appear to interact only weakly with phosphorus. The NMR spectra can be analyzed in terms of the concentration of bridging oxygen atoms per network former species, [O], and divided into the quantitative contributions from P-O-P, P-O-B, and B-O-B linkages. [O] reveals a nonlinear compositional trend showing an excellent correlation with macroscopic properties such as glass-transition temperatures, densities, and ionic-conductivity parameters.