Structure/Property Correlations in Ion-Conducting Mixed-Network Former Glasses:  Solid-State NMR Studies of the System Na₂O−B₂O₃−P₂O₅

Abstract

The structural organization of sodium borophosphate glasses with composition (Na 2 O) 0.4 [(B 2 O 3 ) x (P 2 O 5 ) 1 - x ] 0.6 (0.0 ≤ x ≤ 1.0) has been investigated by 11 B and 31 P 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 ( n ) m B and B ( n ) m P, where n reflects the number of bridging oxygen atoms and m B ≤ n and m P ≤ 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 BO 3/2 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.