The Mixed-Network Former Effect in Phosphate Glasses: NMR and XPS Studies of the Connectivity Distribution in the Glass System (NaPO3)1−x(B2O3)x

The structural organization of sodium borophosphate glasses with composition (NaPO 3 ) 1− x (B 2 O 3 ) x (0.0 ≤ x ≤ 0.3) has been investigated by X-ray photoelectron spectroscopy (XPS), as well as single- and double-resonance 11 B and 31 P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. O-1s XPS data provides a quantitative distinction between B−O−B, B−O−P, and P−O−P linkages as well as nonbridging oxygen atoms. 11 B and 31 P MAS NMR data indicate that within the compositional region 0 ≤ x ≤ 0.20 the entire boron inventory is present in the form of anionic BO 4 − units, resulting in the repolymerization of an equivalent fraction of the phosphate units (conversion of anionic metaphosphate (P (2) ) into neutral branching groups (P (3) ) species. Both XPS as well as 31 P{ 11 B} and 11 B{ 31 P} rotational echo double resonance (REDOR) NMR results reveal strong interactions between the two network formers boron oxide and phosphorus oxide, resulting in the dominant formation of B−O−P linkages. In addition, the shape of the REDOR curve reveals a certain tendency of these linkages to cluster, consistent with a preference of P (3) units to form more than one P−O−B linkage, even at low boron contents. The enhanced degree of network polymerization correlates with a significant increase of the glass transition temperature as a function of boron content.