Cation Field Strength Effects on Boron Coordination in Binary Borate Glasses

The field strength of modifier cations in boron-containing oxide glasses has important but complex effects on boron coordination, and has long been known to have major effects on glass and liquid properties. With well-constrained compositional and fictive temperature information in three binary borate glass series, we report how different modifier cations (Na+, Ba2+, Ca2+) affect boron coordination (B-11 MAS NMR), as well as glass transition temperatures and configurational heat capacities (DSC). Using estimated reaction enthalpies for converting a B-[4] to a B-[3] with an NBO from previous studies, we compare boron coordinations in glasses with different modifier cations on an isothermal basis. Temperature and modifier cation effects can thus be isolated. At low modifier contents [R=(Na-2,Ca,Ba)O/B2O3<0.45], N-4 is systematically higher in the order Na>Ba>Ca, suggesting the enhanced stabilization of NBO for the divalent cations, especially for the smaller Ca2+. At higher R values, N-4 for Na borates drops below values for Ca and Ba borates. The trend in N-4 with modifier field strength reverses at high R values (similar to R>0.7), with Ca>Ba>Na. The transition may be related to the enhanced stabilization of B-[4]-O-B-[4] groups by higher field strength cations in NBO-rich glasses in which boron is the primary network component.