A 11B and 31P MAS NMR study of the impact of Ca2+ and Sr2+ network modifying cations on the structure of borate and borophosphate glasses

A detailed qualitative and quantitative analysis of the short and medium range order of alkaline earth binary borate and ternary borophosphate glasses has been carried out through the use of B-11 and P-31 magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. In order to understand the charge compensation characteristics of B and P within these glasses, a series of calcium and strontium borate and borophosphate glasses were prepared with a constant P2O5:B2O3 ratio of 0.4 and varying alkaline earth loading contents. For the alkaline earth borate glasses with loading values up to R=0.4 (where R is the ratio between alkaline earth oxide and boron trioxide), the glass network preferentially forms negatively-charged four-coordinated borate units to balance the cationic charge of the alkaline earth network modifier. At higher alkaline earth loading values of R>0.5, there is a mixture between three- and four -coordinated borate species balancing the modifier charge. On the other hand, investigation of the alkaline earth borophosphate glass series indicates that these glasses form a complex network, with depolymerisation of the phosphate species while maintaining medium range connectivity of the borate units within the amorphous solid. At low alkaline earth loadings, characteristic of BPO4, there is clustering, while at high loading values, there are bridges from negatively-charged phosphate units to four-coordinated boron species, and a complex mixed former oxide network is formed. Ultrahigh field B-11 MAS NMR helps to determine the varying degrees of ring and non-ring borate species present within these glasses, and confirms the unique structures present in both the binary borate and ternary borophosphate series. The P-31 MAS NMR data suggest that calcium prefers a phosphate chemical environment at high R, whereas strontium appears to maintain a higher fraction of four-coordinated borate species over the same composition range.