Network connectivity in cesium borosilicate glasses: 17O multiple-quantum MAS and double-resonance NMR

17O solid-state NMR was used to study oxygen connectivity in ternary cesium borosilicate glasses. 17O multiple-quantum magic-angle spinning (MAS) NMR and ultrahigh-field 17O MAS NMR provide resolution of non-bridging and bridging oxygens, permitting their quantification for modeling short-range order. 17O{11B} rotational-echo double-resonance (REDOR) NMR is used to verify peak assignments for B–O–B, Si–O–B and Si–O–Si species, which agree well with a body of data from other alkali borosilicate glasses. The resulting bridging oxygen populations are in poor agreement with a model based on random network mixing, suggesting some degree of macroscopic segregation into silica-rich and borate-rich phases. Electron microprobe analysis reveals significant compositional variation between the batch and final products due to cesium volatilization at high temperatures. Both phase separation and elemental boil-off must be considered to account fully for the 17O MAS NMR results.