Tricluster oxygen atoms in crystalline and glassy SrB4O7: High resolution 11B and 17O nuclear magnetic resonance analysis

We present 11B and 17O magic-angle spinning (MAS) and multiple quantum (MQMAS) nuclear magnetic resonance (NMR) data for crystalline strontium tetraborate (SrB4O7) and the glass with corresponding SrO·2B2O3 composition. Crystalline SrB4O7 contains a tricluster oxygen site (3 BO4 neighbors) and can be used as a model compound for observing tricluster oxygen sites in the corresponding glass. NMR site parameters for the tricluster are distinguishable from those of the bridging oxygen atoms in SrB4O7, allowing for visible areas in the 17O spectrum attributed solely to the presence of tricluster oxygen atoms. By overlapping the spectra of the crystalline SrB4O7 and the corresponding glass there are indications for a maximum of a few percent of tricluster oxygen atoms within the glass. This is further confirmed by calculations of expected non-bridging oxygen (NBO) content without triclusters, done using 11B MAS NMR and analyzed composition, in comparison to directly measured NBO content in the SrO·2B2O3 glass, which suggests that up to 2% of the total oxygen atoms within this glass are in triclusters.