Intermediate range order in barium aluminoborate glasses from heteronuclear correlation NMR experiments

A series of barium aluminoborate glasses have been investigated by single and double resonance magic angle spinning (MAS) nuclear magnetic resonance (NMR) techniques. ¹¹B MAS NMR spectra indicate that boron is present in three and four coordinate environments, while ²⁷Al MAS NMR spectra show that aluminium is present in four, five and six coordinate environments (four coordinated aluminium being the predominant aluminium species). Glasses with high BaO content exhibit resonances from two types of trigonal boron species in units containing either two or three bridging oxygens. Structural correlations between ²⁷Al and ¹¹B nuclei have been probed using ¹¹B{Al} and ²⁷Al{¹¹B} transfer of populations in double resonance (TRAPDOR) NMR spectroscopy. Results obtained from TRAPDOR NMR reveal short range couplings between tetrahedral boron and aluminium sites and the tetrahedral to trigonal boron ratio obtained from ¹¹B{²⁷Al} TRAPDOR is higher than that found from quantitative one dimensional ¹¹B NMR.