Crystal chemical characterization of mullite-type aluminum borate compounds

Al-rich aluminum borates were prepared by different synthesis routes using various Al/B ratios, characterized by diffraction methods, spectroscopy and prompt gamma activation analysis. The ¹¹B NMR data show a small amount of BO₄ species in all samples. The chemical analysis indicates a trend in the Al/B ratio instead of a fixed composition. Both methods indicate a solid solution Al_5−xB_1+xO₉ where Al is substituted by B in the range of 1–3%. The structure of B-rich Al₄B₂O₉ (C2/m, a=1488 pm, b=553 pm, c=1502 pm, ß=90.6°), was re-investigated by electron diffraction methods, showing that structural details vary within a crystallite. In most of the domains the atoms are orderly distributed, showing no signal for the postulated channel oxygen atom O5. The absence of O5 is supported by density functional theory calculations. Other domains show a probable disordered configuration of O5 and O10, indicated by diffuse scattering along the b direction. - Graphical abstract: Projections of three-dimensional electron diffraction space of Al₄B₂O₉ along the main directions. - Highlights: • The crystal structure of Al₄B₂O₉ was re-evaluated. • Structural details vary among different crystals and inside Al₄B₂O₉ crystallites. • Diffuse scattering indicate a probable disordered configuration of O5 and O10. • A solid solution series for Al_5−xB_xO₉ is indicated by PGAA and NMR spectroscopy. • The presence of BO₄ groups is confirmed by ¹¹B MAS NMR spectroscopy for Al_5−xB_1+xO₉.