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The crystal structure of Al 4 B 2 O 9 , synthesized from Al(NO) 3 ·9H 2 O and B(OH) 3  via a sol-gel process, is studied and characterized by Rietveld refinements and grid search analyses combined with  11 B and  27 Al MAS NMR spectroscopy. The aluminum borate with a unit-cell composition of Al 32 B 16 O 72  is closely related to the boralsilite (Al 32 B 12 Si 4 O 74 ) structure with Si replaced by B and to mullite (Al 4+2x Si 2-2x O 10-x ). It crystallizes in the monoclinic space group C2/m, a = 14.8056(7) Å, b = 5.5413(2) Å, c = 15.0531(6) Å, β = 90.913(2)°, Z = 8 for Al 4 B 2 O 9 . The main structural units are isolated chains of edge-sharing AlO 6 -octahedra running parallel to b that is a characteristic feature of the mullite-type crystal structures. The octahedral chains are crosslinked by AlO 4 , AlO 5 , BO 3 , and BO 4  groups with two B atoms and one O atom (O5′) disordered on interstitial positions. 27Al and 11B NMR studies confirm the presence of sixfold (octahedral), fivefold, and fourfold (tetrahedral) coordinated Al (sixfold:[fourfold + fivefold] = ~50%:50%) and of threefold and fourfold coordinated B (~80%:20%).

Crystal structure of synthetic Al 4 B 2 O 9 : A member of the mullite family closely related to boralsilite