Crystal structure of synthetic Al₄B₂O₉: A member of the mullite family closely related to boralsilite

Abstract

The crystal structure of Al₄B₂O₉, synthesized from Al(NO)₃·9H₂O and B(OH)₃ via a sol-gel process, is studied and characterized by Rietveld refinements and grid search analyses combined with ¹¹B and ²⁷Al MAS NMR spectroscopy. The aluminum borate with a unit-cell composition of Al₃₂B₁₆O₇₂ is closely related to the boralsilite (Al₃₂B₁₂Si₄O₇₄) structure with Si replaced by B and to mullite (Al_4+2xSi_2-2xO_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 Al₄B₂O₉. The main structural units are isolated chains of edge-sharing AlO₆-octahedra running parallel to b that is a characteristic feature of the mullite-type crystal structures. The octahedral chains are crosslinked by AlO₄, AlO₅, BO₃, and BO₄ 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%).