The crystal structure of boron-doped mullite (B-mullite) was studied by means of neutron powder diffraction and 11B solid state, magic-angle-spinning (MAS) NMR spectroscopy. The samples were prepared from single- phase gels consisting of aluminum nitrate nonahydrate, tetraethoxysilane, and boric acid, annealed at 1200 ºC. Boron resides in a planar, trigonally coordinated position crosslinking the octahedral chains perpendicular to the caxis. The B position was derived from a combination of Rietveld refinements with difference Fourier calculations and gridsearch analyses applied to a series of samples with varying B contents. Based on the sample with the highest B content the local BO3 configuration was determined by distance least squares (DLS) refinements resulting in split positions for two O atoms in the AlO6 octahedron which leads to a local distortion of the octahedral geometry. This model was verified by subsequent Rietveld refinements of several samples, details of the crystal structure are given for the sample with the highest B content. The model for B incorporation is corroborated by the MAS NMR results which clearly show that the 11B isotropic chemical shift (δiso) and quadrupolar coupling constant and asymmetry paramater (CQ, η) describe that of threecoordinate B in a near-trigonal planar environment. Furthermore this crystallographic model provides an explanation for the anisotropic behavior of the lattice parameters upon B-incorporation.
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