Abstract

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 (δ_iso) and quadrupolar coupling constant and asymmetry paramater (C_Q, 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.