Formation, stability and crystal structure of mullite-type Al_6−xB_xO₉

Mullite-type Al_6−xB_xO₉ compounds were studied by means of powder diffraction and spectroscopic methods. The backbones of this structure are chains of edge-connected AlO₆ octahedra crosslinked by AlO- and BO-polyhedra. Rietveld refinements show that the a and b lattice parameters can be well resolved, thus representing an orthorhombic metric. A continuous decrease of the lattice parameters most pronounced in c-direction indicates a solid solution for Al_6−xB_xO₉ with 1.09≤x≤2. A preference of boron in 3-fold coordination is confirmed by ¹¹B MAS NMR spectroscopy and Fourier calculations based on neutron diffraction data collected at 4 K. Distance Least Squares modeling was performed to simulate a local geometry avoiding long B-O distances linking two octahedral chains by planar BO₃ groups yielding split positions for the oxygen atoms and a strong distortion in the octahedral chains. The lattice thermal expansion was calculated using the Grüneisen first-order equation of state Debye-Einstein-Anharmonicity model. - Graphical abstract: Local distortion induced by boron linking the octahedral chains. - Highlights: • Decreasing lattice parameters indicate a solid solution for Al_6−xB_xO₉ (1.09≤x≤2). • B-atoms induce a local distortion of neighboring AlO₆ octahedra. • A preference of boron in BO₃ coordination is confirmed by ¹¹B MAS NMR spectroscopy. • An optimized structural model for Al_6−xB_xO₉ is presented.