Formation, stability and crystal structure of mullite-type Al6−xBxO9

Abstract Mullite-type Al 6−x B x O 9 compounds were studied by means of powder diffraction and spectroscopic methods. The backbones of this structure are chains of edge-connected AlO 6 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−x B x O 9 with 1.09≤ x ≤2. A preference of boron in 3-fold coordination is confirmed by 11 B 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 BO 3 groups yielding split positions for the oxygen atoms and a strong distortion in the octahedral chains. The lattice thermal expansion was calculated using the Gruneisen first-order equation of state Debye-Einstein-Anharmonicity model.