Crystal structure of κ-alumina: an X-ray powder diffraction,TEM and NMR study

The crystal structure of κ-alumina (κ-Al 2 O 3 ) has been determined ab initio from an X-ray powder diffraction pattern (reliability factors: R Bragg =0.046, R p =0.090, R wp =0.115, χ 2 =11.7). The acentric structure (orthorhombic system, space group Pna2 1 , a=4.8437(2) A, b=8.3300(3) A, c=8.9547(4) A, Z=8) is built up from a pseudo-close-packed stacking ABAC of oxygen atoms, with aluminium in octahedral and tetrahedral environments in a 3:1 ratio, which form zigzag ribbons of edge-sharing octahedra and corner-sharing tetrahedra. κ-Alumina was characterized using magic angle spinning (MAS) 27 Al NMR at two fields and multiple quantum magic angle spinning (3Q MQ MAS) 27 Al NMR. A high-resolution electron microscopy study confirmed the structure and showed the presence of two types of defects: antiphase boundaries and 120 ° disorientations. A model is proposed for these two types of defects, which leaves unchanged the pseudo-close-packed arrangement of the oxygen atoms and assumes a shift or 120 ° twinning of aluminium ions.