Crystal structure of kappa-alumina: An X-ray powder diffraction, TEM and NMR study

The crystal structure of kappa-alumina (kappa-Al2O3) 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, chi(2) = 11.7). The acentric structure (orthorhombic system, space group Pna2(1), (a) = 4.8437(2) Angstrom, b = 8.3300(3) Angstrom, c = 8.9547(4) Angstrom, 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. kappa-Alumina was characterized using magic angle spinning (MAS) Al-27 NMR at two fields and multiple quantum magic angle spinning (34 MQ MAS)Al-27 NMR. A high-resolution electron microscopy study confirmed the structure and showed the presence of two types of defects:antiphase boundaries and 120 degrees 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 degrees twinning of aluminium ions.