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

Abstract

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) Å, b=8.3300(3) Å, c=8.9547(4) Å, 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.