A Structural Study of Stacking Disorder in the Decomposition Oxide of MgAl Layered Double Hydroxide: A DIFFaX+ Analysis

The structure of the decomposition oxide of a nitrate-based magnesium−aluminum layered double hydroxide (LDH) was studied in detail by means of the DIFFaX+ refinement program. The DIFFaX+ refinement was based on an in situ X-ray powder diffraction pattern (XRPD) collected at 890 °C and showed that the decomposition oxide is a disorder phase, where periclase-type layers and spinel-OT-type layers with different cation concentration and distribution are stacked along the direction corresponding to the cube diagonal of the ideal cubic periclase and/or spinel structure. The spinel-OT-type layers in the periclase-like decomposition phase are assumed to contain significant amounts of tetrahedrally coordinated aluminum atoms. Furthermore, the DIFFaX+ refinement of the decomposition oxide indicated that the octahedra and tetrahedra of the spinel-OT-type layer and the octahedra of some of the periclase-type layer are distorted due to deviation in the cubic-close packing of the oxygen atoms in the decomposition oxide. Our detailed structural investigation described the disorder present in the periclase-like decomposition oxide and revealed the origin of an additional major diffraction peak in the powder diffraction pattern which has up to now remained unexplained.