NEUTRON-DIFFRACTION STUDIES OF POLYCRYSTALLINE NI MG/AL MIXED OXIDES OBTAINED FROM HYDROTALCITE-LIKE PRECURSORS/

The thermal decomposition of a synthetic hydrotalcite-type (HT) sample [Mg0.71Al0.29(OH)(2)(Co-3)(0.145). mH(2)O] has been investigated by n eutron diffraction using a programmed temperature control facility. Th e evolution from HT phase to mixed oxide leading ultimately to the par tial segregation of a spinel phase is reported. Structural modificatio n ascribed to the interlayer water loss from the HT phase is accompani ed by a reduction of the c-axis parameter and changes in the neutron p owder pattern. The neutron diffraction patterns of five Mg/Ni/Al mixed oxides obtained from FIT samples by calcination at 923 K were subject ed to structural refinement by the Rietveld method. The model that bes t describes the refined data consists of a supercell with an a-axis tw ice that of the stoichiometric rock-salt type oxide, a certain degree of ordered cation vacancies in the octahedral sites, and partially occ upied tetrahedral sites. Although the samples essentially show the typ ical patterns of a rock-salt type structure, the results indicate that they conform to a spinel type structure, which corroborates the deduc tions made by workers using other techniques. A high replacement of al uminium for magnesium in the magnesium-richer samples is proposed.