Ingo Bergmann

High-resolution 27Al MAS NMR spectroscopic studies of the response of spinel aluminates to mechanical action

The response of the local structure of various types of spinel aluminates, ZnAl2O4 (normal spinel), MgAl2O4 (partly inverse spinel), and Li0.5Al2.5O4 (fully inverse spinel), to mechanical action through high-energy milling is investigated by means of 27Al MAS NMR. Due to the ability of this nuclear spectroscopic technique to probe the local environment of Al nuclei, valuable quantitative insight into the mechanically induced changes in the spinel structure, such as the local cation disorder and the deformation of the polyhedron geometry, is obtained. It is revealed that, independent of the ionic configuration in the initial oxides, the mechanical action tends to randomize cations over the two non-equivalent cation sublattices provided by the spinel structure. The response of the spinels to mechanical treatment is found to be accompanied by the formation of a non-uniform core–shell nanostructure consisting of an ordered crystallite surrounded by a structurally disordered interface/surface shell region. Based on the comparative NMR studies of the non-treated and mechanically treated spinels, an attempt is made to separate the surface effects from the bulk effects in spinel nanoparticles. The non-equilibrium cation distribution and the deformed polyhedra are found to be confined to the near-surface layers of spinel nanoparticles with the thickness extending up to about 0.7 nm. The cation inversion parameter of the mechanically treated spinel is compared with that of the non-treated material at non-ambient conditions.

Nanocrystalline Complex Oxides Prepared by Mechanochemical Reactions

The preparation of complex oxides by the conventional solid‐state (ceramic) route requires a number of stages, including homogenization of the powder precursors, compaction of the reactants, and finally prolonged heat treatment at considerably elevated temperatures under controlled oxygen fugacity. One goal of modern materials research and development has been to identify simpler processing schemes that do not rely upon high‐temperature treatments for inducing solid‐state reactions. At present, mechanochemical methods become widely used for the preparation of nanocrystalline materials due to their relative simplicity and availability. In this work, selected examples of the preparation of nanoscale complex oxides via single‐step mechanochemical routes are presented. Nuclear spectroscopic methods are employed to follow the mechanically induced formation of nanooxides and to characterize the nonequilibrium structural state of the resulting nanophases at the atomic level.