phase transformations

The interest in solid state chemistry and mineralogy widely focuses on the reaction products of the synthesis, and these are characterised with methods integrating over larger volumes such as X-ray diffraction, infrared spectroscopy and solid state nuclear magnetic resonance spectroscopy. Then, however, the reaction mechanisms during (solid state) synthesis and possibly formed transition phases remain undiscovered.

The study of such processes and intermediate products in the reacting regions requires methods with high local sensitivity. For that the various methods of TEM are best suited: High resolution imaging (HRTEM) as well as electron diffraction (electron crystallography) on small selected regions yield information on structural changes such as the formation of transition phases - in real space down to atomic dimensions or in reciprocal space, respectively. Microstructural features such as cracks, pores and dislocations, which essentially determine the reaction process, are detected and characterised in detail. The methods of electron spectroscopy and energy-filtered TEM allow highly localised elemental analyses as well as imaging of the elemental distribution at near atomic resolution.

Currently we restrict ourselves on the research of oxide and nitride systems, which are easy to handle and are hardly sensitive to damage during the investigation by TEM. The starting materials have reacted only partially and the developing phase boundaries are specially prepared for and characterised by TEM methods. We increase the reliability and representativeness of the information obtained by using material that is highly homogeneous or even single crystalline.
The systems investigated in our group largely deal with classical problems in solid state chemistry and materials science:

The high temperature reaction of andalusite to the high strength ceramic mullite (both are aluminium silicates) is an important step for the processing of such ceramics. We could show in detail how the transformation proceeds and that the reaction mechanisms strongly depend on the crystallographic orientation in which the transformation proceeds.
Diaspore and goethite are oxide hydroxides of aluminium and iron, respectively, which transform upon dehydration to the stable oxides corundum (Al2O3) and hematite (Fe2O3), without any formation of transition oxides. Our results on these long standing problems associated with these particular transformations shed light on the formation of a highly interesting microstructure of the reaction products, whereby the reaction mechanisms are strongly determined by intermediate products and by the geometrical fitting of the crystal lattices.

© WA

Phase transition upon diffusion of Fe2+ in ZnO: from single defects (left) to phasoids consisting of inversion domains (right). Orientation of polar axis c of ZnO domains is indicated by arrows.


© WA

Transformation of diaspore, AlO(OH), to corundum, Al2O3, at the nanometer scale.
Dehydration of diaspore proceeds via topotactic transformation to corundum preserving the AlO6 octahedrons (left). The corundum is produced as alternating dense and porous lamellar crystal keeping the crystal structure (middle). This nanostructure in density is caused by different spacings of the lattice planes and is generated by alternating good fitting regions (→ dense) and not fitting regions (→ porous) for the AlO6 octahedrons of the two phases at the reaction front (right) .

Wird geladen