Nanoscopy and Materials Engineering

The strive for unification is a mainspring in natural sciences; perhaps the most famous example is the search for the Unified Field Theory by Einstein, Heisenberg and others that – up to date – has not yet been successful. However, on the comparably modest sublevel of unifying two experimental methods as special tools of spectro-scopy and diffractometry, it may be stated that this aim could be achieved by the undersigned unit leader and coworkers.

The result was the creation of the difference electron nanoscope (DEN) and its application „nanoscopy“ as a comparably new hybrid method – however, the DEN is no classical machine as the term might infer. It is a computer program running on a fast computer system displaying 3D difference electron hyperareas floating in space together with the relevant main physical quantity, the electric field gradient efg, as a wire frame model within the unit cell of the sample under consideration. In this sense, the method acts on a sub-nanometer scale (hence the term „nanoscope“) and delivers images of uncompared symmetrical and physical evidence – letting aside the aesthetical aspects. Has anyone seen a real orbital distribution together with its efg within a crystal before? The following image shows an example.

 Barrierefreiheit: Kurzbeschreibung des Bildes

3D difference electron densities (DEDs) (shaded yellow) around the centre position of the FeII-ion (red) with the efg as blue wire frame model in the unit cell of synth. Fe2SiO4. View along the c-direction (space group Pnma); for better spatial impression the surr. O2- octahedron is denoted by green lines, the adjacent silicon-O tetrahedron by yellow ones. Along certain superexchange pathways close to special oxygens, residual DED/magnetic moment densities can be recognized as the theory of magnetism for special solid state materials has predicted. 

From: Werner Lottermoser: The Difference Electron Nanoscope: Methods and Applications. © 2017 PanStanford Publishing Pte. Ltd., Singapore, Ashford Colour Press Ltd. ISBN 978-981-4774-01-7

However, the method is not a purely academic playground. Its application should lead to a better understanding of electric and magnetic interactions in crystals in order to improve the existing technical properties of materials (mat „engineering“) or to derive new compounds with unforeseen features. In this sense, the method fits well in the existing materials research activities of the Department and the Division.

At present, the DEN method consists of the following sub-units, partly available at Salzburg:

  • Classical 57Fe Mössbauer Spectroscopy, in partic. Single Crystal Mössbauer Spectroscopy SCMBS to derive the experimental efg
  • Diffractometry (X-ray, neutron and synchrotron d.) to evaluate the difference electron densities/magnetic electron densities and to derive a semi-empirical efg
  • Density Functional Theory (Self-Consistent-Charge SCC X alpha ­- Method) to determine a full-quantitative efg for comparison and for an important interaction with the experimental team

 

Why is the efg, obviously, so significant? Many physical and technical quantities have tensor character, i.e. they are dependent on size and direction, like the elastic tensor, the dielectric tensor, magnetic susceptibility a.s.o. - in our case the tensor of the electric field gradient. All these have in common that they are dependent on structural and symmetric properties of the solid state, i.e. the sample under consideration. This means that for a given crystallographic structure determined by the atom/ion positions and given properties like, e.g., piezoelectricity, magnetism, elasticity/plasticity, luminescence, conductivity a.s.o., the careful choice of the ions and their relative arrangement in the solid state let us create „designed“ materials (also nanomaterials).

An example is the synthesis and research on special Li ion conductors with garnet structure (battery research) that were investigated by our team.

Last, but not least: the DEN method is not confined to iron containing samples as the application of 57-Fe Mössbauer spectroscopy might infer: An efg can also be derived from NMR/NQR measurements, the DFT is only dependent on the performance of the computer system and diffractometry has no limitations at all. In this sense, the DEN method is rather universal. The „hype“ around it in the preceding years, was, however, merely occurring in foreign countries (USA, China). Werner Lottermoser

 

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