Diffraction at the Edge

 

PCG/CCG joint symposium 10th April 2001

 

This final morning session of the BCA spring meeting held at Reading University comprised five very different and interesting talks loosely connected with photon scattering experiments close to X-ray absorption edges. It is always good to get a tutorial/overview as an introduction to the subject and this was provided by Georgina Rosair (Heriot-Watt) with a superb talk entitled “Structural chemistry from the edge”.

This well illustrated power point presentation opened with the obvious question – why use absorption edges ? and answered it succinctly  by pointing to the ability of the technique to probe the local environment and study the electronic/magnetic structure of materials.

We were reminded of the origins of K edges as core electron ionization from a 1s orbital with good graphics. A typical K-edge X-ray absorption spectrum illustrated the oscillations associated with EXAFS and the audience was left with the memorable quip from Georgina “So much information from wobbles…”. Many of the details discussed concerning anomalous scattering can be found at the following web address http://www.bmsc.washington.edu/scatter/AS_index.html. Some applications of multi-wavelength anomalous dispersion (MAD) were cited from the recent literature these included molecular magnets, excited states, metallo-proteins, catalysis. In conclusion absorption edges were shown to be useful not only for determining the spin states of metals and investigating the local structure around the metal in metallo-proteins but also for following changes during a chemical reaction in the bulk or on a surface. This sharp, lively lecture fitted the billing and had us on the edge of our seats…..

This was followed by Madeline Helliwell (Manchester) who spoke of anomalous dispersion techniques used in small molecule crystallography. In her own words “Synchrotron sources offer the ability to harness intense, tuneable X-rays with a wavelength selected to be close to the absorption edges of specific elements in the sample”. In this case the samples were microporous materials NiAlPO and CoZnPo-CZP. These phosphate based molecular sieves depend on the shape & size of the cavities to control the molecular sieving. The structure was analyzed using Cu rotating anode lab sources and the Brookhaven synchrotron. The beauty of the method is the ability to identify and distinguish between atoms which are close together in atomic number, even when two or more such atoms occupy a single site. This leads to the identification of metal atoms that are incorporated into the framework of the microporous materials as a result of their catalytic properties. An area where there is continued interest we were told is the combined use of XRD/EXAFS where the former monitors changes in structure while the latter identifies the atom edge positions.

Brian Tanner (Durham) switched tack to talk about variable energy (6.5 – 10 keV) grazing incidence X-ray reflectivity and reflection from multiple layer transition metal films. These permalloy NiFe/Cu multilayers with low magnetostriction find application in hard disc drives and are therefore commercially important devices. The interface structure (typically nanometres thick) is not normally easy to see due to small differences in the scattering factor. However each metal possesses characteristic changes in scattering power close to the K absorption edge. So multi-wavelength measurements of reflectivity give more information on the metal multilayer transition, the measurements being taken at a grazing incidence of less than 1o. In the region above the absorption edges an oscillatory component (Krammers-Kronig oscillations) in the spectrum was observed, reminescent of Diffraction Absorption Fine Structure (DAFS), see for example  http://www.cars9.unchicago.edu/dafs/icas/icas.html. This method combines the crystallographic long range order of XRD with local spectroscopic sensitivity of X-ray absorption techniques. This enabled these workers to observe a decrease in the nearest-neighbour distance around the Ni atoms in the multilayers on annealing. With the advent of SRS the combined use of spectroscopic, structural & crystallographic methods has been provided by DAFS.

The next speaker John Goff (Liverpool) continued the theme of magnetic materials by talking on “Resonant X-ray scattering from magnetic multilayers and alloys” usually given the acronym XMRS. The materials used in the study were Nd/Pr thin films manufactured by Molecular Beam Epitaxy (MBE) having tailor made physical properties and Ho-Ce alloys because single crystals were difficult to obtain. During the 1980’s long range magnetic ordering was shown in rare earth systems while traditionally neutron diffraction techniques have been widely used. XMRS offers advantages by tuning of the resonant energies enabling different components of complex systems to be studied separately also there are enormous gains in the signal-noise ratio. This latter point is of crucial importance for epitaxial systems where the volume of the magnetic material is small. XMRS has thus provided new insight into the complex magnetism of supperlattices and alloys.

The final lecture was an extended talk of 1 hour by an invited speaker from Moscow, Vladimir Dimitrienko, on a highly mathematical excursion into the realm of “Anisotropy of anomalous scattering: An interface between crystallography and material science”. This provided an opportunity to renew old acquaintances as I had met Vladimir in Nijmegen, Netherlands for the Aperiodic 2000 conference. We were reminded that X-ray “forbidden” Bragg reflections are excited at energies near absorption edges because the X-ray susceptibility is anisotropic. This anisotropy results from distortion of the atomic wave functions by crystal fields so that near the absorption edges the scattering factors become tensorial. Physical factors that can distort the atomic wave functions include thermal motion or point defects and can thus be studied. Polarization anisotropy of X-ray absorption is similar. The extinction rules for glide-plane and screw-axis need to be relaxed so that the anisotropy induced “forbidden” reflections can be observed. In conclusion this work may find theoretical application to incommensurate crystals while observations have been made in liquid crystals. Anisotropy forbidden reflections have also been observed in a number of materials including NaBrO3, TiO2, Fe2O3 amongst them, to put this work on an empirical basis.

After three days of stimulating talks on a whole range of topics, you could finally say we really were “taken to the edge….”

 

 

 

Jim Kelly

Industrial Materials Group

Birkbeck College