Diffraction

X-ray diffraction is the most widely used approach to imaging substances at atomic resolution and elucidating their structures. These substances include metals and alloys, chemical compounds, minerals, and molecular crystals ranging from several atoms to macromolecular assemblies of several million dalton.

In a crystal the constituent atoms are arranged in regular, though very complex, arrays. When x-rays pass through the crystal they may be scattered elastically by the electronic structure of the atoms. Because of the regular arrangement of the atomic lattice the scattered waves periodically reinforce one another to form a diffracted beam. By rotating the crystal through all angles with respect to the x-ray beam a complete diffraction pattern can be built up that forms a unique, characteristic signature of the crystal structure. From this pattern it is possible to determine accurately the crystal structure by both experimental and theoretical analysis with reference to molecular and crystallographic databases.

Because the x-rays are scattered by the electrons in the crystal structure it is not only possible to compute the atomic structure of the crystal but also its electronic density map.

While single crystal diffraction is the preferred method for determining the structure and electronic density maps, the preparation of diffraction quality crystals often lags months or years behind pioneering scientific breakthroughs, such as high-temperature superconductivity. This is particularly true of many materials of major industrial or commercial importance, including zeolites, polymers, and pharmaceuticals, collectively known as small molecules, as well as many minerals. The ability of the synchrotron to produce intense, highly collimated x-radiation and to focus it with x-ray optics to a very small spot size enables single crystal diffraction to be performed on many of these materials where previously this has been impossible.

Powder diffraction

Alternatively, these powders can be analyzed in bulk by powder diffraction, which is the only available option in studies of the behaviour of the crystal structure under external conditions such as high pressure, high or low temperature, or in the liquid state. In this technique a three dimensional diffraction pattern is collapsed onto one dimension by spherical averaging. As a result, reflections which would otherwise be measured separately are caused to overlap. The resulting pattern is a collection of concentric rings, often with fine-structure from the individual crystallites in each ring. The resolution that can be obtained from powder diffraction is limited by the extent of this overlap, the signal to background noise level and by the range of observations (d-range). The background noise can result from scattering by the cell or by the air. The high intensity and high collimation of synchrotron radiation can be of great assistance in the experimental resolution of these overlaps. These features also reduce the signal to noise ratio, and can increase the range of observations. Consequently, diffraction data obtained from synchrotron based instruments provide much greater accuracy and resolution compared with conventional instruments.

Anomalous dispersion

Another important feature of synchrotron light is its tunability, enabling the exploitation of anomalous scattering effects. This means that by the use of a monochromator the wavelength of the beam can be selected to maximize the visibility of certain parts of the sample to the measurement. Many advanced materials contain elements that may be disordered over a number of sites within the crystalline phases, either as a result of the processing conditions or due to deliberate doping. Such disorder is often inherent in mineral samples. Anomalous dispersion effects produce element-specific information, enabling such disorder to be identified and quantified.

Anomalous dispersion is particularly useful for protein crystallography, especially proteins containing seleno-methionine, where selenium that replaces sulphur in the amino-acid methionine. In this technique, called multi-wave anomalous dispersion (MAD) the wavelength of the x-ray beam is scanned rapidly over a range of frequencies close to the x-ray absorption edge of certain atoms in macromolecules, in particular selenium. The technique has formed the basis of many structural genomics initiatives, where the gene products of entire genomes are produced with seleno-methionine.

Small and wide-angle scattering

Small angle scattering (SAXS) is the process of elastic scattering of x-rays from a sample, where those scattered at a small angle with respect to the original direction of the beam are detected. The information gained is primarily structural, especially for materials whose features are in the length range 500 nm to 0.1 nm. Because of the inverse relationship between scattering and size, x-rays scattered at small angles give information about the size and shape of relatively large structures such as polymers and proteins.

Form many experiments, such as nucleation and crystallation, it is necessary to observe both the the small angle scattering and at the same time the wide angle scattering (WAXS) from growing crystallite phases. These include hydrothermal processes such as zeolite formation or Bayer liquor crystallization. The wide angle diffraction gives information about structure at far smaller scales, similar to that obtained from the more standard x-ray diffraction technique. At a synchrotron beamline it is possible to collect SAXS and WAXS data simultaneous with a specially designed detector.

A major advantage of a synchrotron source is that because of the extremely high beam intensities it is possible to study processes that are changing with time. These dynamic processes are of enormous interest in both the life and physical sciences and they represent an area where synchrotron sources are contributing valuable knowledge, unobtainable by any other experimental technique.

One of the first instruments to be built at the Replacement Research Reactor at Lucas Heights will be a small angle neutron scattering instrument. The equivalent x-ray and neutron scattering methods complement one another and the simultaneous refinement of data produced using a combination of both methods on the one system is very powerful. The major benefit of such analysis is the removal of potential ambiguity in the interpretation of data.