Atom: the whole picture
DETERMINATION of the atomic configurations of materials is of great relevance in studying their properties. From x-ray diffraction to electron microscopy, many tools have been used over the years. But now comes a new technique of x-ray holography which promises to revolutionise the study of materials. Miklos Tegze and Gyula Faigel of the Research Institute for Solid State Physics in Budapest, Hungary, have reported the construction of the first x-ray hologram.
The ability to see inside materials and deduce their structure depends on the tools used. A technique called x-ray diffraction is used in which the sample is irradiated with a focussed monochromatic (single wavelength) beam of x- rays, which then scatter from the atoms in the crystal to produce a pattern of spots on a photographic film. These spots are then used to infer the crystal structure of the specimen.
Though almost eight decades old, x-ray diffraction is still a very powerful tool in the study of atomic structure, specially now with the availability of bright x-ray sources from synchrotrons. But the method suffers from a fundamental limitation. The image formed on ,he film carries information only about the intensity of the scattered x-ray and nothing about its phase. This limitation implies that the structure cannot be uniquely determined. In practice, the observed structure is matches with likely structures (which are calculated) and the one that fits best is chosen.
A way around these problems is to directly obtain a three-dimensional (3-D) image of the atomic configuration by holography. Holography was discovered in 1947 by Dennis Gabor at the Imperial College of Science and Technology in London, UK. Initially it was restricted to being a scientific curiosity but with the advent of lasers in the '60s, the use of holography became widespread. Nowadays, it finds applications in navigation, industry and the entertainment industry among others.
Holographic images which range from credit cards to product identification labels are, however, limited in their resolution by the wavelength of visible light. Visible light has a wavelength of around .00005 cm which is several thousand times the typical atomic diameters.
Thus one cannot use visible laser light to study atomic configurations. A way out, proposed about a decade ago, was to use x-rays or electrons which have much shorter wavelengths. A lot of theoretical work has been done in this field and several experiments using electrons have been carried out.
Tegze and Faigel have now for the first time experimentally demonstrated the use of x-rays in obtaining a 3-D image of a material. A single crystal of perovskite, a compound of strontium, titanium and oxygen, was irradiated with x-rays to generate fluorescent x-rays from the strontium atoms in the sample.
Some of the fluorescent x-rays do not interact with the atoms in the sample while others scatter off them. The interference between these two waves results in a unique 3-D image of the atomic configuration of the sample. Though the effect with x-rays is much smaller as compared to that with electrons, Tegze and Faigel have obtained the first x-ray holographic images of the arrangement of atoms in perovskite.
Although the technique suffers from many shortcomings, it is only a matter of time before the limitations are overcome. The weak diffraction patterns obtained using x-rays necessitate the use of longer counting times. With the availability of very bright synchrotron x-ray sources, this could easily be overcome. Similarly with the fast x-ray cameras which have a good energy resolution, this technique could become easy and its use very widespread not only in the materials industry but also in biology. (Nature, March 7, 1996).