The Nature of X-ray Studies

There are a number of excellent texts devoted entirely to the methods of x-ray diffraction and interpretation of the resulting data. This chapter affords only a quick glimpse into the least complex aspects of this important field. Although x-ray studies of noncrystalline material (gases, liquids, and glasses) have led to some interesting results, such studies are far less important than studies of crystals and will not be discussed in this text. 

The very high energies needed to excite the inner electrons of atoms used as x-ray sources (chiefly Mo, Cu, Ni, Co, Fe, W, and Cr) are obtained by shooting a high-speed beam of electrons from an outside source (a cathode) at a target of the appropriate metal. 

---

With the development of intense synchrotron X-ray sources, it has become possible to use X-ray absorption spectroscopy to structurally characterize the metal clusters in metalloproteins. A wide variety of systems have been studied in this manner in the last ten years. The present article reviews the nature of X-ray absorption spwtroscopy and the information which can be obtain fix)m its study. The strengths and Kmitations of the technique are discussed with reference to the recent literature. 

Having demonstrated that X-rays are a secondary radiation cansed by what was referred to at the time as cathode rays , RSntgen showed that the study of the nature of X-rays had close ties with the determination of the natirre of electronic radiation. After the discovery by Crookes of the existence of a radiation emitted by the cathode and attracted by the anode, the question of the natirre of these cathode rays was the snbject of intense activity. When X-rays were discovered, the two theories clashed. Some considered that this cathode rays was cansed by a process of vibration taking place in the rarefied gas inside the tube (the ether ) [LEN 94, LEN 95], while others thought that this current was the result of the propagation of charged particles emitted by the cathode [PER 95, THO 97a]. 

We mentioned above that, at the dawn of the 20 century, the nature of X-rays was already well known. Evidence of gas ionization by X-rays quickly led to the creation of devices designed to quantitatively measure the intensity of X-ray beams. This enabled researchers at the beginning of the last century to study in detail the interaction between X-rays and solid matter, leading, naturally, to the observation and quantitative analysis of scattering, and then diffraction, of X-rays. 

In 1896, only a few months after Roentgen aimounced the observation of x-rays, Becquerel reported the additional observation of penetrating radiation emitted from certain natural materials, a phenomenon that Marie Curie would later name radioactivity. This phenomenon had a much less glamorous development. Over a three-year period, Becquerel pubHshed three articles, decided there was Htfle else to learn about it, and went on to the study... 

The technique of using the diffraction of X-rays to study the structure of fibres, plastics and elastomers, to determine the crystalline nature of such polymers, and to measure the particle size of finely divided materials. 

As noted earlier, the diffraction of X-rays, unlike the diffraction of neutrons, is primarily sensitive to the distribution of 00 separations. Although many of the early studies 9> of amorphous solid water included electron or X-ray diffraction measurements, the nature of the samples prepared and the restricted angular range of the measurements reported combine to prevent extraction of detailed structural information. The most complete of the early X-ray studies is by Bon-dot 26>. Only scanty description is given of the conditions of deposition but it appears likely his sample of amorphous solid water had little or no contamination with crystalline ice. He found a liquid-like distribution of 00 separations at 83 K, with the first neighbor peak centered at 2.77 A. If the pair correlation function is decomposed into a superposition of Gaussian peaks, the area of the near neighbor peak is found to correspond to 4.23 molecules, and to have a root mean square width of 0.50 A. 

Al-for-Fe substitution in natural goethites was originally discovered, with the aid of X-ray diffraction, in marine, oolithic, iron ores from the Jurassic era by Correns von Engelhardt as early as in 1941 and twenty years later found in soils by Norrish and Taylor (1961). Since then, a large number of studies has revealed that Al substitution in goethites from the weathered zone, e.g. in soils (see chap. 16), appears to be the rule rather than the exception. It should be noted that Al located in the struc-... 

---