section%204.8%20-%20Diffraction

section%204.8%20-%20Diffraction - 2 4.8 Determining Crystal...

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2 θ 4.8 Determining Crystal Structure: Diffraction Objectives Use Bragg’s Law to relate crystal structure and diffraction peaks. Now that we know something about the structure of crystals, how do we actually determine that structure? One of the most powerful and common ways to do it is by using x-ray diffraction. X-rays were discovered in 1895 by Wilhelm Roentgen, a professor of physics in Bavaria. The theory of x-ray diffraction by crystals was first proposed in 1912 by Paul Ewald as part of his PhD thesis; at the time he was only 24 years old. In 1913 the father-son team of William Henry Bragg and William Lawrence Bragg created what has become known as Bragg’s Law (which we will show below). In 1915 they became the only father-son team to share a Nobel Prize (although other pairs of fathers and sons have won separate prizes). William L. Bragg was only 25 when he won the Nobel Prize, another example of someone in their early 20’s making a major contribution to science. This is actually not that unusual; many of the greatest scientific discoveries have been developed by people in their 20’s. The experimental set-up for x- ray diffraction is shown schematically in Figure 4.8.1. Figure 4.8.1: Schematic diagram of an x-ray diffraction experiment. From http://fig.cox.miami.edu/~cmallery/255/255prot/mcb3.38a.xray.jpg . Figure 4.8.2: X-ray diffraction patterns of (a)liquid benzene; (b) glass; (c) polycrystalline quartz powder; and (d) single crystal of zinc dimethyldithiocarbamate. From X-Ray Diffraction Methods in Polymer Science by Leroy E. Alexander. An x-ray source creates a beam of x-rays, which hits the sample. The sample causes the x-rays to be deflected, or scattered , in different directions. The angle between the original beam direction and the scattered beam direction is called the scattering angle , and is given the symbol 2 θ . An x-ray detector measures the intensity of the x-rays at the different values of the scattering angle. The power of x-ray diffraction is that it provides a lot of information which can be shown in many different ways. One way is to collect the scattered x-rays on a piece of film. Doing this can provide information about the overall
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section%204.8%20-%20Diffraction - 2 4.8 Determining Crystal...

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