X-ray Diffraction Tutorial

X-ray Diffraction
	X-ray diffraction is a technique that takes advantage of the crystalline properties of a material to elucidate structure. Powder diffraction is a specific diffraction technique useful for bulk materials. X-rays from a source such as copper, molybdenum or cobalt are directed at a powder sample.
The x-rays are then diffracted by the sample in a manner that is unique to that sample. A moveable scintillating detector or CCD area detector measures the diffracted intensities at specific positions around the sample. When scintillating detectors are used, the intensities are measured as counts. When area detectors are used, the diffraction appears as rings. Because different samples will have unique diffraction patterns, diffraction data can be used to unambiguously characterize a sample.
Diffraction occurs as x-rays are scattered by the planes of the crystalline sample. The incident x-rays are directed at the sample at some angle, theta (θ). The crystal planes scatter the x-rays and the detector measures the intensity of the diffracted beam as a function of 2θ.
	These data can be used to determine phase, characterization and the structure of the sample. Below is a typical diffraction pattern (left) used to determine the structure of a particular chemical (right) (reprinted by permission from Elsevier).

	The different layers in a thin film can also be studied using a process called reflectivity. Thin films have the ability to reflect x-rays. The intensity of the reflected x-ray can be calculated based on the sample. Any deviation from this calculation in the data can be used to obtain density profiles of the thin film (source http://www.ptb.de/).
Single crystal diffraction is another type of diffraction which uses large single crystals to obtain diffraction data. Diffraction occurs exactly the same way as it does in powders, but because single crystals are used, the orientation of the crystal must always be known.
	In addition, area CCD area detectors are used to measure intensities rather than scintillating detectors. This allows for better diffraction data and better structure determination of more complex chemical structures. And instead of rings, diffraction appears as single spots on detector film. Below are some examples of complex chemicals studied using single crystal diffraction (Structure below reprinted by permission from American Chemical Society, Hirani et al, 2007). Images of powder diffraction, Bragg diffraction, and single crystal diffraction obtained from Wikipedia commons public domain.