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Glossary of MinSE


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X

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A general purpose method of phase analysis and lattice parameter determination.  The basis of X-ray diffraction is the Bragg equation:

nl = 2dsinq

where  is the wavelength of the X-ray radiation, n is the order of reflectcion, q is the Bragg angle and d is the inter-planar distance of the planes causing diffraction.   By irradiating a sample surface with monochromatic X-rays  (e.g., Cu Ka1) and measuring q,  d is readily determined.  The intensity  of the diffracted X-rays  are also recorded.    Nowadays, d-spacing and X-ray intensity data is obtained using an X-ray goniometer which uses a proportional counter to measure diffraction intensity; an output of intensity versus 2q is obtained.   For a randomly oriented polycrystalline solid, several diffraction peaks of characteristic intensity will be produced.  The international JCPDS data base should be consulted when undertaking phase analysis; this is a major collection of diffraction data.   Much greater detail on X-ray diffraction principles and methods are available in standard texts, e.g., see B. D. Cullity: "Elements of X-ray Diffraction", 2nd ed; 1978, Addison-Wesley, Reading MA.   Also see glancing angle X-ray diffraction.

Hungary flag Röntgensugár elhajlás

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X-ray photoelectron spectroscopy.   The surface under interrogation is irradiated by monochromatic low energy ("soft") x-rays, e.g., Al Ka,  under ultra high vacuum conditions.   This produces an effect termed photoionisation which leads to the emission of photoelelctrons from the atomic cores of the surface atoms.     The latter have a kinetic energy (Ek) that is realted to the incident x-ray energy (hn) and the core binding energy of the elelctron (Eb) by the Einstein relation:

Ek  =  hn - Eb

In XPS, a kinetic energy distribution is obtained, such that a plot of intensity versus binding energy can be derived.  Binding energies corresponding to, for example, 3d, 3p and 3s transitions are obtained.  Their energies are unique to a given element, enabling precise elemental identification.  It is also possible to use this information to gain chemical bonding information, e.g., to demonstrate whether a given element is bonded to another, or not.   XPS can be used on non-conducting surfaces but cannot be generally applied to sample surface areas of less than 1 cm2.   However, at the time of writing, attempts are being made to improve the capability of XPS to enable selected area analysis.

Hungary flag XPS


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