International
Tables for
Crystallography
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk

International Tables for Crystallography (2018). Vol. H, ch. 2.8, p. 174

Section 2.8.1. Introduction

H. Ehrenberg,a* M. Hinterstein,a A. Senyshynb and H. Fuessc

aInstitut für Angewandte Materialien (IAM-ESS), Karlsruhe Institut für Technologie (KIT), Eggenstein-Leopoldshafen, Germany,bTechnische Universität München, Garching b. München, Germany, and cTechnische Universität Darmstadt, Darmstadt, Germany
Correspondence e-mail:  helmut.ehrenberg@kit.edu

2.8.1. Introduction

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The functionality of materials depends strongly on the crystalline structure and structural changes during operation. The term `structure' usually refers to the ideal structure, which specifies the positions of the atoms in a lattice, and thus the distances and angles between them. This idealized model is, however, far too simple to describe the full functionality of a material in a device. Many types of defects, such as point defects, dislocations or grain boundaries, are essential to the functionality and have to be taken into account. As the length scales of defects range from atomic bond lengths via nanometres to micrometres, different methods have to be used for comprehensive structural characterization. High-resolution transmission electron microscopy (HRTEM) is the ideal tool for studying a material at the atomic scale, as it gives direct evidence of the arrangement of atoms. In addition, information on the chemical composition can be provided through X-ray or electron spectroscopies. However, in many cases electron microscopy requires a tremendous effort in sample preparation. Furthermore, the application of electric fields in a TEM column is a serious challenge with significant limitations. While electron microscopy will provide information on small sample volumes, diffraction methods probe larger quantities of samples, but give average information. In general, diffraction methods are based on electromagnetic or particle waves. X-ray photons with energies in the keV range have wavelengths similar to interatomic distances and, therefore, X-rays from laboratory sources or synchrotrons are the most widely used. Thermal neutrons with meV energies have complementary properties suited for other applications. While electrons are usually used for microscopy techniques, the field of electron crystallography has developed in recent years. However, given the very small size of an electron beam, its short wavelength (circa 0.03 Å) and high absorption, most particles studied by electron crystallography can be considered as single crystals. The combination of electron crystallography and powder diffraction is a powerful tool for tiny crystalline samples, especially inclusions (Weirich et al., 2006[link]).

In the field of in situ materials research, multiparametric measurements as functions of three or more external parameters, e.g. temperature–magnetic field–pressure or temperature–magnetic field–electric field, have been reported. However, the majority of so-called in situ studies are carried out as a function of temperature and sometimes of external pressure. Studies of structural changes under electric fields are relatively rare. Studies of changes due to magnetic fields almost entirely lie in the domain of neutron scattering, where single-crystal experiments usually give more details on the evolution of the magnetic structure. The challenges, necessary instrumentation and some examples of in situ diffraction measurements are described in Chapter 16 of the book Modern Diffraction Methods (Mittemeijer & Welzel, 2012[link]).

References

Mittemeijer, E. J. & Welzel, U. (2012). Editors. Modern Diffraction Methods. Weinheim: Wiley-VCH.Google Scholar
Weirich, T. E., Lábár, J. L. & Zuo, X. (2006). Editors. Electron Crystallography. Nato Science Series, Series II: Mathematics, Physics and Chemistry, Vol. 211. Heidelberg: Springer-Verlag.Google Scholar








































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