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. 3.2, p. 252

Section 3.2.1. Introduction

P. W. Stephensa*

aDepartment of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794–3800, USA
Correspondence e-mail: peter.stephens@stonybrook.edu

3.2.1. Introduction

| top | pdf |

The term powder is used here as a label for polycrystalline samples, but they may not be powdery at all, e.g., metallurgical samples or chocolate. The central premise of powder diffraction is that the sample consists of a sufficiently large number of independent particles (Smith, 2001[link]). In that case, the diffraction pattern will consist of a series of peaks corresponding to the Bragg reflections from each component of the sample. This chapter starts with an idealized description of the data collected in a powder-diffraction measurement and how it relates to the physical properties of the sample and the diffractometer. In real experiments, many of the idealizations of this description are not satisfied, and the influence of those deviations from simple model behaviour are discussed in turn. To a greater or lesser extent, those confounding factors are under the control of the experimenter, and so it is important to understand how they can be optimized or avoided in the experimental design, as well as how to deal with situations where they cannot be (or have not been) avoided and deal with them in real data that have been collected.

The diffraction probes discussed here are X-rays and neutrons. Bragg's equation, λ = 2d sin θ, relates properties of the measurement (wavelength of the diffracted radiation λ and scattering angle 2θ) with a property of the sample (the d-spacing of the particular reflection observed). Data may be collected with radiation of fixed wavelength as a function of angle, or as energy-dispersive measurements at fixed angle. The former case is perhaps the most familiar, exemplified by a laboratory powder diffractometer using characteristic radiation from an X-ray tube, but it is frequently performed with X-rays or neutrons from a continuum source (e.g., storage ring or nuclear reactor) selected by a suitable monochromator. Energy-dispersive X-ray experiments are performed by illuminating the sample with a continuum spectrum from a synchrotron source or Bremsstrahlung from an X-ray tube, whereas time-of-flight methods are in use at pulsed spallation neutron sources. Convenient conversions are that X-rays of energy E (keV) have wavelength λ (Å) = 12.398/E, and neutrons with a speed v (m s−1) have wavelength λ (Å) = 3956/v.

References

Smith, D. K. (2001). Particle statistics and whole-pattern methods in quantitative X-ray powder diffraction analysis. Powder Diffr. 16, 186–191.Google Scholar








































to end of page
to top of page