Section 3.3.1. Introduction

Powder diffraction is used for many purposes. One important use is the determination of crystallographic models (crystal structures), particularly when single crystals are not available. Other common uses of powder diffraction include the identification of unknown materials; determining the amounts of different crystalline phases in a mixture; studies of phase transitions; and measuring changes in lattice constants with composition, pressure or temperature. Residual stress measurements are also frequently made using powder diffraction. Measurements of the degree of preferred orientation of grains in a processed material (texture or pole-figure measurements) are also carried out using powder diffraction; these are useful as they can be used to relate the engineering properties of a material to its processing conditions.

A wide range of instrumentation is used for powder diffraction. Synchrotron, sealed-tube and rotating-anode X-ray sources and both spallation and reactor neutron sources are used. When monochromatic radiation is used, diffraction intensities are measured as a function of the detector setting angle, . When polychromatic radiation is used, however, energy-dispersive detectors at fixed angles are used. For a pulsed neutron source, the neutron time-of-flight (TOF) is measured to provide energy-dispersive detection. Energy-dispersive X-ray diffraction is not common, but is used for certain specialized measurements such as in situ high-pressure studies. Most laboratory powder diffractometers have a single discrete detector, but a few use position-sensitive detectors (PSDs). Multiple detectors or PSDs are common for specialist instruments, for example at large national user facilities.

Historically, X-ray sensitive film in a diffraction camera was used to measure the intensities. At present, diffractometers are much more common than cameras, but film is an inexpensive method of area detection and cameras continue to have some advantages over diffractometers. X-ray film, however, has a nonlinear sensitivity and a limited dynamic range, so electronic area detectors and image plates are replacing film for many applications.