Combined Analysis

Chapter 1

Some Basic Notions About Powder Diffraction

1.1. Crystallite, grain, polycrystal and powder

A polycrystal, dealing with diffraction, is a solid substance divided into very small homogenous particles, elementary single crystals called crystallites (Figure 1.1). Crystallites are tridimensional domains which propagate an incident x-ray or neutron wave in a coherent way without phase loss. Crystallites are also called “coherent domains” in crystallography. A grain (such as the ones we can observe using a microscope for instance) can be made up of many crystallites or only a single crystallite. A single crystal is then composed of only one crystallite, and a polycrystal of an ensemble of crystallites.

Figure 1.1. Definitions of grains (bold), crystallites (hatched zones) and crystallographic planes (hatchings) in a polycrystalline sample

A powder is an aggregate of crystallites (metal piece, ceramic, polymer or simply a compacted or not compacted powder) of varying number, shape, size and crystalline state, it is thus a polycrystal. But an ideal powder or “standard”, again concerning diffraction experiments, is made up of a large number (several tens of thousands at least) of crystallites in a perfect crystalline state (without microdistortion or any other defects) which exhibit a very narrow monomodal size and shape distribution. Furthermore, the standard powder has crystallites ...