Chemistry Reference
In-Depth Information
to von Laue and the senior and junior Bragg for the discovery and explanation
of X-ray diffraction. X-ray diffraction techniques are based on elastic scatteredX-rays
frommatter. Due to the wave nature of X-rays, the scattered X-rays from a sample can
interfere with each other such that the intensity distribution is determined by the
wavelength and the incident angle of the X-rays and the atomic arrangement of
the sample structure, particularly the long-range order of crystalline structures.
The expression of the space distribution of the scattered X-rays is referred to as an
X-ray diffraction pattern. The atomic level structure of the material can then be
determined by analyzing the diffraction pattern. Over its hundred-year history of
development, X-ray diffraction techniques have evolved into many specialized areas.
Each has its specialized instruments, samples of interests, theory, and practice. Single-
crystal X-ray diffraction (SCD) is a technique used to solve the complete structure of
crystalline materials, typically in the form of single crystals. The technique started
with simple inorganic solids and grew into complex macromolecules. Protein
structures were first determined by X-ray diffraction analysis by Max Perutz and
Sir John Cowdery Kendrew in 1958 and both shared the 1962 Nobel Prize in
Chemistry. Today, protein crystallography is the dominant application of SCD.
X-ray powder diffraction (XRPD), alternatively powder X-ray diffraction (PXRD),
got its name from the technique of collecting X-ray diffraction patterns from packed
powder samples. Generally, X-ray powder diffraction involves the characterization
of the crystallographic structure, crystallite size, and orientation distribution in
polycrystalline samples [2-5].
X-ray diffraction (XRD), by definition, covers single-crystal diffraction and powder
diffraction aswell asmanyX-ray diffraction techniques. However, it has been accepted
as convention that SCD is distinguished from XRD. By this practice, XRD is
commonly used to represent various X-ray diffraction applications other than SCD.
These applications include phase identification, texture analysis, stress measurement,
percent crystallinity, particle (grain) size, and thin film analysis. An analogous method
to X-ray diffraction is small-angle X-ray scattering (SAXS) technique. SAXS mea-
sures scattering intensity at scattering angles within a few degrees from the incident
angle. SAXS pattern reveals thematerial structures, typically particle size and shape, in
the nanometer to micrometer range. In contrast to SAXS, other X-ray diffraction
techniques are also referred to as wide-angle X-ray scattering (WAXS).
1.2 GEOMETRY OF CRYSTALS
Solids can be divided into two categories: amorphous and crystalline. In an amor-
phous solid, glass, for example, atoms are not arranged with long-range order. Thus,
amorphous solids are also referred to as “glassy” solids. In contrast, a crystal is a solid
formed by atoms, molecules, or ions stacking in three-dimensional space with a
regular and repeating arrangement. The geometry and structure of a crystalline
solid determines the X-ray diffraction pattern. Comprehensive knowledge of crystal-
lography has been covered by many topics [2,5-9]. This section gives only some
basics to help further discussion on X-ray diffraction.
