Now that the effect of diffraction on a crysalline sample has been described, it is time to discuss the core concept of this module: how diffraction of a polycrystalline sample (usually a powder) works.
A powder pattern is like a 'spectrum' of d-spacings in the crystal structure and is usually presented in the form of a line trace. A finely ground crystalline powder contains a very large number of small crystals known as crystallites, which should be oriented randomly to one another. When this sample is placed in the path of a monochromatic X-ray beam, diffraction will occur from planes in those crystallites, which happen to be oriented at the correct angle to fulfill the Bragg condition. The effect of this is that each lattice space in the crystal will give rise to a cone of diffraction. Every cone consists of a set of closely spaced dots, each of which represents diffraction from a single crystallite within the powder sample.
The goal of performing X-ray diffraction on a polycrystalline sample is to capture all possible orientations of the sample crystal structure using a minimal amount of change in the relative orientation of the sample. This has the benefit of requiring far less time and resources to obtain the same information. It also allows observation of a much more diverse faimly of types of samples. Many materials only ever appear as a residual polycrystalline signature within another matrix, and there are difficulties in growing crystals of sufficient dimensions for single crystal diffraction analysis.
In order to obtain powder x-ray diffraction data in a useful format necessary for analysis, the positions of the various diffraction cones need to be determined. This can be achieved by using photographic film or a detector sensitive to x-ray radiation. Both techniques allow us to determine the angle (2θ, referring to Bragg's law) of the diffracted beam of the various diffraction cones.
Limitations of powder diffraction:
- Single crystal techniques utilize mathematical algorithms and accurate peak intensities to solve structures
- The 3D collection of spots and intensities of a single crystal experiment are reduced to a 1D pattern. This causes peak overlap and makes accurate peak intensities problematic.
- Crystal symmetry is not obvious from the pattern.
- Multiphase mixtures complicate the issue.
- Preferred orientation leads to inaccurate peak intensities.
One improtant thing to note about powder diffraction is that sample preparation must be done carefully and precisely to get a good result.