12.1.7: Single Crystal Diffraction

Figure 12.32: A single crystal diffractometer

For powder diffraction, we grind a sample so we have many crystallites contributing to the diffraction pattern. Single-crystal diffraction instead, as the name implies, uses only a single crystal. The technique involves measuring directions and intensities of diffraction when a collimated X-ray beam strikes a carefully oriented small crystal. Crystals are typically 50 to 250 microns (0.05 to 0.25 mm) in longest dimensions. The diffractometer rotates the crystal and moves the detector to measure intensities of all diffraction peaks in 3D space. Typically, a detector moving in a sphere or hemisphere around the crystal measures diffraction intensity. Figure 12.32 shows an example of such a diffractometer. Data collecting can be time-consuming, but modern single crystal diffractometers are computer controlled and can collect diffraction intensities for thousands of directions – sometimes as many as 20,000 – in a few hours. Computers store the data and process it automatically.

We use the data obtained from single-crystal diffraction studies to determine crystal structures. If we know the composition of a mineral, we know how many atoms of which elements are present in a unit cell. Computer programs figure out where atoms are relative to each other, based on the diffraction data. We call this process a crystal structure determination or crystal structure refinement. Crystal structure determination involves many complexities, but automation and high-speed computers have simplified the process considerably since the Braggs determined the first crystal structure in 1913.