Techniques and facilities where you can find them
In classic crystals, atoms and molecules are ordered in a three-dimensional periodic manner. In this crystal lattice, each lattice plane behaves like a semi-reflecting mirror. Constructive interference occurs only if the contributing planes fulfill Bragg's law. This relationship between the wavelength of the incoming beam, the lattice distances and the diffraction angle leaves only a number of directions in which so called Bragg reflexions can be found.
The diffraction angles contain the information about the unit cell, the smallest unit to describe the order of the whole crystal. The intensities of the Bragg reflections contain the information about the atomic/molecular distribution inside the unit cells and the motion of the atoms (Debye-Waller factor).
Besides the positions of the atoms (crystallographic structure) also the ordering of magnetic moments (magnetic structure) can be obtained. The knowledge of the structure is crucial to understand structure – properties – relationships in any material.
A powder or polycrystalline material contains a large number of small randomly oriented single crystals. All the possible atomic planes will simultaneously diffract and the beams are distributed along cones. Their projections on a detector are the so called Debye-Scherrer circles. A huge practical advantage of powder diffraction is that chemical compounds are much more easily obtained in the form of a powder than in the form of a single crystal and measurements are much faster.
The majority of powder diffraction studies targets the analysis of phase transformations under the influence of external parameters (temperature, pressure, magnetic field…) together with the refinement of structural parameters. In particular, functional materials can be investigated in conditions close to those of their application. However, a powder provides less information for structural solution than a single crystal of the same material. The reason for this is that Debye-Scherrer circles do not distinguish between different lattice planes if they share the same diffraction angle.
Where to find this technique?
Powder diffraction can be found at:
Single crystal diffraction
A single crystal (monocrystalline solid) is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries. A single crystal yields diffraction spots instead of Debye-Scherrer circles. Each spot represents a unique lattice plane (in the sense of group of identically oriented lattice planes). In a single crystal experiment the orientation and intensity of each lattice plane is measured separately. The price for this significant gain of information for structure solution is the severe amount of time to be spent for such an experiment and the need for sufficiently large single crystals.
Where to find this technique?
Single crystal diffraction can be found at: