Glossary
Electron diffraction & crystallography, defined.
A plain-language reference for the terms behind nanocrystal structure determination.
Asymmetric unit
An asymmetric unit of a space group is the simply connected smallest group of atoms – closed part of space from which, by application of all symmetry operations of the space group, the whole of space is filled. This implies that:
Bragg’s law
Bragg’s Law is an equation formulated by Sir W. H. Bragg and his son, which predicts whether diffraction can take place or not. The equation provides the condition for a plane wave to be diffracted by a family of lattice planes:
Crystal Lattice
To describe the structure of a crystal, it is only necessary to know the simplest repeating “motif” and the length and directions of the three vectors which together describe its repetition space. The motif can be a molecule or the building block of a network structure. Normally, it consists of several such units, which may be converted into one another by symmetry operations.
Cryogenics
The field of cryogenics studies the regularities of properties alternation of various substances under conditions of extremely low (“cryogenic”) temperatures; involves technologies and hardware-methodical means of work at low temperatures. Temperatures below -180°C (93 K; −292 °F) are considered as cryogenic.
Nanocrystal
A material particle having dimensions in the range form 1 – < 1000 nm (i.e, < 1 x 10-6 m) and composed of atoms in either a single- or poly-crystalline arrangement. Nanocrystals of certain substances may have beneficial (or different) chemical and physical properties comparing to larger crystals. Especially if their dimensions range from 1 – 100 nm (quantum dots effects).
Polymorphism
The phenomenon in which the same solid chemical compound can adopt different forms and crystal structures. In 1969, Rosenstein & Lamy propose a definition: ‘‘When a substance can exist in more than one crystalline state it is said to exhibit polymorphism.’’ Polymorphs can provide valuable insights into crystal packing and structure-property relationships.
Powder material
A powder is a large number of crystallites or particles (grains, agglomerates or aggregates; crystalline or non-crystalline) irrespective of any adhesion* between them and thus can be a loose powder, a solid block, a thin film or even a liquid. An ideal powder is represented by a virtually unlimited number of sufficiently sized, randomly oriented and spherical crystallites.
Radiation damage
Damage caused to the crystals by the X-Ray beam , resulting into the change of the ordered structure of crystalline material. Very rarely these changes may be beneficial, but generally they cause harmful modifications of properties. Electrons, which have stronger interaction with matter (as X-rays for example), can produce more radiation damage on organic samples than commonly used sources of X-rays.
Single crystal
A single-crystal is a solid material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries. Single crystals are a requirement for X-ray diffraction experiments.
Structure determination
Crystal structure determination is a two-part process: (a) the determination of the size and shape of the unit cell (i.e. the lattice parameters) from the geometry of the diffraction pattern and (b) the determination of the lattice type and distribution of the atoms in the structure from the (relative) intensities of the diffraction spots.
Synchrotron radiation
Electromagnetic energy emitted by charged particles which are accelerated at speeds close to the speed of light in a curved orbit. Instead of using the characteristic X-radiation produced by X-ray tubes or microfocus sources, it is possible to make use of the radiation produced as a by-product of particle acceleration in a synchrotron.
Twinning
A twin consists of two or more single crystals of the same species but in different orientation, its twin components. They are intergrown in such a way that at least some of their lattice directions are parallel. The twin law describes the geometrical relation between the twin components.
Have a sample these terms don't quite cover?
Start with a feasibility study – we'll tell you whether electron diffraction can resolve it.
Discuss feasibility study