Crystallography

Crystallography is the scientific study of molecular and crystalline structures. It examines how atoms and molecules are arranged in solids. The name comes from the Ancient Greek words for "clear ice" and "to write." This science explores everything from basic crystal structures to complex mathematics. It even includes the study of quasicrystals, which are not periodic. Because it is so vital to many fields, the United Nations proclaimed 2014 as the International Year of Crystallography.

To see these structures, scientists use beams of radiation to create diffraction patterns. X-ray crystallography is a very common method. In this process, X-rays interact with the spatial distribution of electrons in a sample. Another method is neutron diffraction. Neutrons are scattered by the atomic nuclei through strong nuclear forces. Because neutrons have a non-zero magnetic moment, they also scatter from magnetic fields. This makes them useful for studying magnetic properties.

Scientists also use electrons to study crystals through electron diffraction. Electrons are charged particles. This charge allows them to interact with both the atomic nuclei and the electrons in a sample. Because they are charged, electrons can be focused using lenses. This allows researchers to use electron microscopes to create highly magnified images. Some techniques, like high-resolution electron microscopy, can achieve atomic resolution. This provides incredibly detailed crystallographic information.

Before the 20th century, crystallography relied on physical measurements of geometry. Scientists used a tool called a goniometer to measure crystal faces. They measured the angles of these faces relative to each other and to crystallographic axes. These points were then plotted on a stereographic net, such as a Wulff or Lambert net. Each point was labeled with a Miller index to establish symmetry. The modern era began in the late 1800s with the discovery of X-rays and electrons. Max von Laue conducted the first X-ray diffraction experiment in 1912. Later, in 1927, the Davisson–Germer experiment and work by George Paget Thomson and Alexander Reid proved electron diffraction was possible.

In materials science, crystallography helps explain why materials behave the way they do. In single crystals, the atomic arrangement often dictates the macroscopic shape. Most materials are actually poly-crystalline, meaning they are made of many small crystals. Scientists use powder diffraction to study these aggregates. Crystallography also explains phase transformations in metals. For example, when iron is heated, it changes from a body-centered cubic structure called ferrite to a face-centered cubic structure called austenite. Because the austenite structure is more close-packed, the volume of the iron actually decreases during this change.

Biology relies heavily on these techniques to understand life at the molecular level. X-ray crystallography is the main way scientists find the conformations of biological macromolecules. This includes proteins and nucleic acids like DNA and RNA. In 1958, the first macromolecule structure was solved using X-ray analysis. This was a three-dimensional model of the myoglobin molecule. Today, these structures are stored in the Protein Data Bank (PDB). This is a free repository where researchers can access biological data.

Crystallography is a deeply interdisciplinary field. It connects to chemistry, physics, biology, and geology. It even relates to group theory through the study of symmetry patterns. Scientists use a wide range of tools for this work. Some use small laboratory diffractometers or electron microscopes. Others use massive facilities like synchrotrons, photoinjectors, or free-electron lasers. These advanced tools allow us to continue discovering how the microscopic world builds the macroscopic world.