How Are Materials Structured?

Structures of materials

Every solid is made up of crystal structures. Solids have many of these structures placed next to each other in a periodic array throughout the material. Amorphous solids, like glass and polymers, do not exhibit this periodicity. Those that do are called crystalline solids. For a crystalline material, one could break up its structure into three fundamental components: lattice, basis, and crystal structure. A lattice is an infinite periodic array of points in space. The atoms that take the spots of these points are called the basis. The crystal structure is just a lattice with a basis in its points.

While crystal structures tell us how the atoms are repeated in space, crystal systems show other types of symmetry. A simple way of looking at crystal systems is that they are periodically repeating shapes. While there are 14 ways that atoms are placed in space (the 14 Bravais lattices), there are 7 crystal systems. These systems are cubic, hexagonal, rhombohedral (or trigonal), tetragonal, orthorhombic, monoclinic, and triclinic. In the diagram below, every shape is the unit cell of the structure. Unit cells are the smallest parts of the material structure that are repeatable. Alpha-iron, for example, has a face-centered crystal structure, cubic crystal system, and a basis of just iron atoms. A simple way of saying this is that alpha-iron is face-centered cubic, or FCC. In the bulk of the iron, we can imagine all of these face-centered cubes (unit cells), stacked on each other.

Special structures

There are some special materials that have periodic structures that are not listed above. Those structures include rock salt, CsCl (cesium chloride), diamond cubic, ZnS (zinc blende), and perovskites. While these structures all belong to cubic systems, they are unique in either how their basis is organized or their lattices. For example, the only difference between zinc blende and diamond cubic is that the atoms which are not in the FCC lattice of zinc blende are different than the one’s which are. All of the atoms in diamond are carbon, while the basis of zinc blende has sulfur atoms in the FCC lattice and zinc in the rest of the spots. These structures all have different properties that will be discussed in depth in other posts.

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