Definition - What does Crystalline mean?

Crystalline is an adjective that describes the periodic translational ordering of atoms or molecules within a solid. The atoms or molecules form a three-dimensional arrangement within a single repeating unit called a unit cell. The unit cell structure repeats in all directions at regular spacing, filling a regular three-dimensional grid called a lattice. A high degree of this ordering, or crystallinity, is the property required for a solid to be classified as a crystal.

Crystalline solids can be of the following types, depending on the nature of bonding between unit cells within the lattice:

  • Metallic
  • Ionic
  • Network (extended covalent)
  • Molecular

The degree of crystallinity of a solid affects properties of the solid such as its density, hardness, transparency and diffusion rate. Although crystals are often associated with transparency, non-crystalline (amorphous) solids such as glass can also be crystalline. To the contrary, although a piece of metal does not look like a crystal, it may in fact be in a crystalline state.

Crystalline states are extremely useful when studying a material's structure and looking for defects and dislocations within the crystalline state or to understand the desired properties of the material such as band gaps in semiconductors and electrical conductivity in other materials.

Corrosionpedia explains Crystalline

Understanding the property of crystallinity starts from understanding the basic properties of a lattice. Lattices can exist in any dimension. A one-dimensional lattice is most easily explained as a line of dots spaced out in equal distances. One unit cell is the space between two dots.

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Moving to two-dimensional lattices, imagine a series of one-dimensional lattices lined up, row by row, across a two-dimensional plane. The dots in each row do not need to line up, making it possible to have different two-dimensional lattices associated with different angles. The unit cell comprises the box created by four points.

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Three-dimensional lattices repeat these two-dimensional lattices at regularly spaced distances above and below the plane, again at different angles. The boxes created by eight proximal points creates the unit cell repeating unit. In three-dimensional space, there are seven unit cell shapes, which arrange to form 14 possible types of lattices (Bravais lattices). In crystalline solids, each of these unit cells contains the same atoms or molecules arranged in the same way. The contents may be oriented in various ways in the special case that symmetry elements are present. It is these ordered fillings of lattices that create the crystalline state.

The lattice structure, unit cell and unit cell contents and composition are determined through a diffraction experiment, usually using X-rays, but neutron diffraction is used in certain cases. The wavelength of the X-ray chosen is small enough to determine the distances between atoms in a unit cell. Using diffraction, scientists can determine the chemical structure of molecules, metallic materials, ceramic materials and even biological molecules such as DNA or proteins.

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