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Unit Cells

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The Same But Different

The Same But Different

Credit: NASA/Marshall
Source: http://science.nasa.gov/media/medialibrary/1999/09/10/msad20sep99_1_resources/9901879.jpg
License: CC BY-NC 3.0

Small molecules are not the only ones that can form crystals. A protein such as insulin will also form regular structures in the solid phase. These crystals can then be studied using X-ray crystallography to determine the three-dimensional configuration of the molecule.

Why It Matters

  • When we talk about crystals, we usually see something like the very orderly sodium chloride crystal. Our textbooks tell us about the structure of the crystal and stress the regular, three-dimensional arrangement of components of the crystal system. However, life does not always work according to the textbook, and crystal structure is no different.
  • Can we get a true copper metal crystal? If we are working with a very small amount of material, we may be able to. But when we work with larger amounts, as the melted metal solidifies, we will get polycrystals (the fusion of several adjacent crystals). The entire structure will then not be a single orderly crystal. All metals form this polycrystalline type of structure, as do many ionic compounds.The polycrystals tend to form when the material is cooled rapidly. Crystal formation occurs simultaneously at many sites and the ordered regions grow into one another.
  • Credit: CORE-Materials
    Source: http://www.flickr.com/photos/core-materials/3838569603/
    License: CC BY-NC 3.0

    Tetragonal Zirconia polycrystals are extremely tough and dense [Figure2]

     

  • Quasicrystals are yet another anomaly in the world of crystal science. These crystals have order to them, but do not repeat in a periodic fashion. The NaCl crystal will look the same and have the same structure wherever you look in the crystal. Quasicrystals have a different organization of the units that depends on the orientation of the material. These crystals tend to have high mechanical strength and poor electrical conductivity.
  • Watch a video about NaCl crystals at the link below:

https://www.youtube.com/watch?v=csfOBynrF8E

Show What You Know

Use the links below to learn more about crystals. The answer the following questions.

  1. What are the individual components of a polycrystal called?
  2. What are grain boundaries?
  3. How can grain boundaries be observed?
  4. Who discovered quasicrystals and when?
  5. What did scientists at the DOE Ames lab discover?

Image Attributions

  1. [1]^ Credit: NASA/Marshall; Source: http://science.nasa.gov/media/medialibrary/1999/09/10/msad20sep99_1_resources/9901879.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: CORE-Materials; Source: http://www.flickr.com/photos/core-materials/3838569603/; License: CC BY-NC 3.0

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