PHY.K02UF Molecular and Solid State Physics

Crystal structure

In a crystal, atoms are arranged in straight rows in a three-dimensional periodic pattern. A small part of the crystal that can be repeated to form the entire crystal is called a unit cell.

  Asymmetric unit  

  Primitive unit cell  

  Conventional unit cell  


Devices such as solid state transistors, lasers, solar cells, and light emitting diodes are often made from single crystals. Many materials, including most metals and ceramics, are polycrystaline. This means there are many little crystals packed together where the orientation between the crystals is random. When the atoms of a material are not arranged in a regular pattern, it is called an amorphous material. An example of an amorphous material is glass. Even though not all solids are crystals, we will spend most of our time studying crystals since the translational symmetry makes them easier to decribe mathematically. Describing the behavior of more complicating materials usually builds on the understanding that has been acquired by studying crystals.

Some common crystal structures you should know

 Simple Cubic 

 Face Centered Cubic 

 Body Centered Cubic 

 Hexagonal Close Packed 







Kittel Chapter 1: Crystal Structure or R. Gross und A. Marx: Kristallstrucktur 1.1 - 1.2
You don't need to know all of the details of the symmetries in section 1.1.2 of Gross und A. Marx. We will deal with the symmetries in more detail in the lecture on crystal phyiscs. I will introduce the concept of the asymmetric unit which is not in these books but is an important concept for crystallography.