Unit cells (crystal structure)

[raintrek]

I'm trying to clarify for myself the difference between a Unit Cell, a Primitive Unit Cell and a Conventional Unit Cell.

As far as I know,

• Primitive Unit Cells contain only one lattice point and are the smallest possible unit cell
• Unit Cells are the crystal's smallest building block, and when stacked represent the bulk crystal
• A Conventional Unit Cell displays the full crystal symmetry by using the conventional choice of lattice vectors, but isn't automatically the smallest (ie, primitive)

Is that concise enough? To me it seems as though the lines between them are somewhat blurred...

 

[Jhero]

You are correct in your understanding of the differences between a Unit Cell, a Primitive Unit Cell, and a Conventional Unit Cell. However, let me provide some additional clarification to help solidify your understanding.

A Unit Cell is the smallest repeating unit in a crystal lattice. It contains the basic structural and symmetry elements of the crystal and can be replicated in three dimensions to create the entire crystal structure. It is important to note that a Unit Cell is not necessarily the smallest possible unit cell, as there are variations that can be made to it while still maintaining the crystal structure.

A Primitive Unit Cell is a specific type of Unit Cell that contains only one lattice point at each of its corners. This means that it is the smallest possible unit cell for a particular crystal structure and has the simplest symmetry elements. It is often used as a reference point for other types of Unit Cells.

A Conventional Unit Cell is a Unit Cell that displays the full crystal symmetry by using the conventional choice of lattice vectors. This means that it may contain more than one lattice point at each of its corners, but it accurately represents the crystal's symmetry and can be used to easily determine the crystal's properties and orientation.

In summary, a Unit Cell is the basic repeating unit of a crystal, a Primitive Unit Cell is the smallest possible unit cell for a specific crystal structure, and a Conventional Unit Cell displays the full symmetry of the crystal. While there may be some overlap between these definitions, understanding the distinctions can help in accurately describing and analyzing crystal structures. I hope this helps to clarify any confusion.