Question: What does 'crystallographer' mean?

Andrew Cairns answered on 16 Nov 2011:

A crystallographer is someone who works out the structures of any substance around you. Both elements and compounds are made up of atoms arranged in 3D space in a specific way that is a characteristic of that substance. We use several tricks to help us in ‘seeing’ the 3D structure of things:

1) The unit cell. You can tile a surface with certain shapes, so example pavements have rectangular or hexagonal tiles placed so they completely cover the surface. Triangular tiles are also possible, but it is impossible to completely cover a surface with pentagons (ALMOST! SEE BELOW!)

This idea is the same in 3D space, we can ’tile’ a volume with different smaller 3D shapes.

The unit cell is like the lego building blocks that make up our structure. How the atoms are placed in the unit cell are worked out by passing x-rays or neutrons through the substance (either a powder or a single crystal) and using a computer to work out what is happening!

2) symmetry. This is a very powerful idea – within the unit cell some kind of symmetry usually exists, so an atom in one corner will be reproduced at all other corners. In very large structures it may be possible to define a unit cell with 100s of atoms by describing 10 atoms and how the symmetry works!

Being a ‘crystallographer’ also means that you have to grow crystals. These are very beautiful! I know I have mentioned this before, but they are very beautiful! Like diamonds!

There are different types of crystallographer as well, some do solids like the steel making your table legs, some do frameworks like I do, some do proteins, some do small molecules like medicines you might take, and other crazy people do structures that do not conform to the usual ‘rules’ of crystallography.

The crazy side is maybe the most interesting! We can work out the structures of things with no structure like methanol and water.

Now, that pentagon tiling. The Nobel Prize was awarded this year for ‘quasi crystals’ – crystals that have 5-fold symmetry. The crystallographer who won the prize was told for many years that he was wrong, but he believed in his own work. In 2D, we can tile pentagons in a pattern known as penrose tiling.

http://en.wikipedia.org/wiki/File:Penrose_Tiling_(Rhombi).svg
http://www.bbc.co.uk/news/science-environment-15181187