It's a bird… it's a plane… it's a superstructure!

What does it look like?

   

This structure consists of four unit cells of SrTiO₃ alternated with three unit cells of PbTiO₃. The Sr atoms are green, Pb atoms are grey, Ti atoms are blue, and O atoms are red. Image generated by the VESTA (Visualisation for Electronic and STructural analysis) software http://jp-minerals.org/vesta/en/

What is it?

Regular Crystallography365 readers may remember the perovskite structure. Since this is a relatively simple crystal structure and can be found with a wide variety of atoms on the cube corners and centers, it's relatively easy to build "stacks" of alternating layers of different perovskites. When these stacks are layered in a repeated manner, this is called a superstructure.

Superstructures are exciting because of the many effects that occur due to the individual properties of the constituent layers and also because of how those layers interact. One of the most well-studied systems has been LaAlO₃/SrTiO₃. Even though both materials are insulators by themselves, when you alternate them in thin layers superconductivity appears at the interfaces!

The example above shows a superstructure alternating layers of PbTiO₃/SrTiO₃, and is actually an example of diarchetypal modular structure, being built by modules (layers here) taken from two different structures. PbTiO₃ is a ferroelectric, meaning it has a native electric polarization (this comes from the O and Ti atoms being displaced from the unit cell center – which you can see if you look closely). When you reduce the number of PbTiO₃ layers you may expect the superstructure to become less ferroelectric, but competing structural distortions of the individual PbTiO₃ and SrTiO₃ layers at the interface actually allow the whole system to sustain ferroelectricity even when the sample is made up of mostly non-ferroelectric SrTiO₃.

   

A TEM image of a perovskite superstructure

Of course, superstructures don't have to just be made of perovskites. However, because all perovskites share a general form it is easy to alternate thin layers of different materials while ensuring that they maintain good crystal quality.

Where did this structure come from?

Both the crystal structure module and TEM image show PbTiO₃/SrTiO₃ superstructure. It was made by starting with the SrTiO₃ structure (#9006864 in the Crystallography Open Database) and then adding a PbTiO₃ layer (#9011192 in the Crystallography Open Database). The in-plane cell parameters of the PbTiO₃ layer were confined to those of the SrTiO₃ layer to simulate epitaxial strain. As a result the out-of-plane cell parameter of the PbTiO₃ layer was made larger, which is observed experimentally.

Tags: superstructure   perovskite   physics