Perovskite – A very useful material
What am I looking at?
This is a super magnified version of a crystal known as a perovskite. Perovskite is the name of both a mineral and other crystal structures with the same simple shape.
To understand crystal structures, scientists need to use high powered microscopes to look at the way atoms and elements fit together. Millions of these tiny crystal structures joined together make materials that we know and use. Diamond, for example, is a special crystal structure made from carbon, shaped in a way that makes it both beautiful and strong.
The atoms that make up perovskite line up to make a box shape like the one you are looking at. Elements such as calcium, strontium or iron can occupy the shapes inside that you can see lit up in different colours.
Perovskite structures are one of the most abundant on Earth. Technology has benefited from these substances that are suited to making sensors, amplifiers, fuel and solar cells.
This is a scale model of the smallest building blocks of the material strontium titanate, which arranges itself into a structure known as the perovskite structure. What’s the scale of this model? The atoms are contained in a box that is 1 metre x 1 metre x 1 metre. In reality each side of this box would only be about 0.3 nanometers in length. That’s 0.00000003 metres. The boxes of atoms are so small that millions of them could fit across one of your eyelashes.
The cubic perovskite, as shown here, features the general formula ABX3 . The central sphere shows the many types of elements that can be placed at this site, which is known as the A site. Watch as it flashes through a number of colours and these colours could represent atoms such as calcium or strontium. In the corners of the structure are purple triangles which connect to red semi-circles. This represents another part of the perovskite structure, the B sites, which are connected to the red X sites. Together these make an octahedral shape in 3D, and like the A site can be made up of a number of elements. For example Ti atoms on the B site and oxygen atoms on the X sites.
The diagonal line of LEDs on one face of the cube represents a plane in the crystal structure from which X-rays, electrons and neutrons can diffract from. The strength of the diffraction signal will depend on the properties of the radiation used and the atoms on this plane. The resulting diffraction from this plane is observed in detectors. Over the course of a measurement many planes in the structure diffract resulting in an overall diffraction pattern as shown below. The plane represented by LED lights is labelled.
Why is this crystal structure important?
Perovskites are named after Russian mineralogist Lev Perovski and were first found in the Ural mountain range in Russia. Pervoskites are interesting due to the simplicity of their atomic arrangement, the number and subtleties of atoms that can be swapped into the structure, and the large range of similar structures). The structure of the perovskite is shown below and takes the general formula ABX3 where B-X bonds are represented as an octahedral shape (BX6) and the central sphere is the A-site.
One of the cool aspects of pervoskites is that the octahedral units can tilt, leading to a range of changes that this structure can undergo. These changes can alter the physical properties of the overall material, which allows us to control things like how strong a magnetic material can be.
Perovskites can also be used as building blocks, much like Lego®, for the construction of all kinds of structures. These are often referred to as perovskite-derived structures which often form a series depending on the number of perovskite blocks (n = 1, 2, etc…). A few examples are shown below:
The perovskite structure is one of the most abundant structures found in materials on Earth. Perovskites and perovskite-derived structures display a variety of cool physical properties such as superconductivity, colossal magnetoresistance and ferroelectricity. They are found in applications such as sensors, amplifiers, fuel and solar cells, electrolytes in batteries, memory and electro-optical devices.
How do we actually know that Perovskite looks like this?
The general crystal structure of all perovskite materials was discovered by an Irish Scientist, Dr Helen Megaw who made a great contribution to our understanding of these materials, particularly their important electronic properties - why they are widely used. Dr Megaw was the first to work out the structure of a perovskite, from her work on barium titanates. This pioneering work led to this perovskite material, calcium tin oxide, being name Megawite after her. She also made a great contribution to the promotion of crystallography to a wider audience, instigating the ‘Atoms to Patterns’ exhibition which featured in the 1951 Festival of Britain. Bringing together the big name science of the day, the exhibition brought atomic drawings to patterns on everyday objects – curtains, plates, ties and ashtrays.