What am I looking at?
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 real life, each side of this box would only be about 0.3 nanometres 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 which represent various different atoms such as calcium or strontium.
In the corners of the structure you can see the purple triangles which connect to red semi-circles. These represent other parts 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, there can be titanium atoms on the B sites 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.
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 clearly 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.
A diagram of the structure of the perovskite is shown below and you can also see 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 properties such as how strongly magnetic a 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 used in sensors, amplifiers, fuel and solar cells, electrolytes in batteries, and memory and electro-optical devices.
The flexibility in the choice of atoms, the distortions and the structural permutations that occur are the reasons why compounds that form perovskites show a variety of properties.
What’s going on in Australia with this material?
A number of Australian researchers have studied perovskite-based materials including, but not limited to, looking at its structural chemistry (Prof. Brendan Kennedy at the University of Sydney), applications in ferroelectrics (Dr. John Daniels at UNSW Australia and Professor Eric Kisi at the University of Newcastle), magnetic properties (A/Prof. Chris Ling at the University of Sydney, A/Prof. Clemens Ulrich at UNSW Australia and Dr. Maxim Avdeev at ANSTO), use in solar cells (Prof. Martin Green at UNSW Australia) and perovskite-derived structures and their properties (A/Prof. Chris Ling at the University of Sydney and Dr. Neeraj Sharma at UNSW Australia). These solid-state, inorganic and materials scientists have made new perovskites, determined the subtleties in their atomic arrangements or used them in a range of applications.