A variation on a theme

SrFeO3

What am I looking at?

This is a scale model of the smallest building blocks of the perovskite structure.  What’s the scale of this model?  The atoms are contained in  a box that is 0.6 metre x 0.6 metre x 0.6 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 the hairs in your nostril. 

The cubic perovskite has the general formula ABX3. The central sphere is showing 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 - say blue representing calcium and green representing strontium.

You would choose this element based on how you want to use the material that you make. In the corners of the structure are orange/yellow triangles which connect to red semi-circles. These represent other parts of the perovskite structure - the B sites which are connected to the the red X sites. Together these make an octahedral shape and, like the A site, can be made up of a number of elements. An example might be titanium atoms on the B sites and oxygen atoms on the red X sites.   

Now if we only consider the orange/yellow triangles this gives a modified perovskite structure which has the composition SrFeO2. As you can see in moving from SrFeO3 to SrFeO2 we have lost an oxygen atom in the formula unit. How is this possible?

Well, there is an aspect of elements termed oxidation state and it refers to the amount of electrons on a particular species. So, metallic iron (Fe) has an oxidation state of zero, which can be written as Fe0. In SrFeO3, strontium (Sr) has an oxidation state that is very difficult to change 2+ or Sr2+ and oxygen is similarly difficult to change 2- or O2-.

Now if we do some maths (charge balancing) we find Fe has an oxidation state of 4+ or Fe4+ in SrFeO3 (c.f. Sr2+ + Fex + 3 x (O2-) = 0) while in SrFeO2 Fe has an oxidation state of 2+ or Fe2+. The SrFeO3 structure is interesting as the B-site or Fe does not exist with an octahedral arrangement of oxygen atoms around it; rather there is a plane of atoms. 

Why is this crystal structure important?

Perovskites are an important class of minerals and more information can be found here. What makes them so cool and interesting from a scientist’s perspective is the ability to make subtle changes to the atoms involved by substitutions, for example, and the range of related structures that are formed. One important question at the moment is how we form SrFeO2. Or how we take this:

small perovskite image 1

and make this:

small perovskite image 2

It’s the first time anyone has been able to make a synthetic mineral based on iron in a networked square planar configuration. This means that each iron atom is surrounded by four oxygen atoms that form a 2D square around it, rather than spacing themselves equally in a 3D tetrahedral arrangement as would usually be expected.

Each of the squares shares its corners with four others, resulting in networked sheets of FeO2 that sandwich the Sr atoms in the crystal. It’s such an unusual configuration for iron that it has never been found in nature. In fact, SrFeO2 is the first Fe-based structure of its kind, and was only achieved in 2008 by the use of novel synthetic methods.

We don’t really know how networked square planar iron should behave, so the discovery of this structure gives us our first chance to find out. Similarly-layered copper compounds were the basis of early superconductors discovered in the late 1980s. Unfortunately, despite several studies which have doped SrFeO2 with other metals such as Ca, Ba, Mn, Co and even Eu, superconductivity has not yet been achieved in this material.

Still, some interesting and useful behaviour has been identified so far. SrFeO2 is an electrical insulator and an antiferromagnet, but under enormous pressure (34 GPa), it becomes a ferromagnet (attracted to magnets) and conducts like a metal. Scientists very recently found that thin films of SrFeO2 grown on KTaO3 substrates were conductive even at atmospheric pressure.

Heating the structure to “buckle” it drastically disrupts the magnetism. Similarly, replacing small amounts of iron with manganese affects its magnetic properties considerably. Physicists especially are interested in such “couplings” - controlling the magnetism of a substance through changes to its crystallographic or compositional properties.

SrFeO2 has also been used as a reaction intermediate in the synthesis of new and interesting fluorinated compounds like SrFeO2F. Fluorination reagents are notoriously hazardous, so safer and more user-friendly alternatives like the SrFeO2 derivatives should make the discovery and production of new oxyfluoride materials much more accessible. 

What’s going on in Australia with this material?

Modified perovskites based on Sr and Fe have been studied extensively because they have the potential to replace more expensive and toxic cobalt-based materials in certain applications, most importantly as oxide ion conducting electrodes in solid oxide fuel cells (SOFCs).

For example, Australian scientists at the University of Sydney and ANSTO have been studying the crystallography and physical properties of SrFeO2.5, another perovskite-derived compound halfway between SrFeO3 and SrFeO2, to learn more about its conductive behaviour at high temperatures (>600 °C).


 

Location

Questacon, Parkes, ACT
From the 18th August