Distortion-detecting microscopy to shed light on material properties

Researchers from North Carolina State University are using a technique they developed to observe minute distortions in the atomic structure of complex materials.

By doing so, they expect to find out the causes these distortions and facilitate studies on how such atomic-scale variations can influence a material’s properties.

Researchers have long known that the properties of complex materials, such as alloys, are influenced by how the material’s component atoms are organised.

‘We knew where the atoms were on average, but we also knew that there were variations in a material – there can be significant displacements, where atoms don’t fit into that average pattern,’ said Dr. Doug Irving, an associate professor of materials science and engineering at NC State and co-author of a paper describing the new work.

‘However, detecting these distortions required indirect methods that could be difficult to interpret, so we couldn’t fully explore how a material’s atomic structure affects its properties,’ said Dr. James LeBeau, an assistant professor of materials science and engineering at NC State and corresponding author of a paper describing the new work.

‘Now we’ve come up with a way to see the distortions directly, at the atomic scale,’ LeBeau said in a statement. ‘We can create a precise map of atomic organization, including the distortions, within a material. Not only which atoms fit into the structure, but how far apart they are, and how distortions in the structure are related to the chemistry of the material.’

The work builds on a technique LeBeau developed called revolving scanning transmission electron microscopy (revolving STEM).

To test the technique and learn more about the links between structural distortions and chemical bonds, the researchers looked at lanthanum strontium aluminium tantalum oxide (LSAT) due to the significant variability in the nature of the chemical bonds within the material.

‘It’s a mess,’ LeBeau said. ‘We didn’t know how the complexity of those bonds influenced structural distortions, and we wanted to see if revolving STEM would give us any insights.’

On the left, the image shows the distortion of lanthanum and strontium directly resolved at the atomic scale. Blue and red colours indicate contraction and expansion of the local structure respectively. On the right, the aluminium and tantalum sites exhib
On the left, the image shows the distortion of lanthanum and strontium directly resolved at the atomic scale. Blue and red colours indicate contraction and expansion of the local structure respectively. On the right, the aluminium and tantalum sites exhibit dramatically different distortion behaviour due to bonding. These images are made possible by a technique developed at North Carolina State University called revolving scanning transmission electron microscopy

The researchers found that the weaker chemical bonds that hold lanthanum and strontium in place in LSAT’s atomic structure made them more susceptible to being pushed or pulled by small variations in their chemical environment.

‘We never would have been able to directly see the extent of that variation before,’ LeBeau said.

‘Now that we can see these subtle distortions, and know what causes them, the next step is to begin work to understand how these structural differences affect specific properties. Ultimately, we hope to use this knowledge to tailor a material’s properties by manipulating these atomic distortions.’

The paper, Direct observation of charge mediated lattice distortions in complex oxide solid solutions, has been published online in Applied Physics Letters