Researchers observe atomic movements to better understand dielectric materials

The work advances understanding of dielectric materials, which are used in a wide variety of applications from handheld electronics to defibrillators.

“Dielectric materials are insulators that can store and manage electric charge. But we hadn’t yet directly measured how atoms move in dielectric materials in order to store that charge,” said Tedi-Marie Usher, a Ph.D. candidate in materials science and engineering at NC State and lead author of a paper on the work.

The researchers applied voltage to a dielectric material and simultaneously bombarded the material with X-rays from a synchrotron at Argonne National Laboratory’s Advanced Photon Source. When the X-rays hit the material, they scatter into a pattern of bright rings. Typically, to figure out the arrangement of atoms in a material, the positions and intensities of these bright rings are analysed.

NC State said that by applying new mathematical techniques that are more sensitive to the weak (dim) scattered X-rays, the researchers could determine changes in the placement of specific atoms within the crystalline structure of the material.

“What’s really new here is that this technique is much more sensitive to the behaviour of select atoms relative to their neighbouring atoms, rather than looking at an average of all the atoms in a sample,” said Jacob Jones, a professor of materials science and engineering at NC State and corresponding author of the paper.

According to the university, the work uses a pair distribution function, which allows researchers to extract information about how atoms are arranged at extremely small length-scales based on the weak intensity X-rays diffracted from a sample. The researchers evaluated three different dielectric materials for this study.

“One of the interesting findings here is that each of the three dielectric materials we tested exhibited very different behaviours at the atomic level - there was no single atomic behaviour that accounted for dielectric properties across the materials,” Jones said in a statement.

The researchers tested sodium bismuth titanate - a non-toxic material that is thought to be promising for use in dielectric devices. In the absence of an electric field, researchers knew that the bismuth ions are off-centre relative to neighbouring atoms. But different bismuth ions would be off-centre in different directions. However, when an electric field is applied, virtually all of the bismuth ions shifted so they were off-centre in the same direction as the electric field.

“Neither of the other dielectric materials exhibited similar behaviour,” Usher said. “One of our questions for future work is whether the bismuth behaviour we saw in sodium bismuth titanate is consistent across bismuth-based dielectrics.”

“We also want to know how dielectric materials and other complex materials, such as high-entropy alloys, behave at the atomic scale when under mechanical stress,” said Jones.

The paper, “Electric-field-induced local and mesoscale structural changes in polycrystalline dielectrics and ferroelectrics,” is published online in Nature’s Scientific Reports.