Scientists have long sought to better understand the “local structure” of materials, meaning the arrangement and activities of the neighboring particles around each atom. In crystals, which are used in electronics and many other applications, most of the atoms form highly ordered lattice patterns that repeat. But not all atoms conform to the pattern.
When some atoms take up local arrangements that are different than that implied by the overall structure of the crystal, studying the local structure gets more difficult — especially when the atoms are moving. In fact, the inability to clearly see these local effects means researchers are often not aware they can happen.
Now researchers using the Spallation Neutron Source at Oak Ridge National Laboratory have developed a new method of studying the local structure of materials in detail and in real time.
The team developed a variable-shutter pair distribution function, or vsPDF, technique in which neutrons function like a camera but at timescales that are a trillion times faster.
Results of the research, led by Columbia University and scientist Simon Kimber, demonstrate a unique use of neutrons that could become a standard method for reconciling local and overall structures in energy materials. The research also revealed a key mechanism behind the thermoelectric effect, in which temperature differences in a material can be converted into an electric voltage or, conversely, the material can be used to heat or cool when a voltage is applied to it. The new local structure analysis method uses the ultrabright flashes of neutrons produced by SNS. When these neutrons pass through a material, the resulting scattering patterns yield information about the material’s atomic arrangement. Using a novel energy filtering technique, the team determined how to change the effective shutter speed of the energy to produce representations — or images — of the atomic arrangement. In conventional cameras, images of moving objects blur at slower shutter speeds. Gradually increasing the shutter speed will eventually freeze the moving atoms in an unblurred image.
The researchers were able to demonstrate the same effect on a timescale that allowed them to observe atomic motions in the material. This capability offered key insights into how the material produces its outstanding thermoelectric performance.
“Neutrons have exceptional properties as a probe of materials,” said Simon Kimber, lead scientist on the project and the paper’s lead author. “They have wavelengths comparable to the spacing between atoms, but they also have an energy similar to the vibrations of the atoms. This unique combination, along with the advanced instrumentation at SNS, allowed us to use the variable shutter speed method.”
The team developed the new technique while investigating the material properties of germanium telluride, or GeTe, which is closely related to materials in thermoelectric-based generators such as those used to power deep space missions like Voyager and the Mars rover.
“We are tantalizingly close to having quiet, energy-efficient and thermoelectric solid-state refrigerators in our houses, replacing the noisy and energy-gobbling compressor fridges we have now,” said Simon Billinge, professor of materials science, applied physics and applied mathematics at Columbia and a physicist at Brookhaven National Laboratory. “Similar to the solid-state lighting revolution, we just need materials with slightly better properties for this to happen.
“This study gave us key insights into the basic physics of the nanoscale atomic dance that is happening in GeTe that can hopefully guide us toward engineering better materials. The variable-shutter method allowed us to see which atoms are in the dance and which are sitting it out,” he added.
Thermoelectric effects are produced by a heat gradient — hotter to colder — in materials that resist heat flow while retaining electrical conductivity. Using the vsPDF technique, the research team found that at slower shutter speeds, the atomic structure of GeTe looks highly crystalline. Yet faster exposures revealed an intricate pattern of dynamic displacements that disrupt heat flow.
“Using computer modeling, we calculated the motions of the atoms to visualize and understand how they are moving in the material. We used open-source software developed at ORNL to analyze the data and validate the experimental findings,” said Timmy Ramirez-Cuesta, SNS Instrument Development group leader.
The team collaborated with theoreticians at Argonne, the University of Chicago and the University of Costa Rica. Their work detected generic signatures of changes in chemical bonding induced by atomic motions within the GeTe. These findings could lead to improved thermoelectrics and new types of heating and cooling devices.
The neutron scattering research was supported by DOE’s Office of Science. Other experiments were performed at the European Synchrotron Radiation Facility in Grenoble, France. The GeTe material was synthesized by Northwestern University.
SNS is a DOE Office of Science user facility. ORNL is managed by UT-Battelle for DOE’s Office of Science, the single largest supporter of basic research in the physical sciences in the United States. DOE’s Office of Science is working to address some of the most pressing challenges of our time. For more information, visit energy.gov/science.