In the world of materials that usually expand when heated, the one that compresses along one 3D axis while expanding along the other stands out. This is especially true when unusual shrinkage is due to a property important for thermoelectric devices that convert heat into electricity or electricity into heat.
In an article just published in a journal Advanced materialsA team of scientists from Northwestern University and the Brookhaven National Laboratory of the U.S. Department of Energy describe previously hidden subdimensional origins of both the unusual shrinkage and the exceptional thermoelectric properties of this material, silver gallium telluride (AgGaT2). The discovery shows a quantum-mechanical turn in what drives the emergence of these properties – and opens up a whole new direction for the search for new high-efficiency thermoelectrics.
“Thermoelectric materials will be transformative in clean and sustainable energy technologies for heat collection and cooling, but only if their performance can be improved,” said Hunyao Xie, a Northwestern doctoral student and lead author. “We want to find basic design principles that will allow us to optimize the performance of these materials,” Xie said.
Thermoelectric devices are currently used in limited niche applications, including NASA’s rover, where the heat released by the radioactive decay of plutonium is converted into electricity. Future applications may include voltage-controlled materials to achieve very stable temperatures critical to the operation of high-tech optical detectors and lasers.
The main obstacle to wider distribution is the need for materials with the right properties, including good electrical conductivity but heat resistance.
“The trouble is that these desirable traits tend to compete,” said Mercury Canadidis, a professor in the Northwest region who initiated the study. “Most materials combine electronic and thermal conductivity, and both are high or low. Very few materials have a special “high-low” combination.
Under certain conditions, silver gallium telluride seems to have the right material – very mobile electrons and ultra-low thermal conductivity. In fact, its thermal conductivity is much lower than theoretical calculations and comparison with similar materials such as copper-gallium telluride.
Northwestern scientists turned to colleagues and tools at the Brookhaven Lab to find out why.
“It took a thorough X-ray study at Brookhaven’s National Synchrotron Light Source II (NSLS-II) to detect previously hidden subdimensional distortions in the position of silver atoms in this material,” said Brookhaven Laboratory Physicist Emil Bozin. structural analysis.
Computational simulations have shown how these distortions cause the crystal to shrink along one axis – and how this structural shift dissipates atomic vibrations, thereby blocking the propagation of heat in the material.
But even with this understanding, there was no clear explanation for what caused the subdimensional distortion. Additional computational modeling by Christopher Wolverton, a Northwestern professor, revealed a new and subtle quantum mechanical origin of the effect.
Together, the findings point to a new mechanism for reducing thermal conductivity and a new guiding principle in the search for better thermoelectric materials.
Display of atomic positions
The team used X-rays on the NSLS-II pairwise distribution line (PDF) to reflect the “large-scale” arrangement of atoms in both copper-gallium and silver gallium telluride in the temperature range to see if they could detect why these the two materials behave differently.
“The flow of hot air heats the sample with degree accuracy,” said Milinda Abeikun, a lead scientist on PDF Beamline. “At each temperature, when X-rays are reflected from atoms, they create patterns that can be translated into measuring the distances between each atom and its neighbors (each pair) with high spatial resolution. Computers then collect measurements into the most likely 3D atoms.” .
The team also conducted additional measurements over a wider temperature range, but with a lower resolution, using a light source at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany. And they extrapolated their results to an absolute zero temperature, the coldest that could be.
The data show that both materials have a diamond-shaped tetragonal structure of angled tetrahedra, one with a copper atom, and the other with silver in the center of the tetrahedral cavity of the 3D object. Describing what happened when these diamond-shaped crystals heated up, Bozin said, “We immediately saw a big difference between the silver and copper versions of the material.”
The copper-based crystal expanded in all directions, but the silver-containing crystal expanded along one axis until shrinking along with another.
“It turns out that this strange behavior comes from the silver atoms in this material, which have a very large amplitude and chaotic vibrations inside the structural layers,” said Simon Billing, a professor at Columbia University with a joint appointment as a physicist in Brookhaven. “These vibrations cause the connected tetrahedra to sway and jump with great amplitude,” he said.
This was a hint that symmetry – the regular arrangement of atoms – could be “broken” or broken on a more “local” (smaller) scale.
The team turned to computational simulation to see how different local symmetry distortions of silver atoms would match their data.
“The one that worked best showed that the silver atom moves away from the center of the tetrahedron in one of four directions, to the edge of the crystal formed by the two tellurium atoms,” Bozin said. On average, random shifts due to the center are leveled, so the overall tetragonal symmetry is maintained.
“But we know that the larger structure is also changing, shrinking in one direction,” he said. “As it turned out, local and larger-scale distortions are interrelated.”
Twisting of tetrahedra
“Local distortions are not accidental,” Bozin explained. “They correlate between adjacent silver atoms – those that are connected to the same tellurium atom. These local distortions cause neighboring tetrahedra to rotate relative to each other, and this twisting causes the crystal lattice to contract in one direction.”
When the shifting atoms of silver twist the crystal, they also dissipate certain undulating oscillations called phonons, which allow heat to propagate through the lattice. Scattering of AgGaTe2Energy-carrying phonons prevent the spread of heat by dramatically reducing the thermal conductivity of the material.
But why do silver atoms shift in the first place?
Scientists from Brookhaven saw similar behavior a decade earlier in a rock salt material similar to lead telluride. In this case, when the material was heated, “lone pairs” of electrons formed, creating tiny areas of split electric charge, called dipoles. These dipoles distracted the centrally located lead atoms from the center and scattered phonons.
“But there are no single pairs of silver gallium telluride. So there must be something else in this material – and probably other” diamond “structures,” Bozin said.
Bending behavior of bonding
Christopher Wolverton’s calculations at Northwestern showed that “something else” is the bonding characteristics of electrons orbiting silver atoms.
“These calculations compared silver and copper atoms and found that there is a difference in the placement of electrons in the orbitals, so silver tends to form weaker bonds than copper,” said Xie of Northwestern. “Silver wants to bind to fewer neighboring tellurium atoms; it wants a simpler communication environment. ”
Thus, instead of bonding equally to all four surrounding tellurium atoms, like copper, silver tends to predominantly (but randomly) approach two of the four. These binding electrons are what distract the silver atom from the center, causing twisting, shrinkage, and vibrational changes that ultimately reduce the thermal conductivity of AgGaTe2.
“We came across a new mechanism that can reduce the thermal conductivity of the lattice,” said Mercury Canadidis of Northwestern. “Perhaps this mechanism can be used to develop or search for other new materials that have such behaviors for future high-efficiency thermoelectrics.”
This study was primarily supported by the US Department of Science. NSLS-II is a user facility of the Ministry of Science of the Ministry of Science.