Hidden distortions activate promising thermoelectric property

Brookhaven Lab members of the research team: Simon Billinge, Milinda Abeykoon, and Emil Bozin adjust instruments for data collection at the Pair Distribution Function beamline of the National Synchrotron Light Source II. In this setup, a stream of hot air heats samples with degree-by-degree precision as x-rays collect data on how the material changes. Credit: Brookhaven National Laboratory

In a world where most materials expand when heated, one that contracts along one 3-D axis while expanding along another stands out. This is especially true when the anomalous shrinkage is associated with a crucial feature for thermoelectric devices, which convert heat to electricity or electricity to heat.

In a paper just published in the journal Advanced Materials, a team of scientists from Northwestern University and the U.S. Department of Energy’s Brookhaven National Laboratory describe the previously unknown sub-nanoscale origins of both the unusual shrinkage and the exceptional thermoelectric properties in this material, silver gallium telluride (AgGaTe2). The study reveals a quantum mechanical twist on what drives the creation of these features and opens an entirely new avenue for the quest for new high-performance thermoelectrics.

“Thermoelectric materials will be transformative in green and sustainable energy technologies for heat energy harvesting and cooling, but only if their performance can be enhanced,” said Hongyao Xie, a postdoctoral researcher at Northwestern and the paper’s lead author. “We want to identify the fundamental design concepts that will allow us to optimise the performance of these materials,” Xie explained.

Heat generated by the radioactive decay of plutonium is converted to energy in a handful of niche applications, including NASA’s Mars rover. Future applications may involve the use of voltage-controlled materials to obtain the extremely stable temperatures required for the functioning of high-tech optical detectors and lasers.

The requirement for materials with the ideal combination of qualities, particularly good electrical conductivity and resistance to the flow of heat, is the primary obstacle to widespread adoption.

This project was conceived by Mercouri Kanadzidis, a professor at Northwestern University. “In the majority of materials, the electronic conductivity and thermal conductivity are related and are either high or low. Very few materials possess the unique high-low mix.”

Under some situations, silver gallium telluride appears to possess the ideal characteristics: highly mobile conducting electrons and ultralow thermal conductivity. In actuality, its thermal conductivity is substantially lower than predicted by theoretical calculations and comparisons with related materials such as copper gallium telluride.

The Northwestern scientists consulted Brookhaven Lab colleagues and resources to determine why.

“It took a rigorous X-ray inspection at Brookhaven’s National Synchrotron Light Source II (NSLS-II) to disclose a previously undetected sub-nanoscale distortion in the locations of the silver atoms in this material,” said Brookhaven Lab physicist Emil Bozin, who led the structural investigation.

Computational modelling demonstrated how these distortions drive the one-axis crystal contraction and how this structural shift scatters atomic vibrations, so preventing the transport of heat in the material.

Even with this knowledge, it remained unclear what caused the sub-nanoscale aberrations. Professor Christopher Wolverton of Northwestern University’s complementary computational modelling suggested a fresh and subtle quantum mechanical cause for the phenomenon.

The findings hint to a novel mechanism for decreasing thermal conductivity and a new guiding principle in the hunt for superior thermoelectric materials.

Nanoscale distortions: The side view of a basic AgGaTe2 construction block (left) reveals the silver (Ag) atom in the heart of a three-dimensional tetrahedron. In the top-down image (middle), heating causes Ag to shift in one of the four directions indicated by the black arrows. A movement toward a certain edge (bold arrow) causes the tellurium (Te) atoms on that edge to move apart (purple arrows), while the Te atoms on the opposite edge move closer together. In the larger crystal lattice, where connected tetrahedra share Te atoms at their corners (right), the atomic shifts (black and purple arrows) become correlated, forcing adjacent tetrahedra to spin relative to one another (red curved arrow). Brookhaven National Laboratory

Mapping atomic positions

The scientists used x-rays at NSLS-Pair II’s Distribution Function (PDF) beamline to map the “large” scale arrangement of atoms in copper gallium telluride and silver gallium telluride over a variety of temperatures to determine why these two materials behave differently.

“A jet of hot air precisely heats the material,” explained Milinda Abeykoon, the head scientist of the PDF beamline. “As the x-rays bounce off the atoms at each temperature, they form patterns that can be translated into high spatial resolution measurements of the distances between each atom’s neighbours (each pair). The observations are then assembled by computers into the most probable three-dimensional configurations of the atoms.”

Using the light source at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany, the researchers performed additional observations across a broader temperature range but with lesser precision. And they projected their results down to absolute zero, the coldest possible temperature.

The data indicate that both substances have a diamond-like tetragonal structure of corner-connected tetrahedra, one with a single copper atom and the other with silver at the core of the tetrahedral cavity of the three-dimensional entity. Bozin stated, “As soon as these diamond-like crystals were heated, we noticed a significant difference between the silver and copper forms of the material.”

The crystal containing copper expanded in all directions, whereas the crystal containing silver expanded along one axis while contracting along another.

Simon Billinge, a professor at Columbia University with a combined position as a physicist at Brookhaven, explained that this peculiar behaviour was caused by the silver atoms in this material having very wide amplitude and chaotic vibrations within structural layers. “These vibrations force the coupled tetrahedra to bounce and leap with great amplitude,” he explained.

This indicated that the symmetry—the orderly arrangement of atoms—could be “broken” or disrupted on a more “local” (smaller) scale.

The team utilised computer modelling to determine how possible local symmetry aberrations of silver atoms would correspond with their observations.

“The one that worked best demonstrated that the silver atom moves out from the centre of the tetrahedron in one of four ways, toward the edge of the crystal produced by two tellurium atoms,” Bozin added. The random, off-center shifts cancel out on average, preserving the overall tetragonal symmetry.

“However, we are aware that the larger scale structure also alters by contracting in one direction,” he added. As it turns out, local and global distortions are related.

In the undistorted large scale AgGaTe2 crystal (left), a single silver atom (grey) resides in the centre of each tetrahedral cavity. As the off-centering shifts of silver atoms within adjacent tetrahedra become linked during heating, the ensuing rotation causes the entire macroscopic crystal to contract in one direction (big black arrows) while expanding in another (small black arrows) (not shown). These distortions disperse vibrations that transfer heat through the material, giving it a low thermal conductivity that makes it a viable thermoelectric material. Brookhaven National Laboratory

Macroscale contraction: In the undistorted large scale AgGaTe⌄2 crystal (left), a single silver atom (gray) sits at the center of each tetrahedral cavity. Upon heating, as the off-centering shifts of silver atoms within adjacent tetrahedra become correlated, the resulting rotation causes the whole macroscopic crystal to shrink in one direction (large black arrows) while expanding in another (not shown). These distortions scatter vibrations that propagate heat through the material, giving it the low thermal conductivity that makes it a promising thermoelectric material. Credit: Brookhaven National Laboratory

Twisted tetrahedrons

“Local distortions are not fully random,” Bozin explained. “They are correlated between adjacent silver atoms that are coupled to the same tellurium atom. These local distortions lead adjacent tetrahedra to rotate relative to one another, and this twisting causes the crystal lattice to contract in one direction.”

As the moving silver atoms twist the crystal, they scatter phonons, which allow heat to flow through the lattice. The thermal conductivity of AgGaTe2 is drastically reduced by the scattering of its energy-carrying phonons.

But why do silver atoms change in the first place?

In a rock-salt-like lead-telluride compound, Brookhaven scientists saw identical phenomena a decade earlier. In this instance, as the material was heated, “lone pairs” of electrons developed, resulting in the formation of dipoles, which are minute regions of split electric charge. These dipoles displaced lead atoms from their central positions and dispersed phonons.

“In silver gallium telluride, however, there are no isolated pairs. Therefore, there must be something else in this substance, as well as other ‘diamondoid’ structures “Bozin stated.

Altering bonding conduct

Christopher Wolverton’s computations at Northwestern revealed “something else” to be the bonding characteristics of the electrons orbiting silver atoms.

“These computations contrasted the silver and copper atoms and determined that there is a difference in the configuration of electrons in the orbitals, such that silver tends to establish weaker connections than copper,” stated Xie of Northwestern. “Silver prefers to form bonds with fewer surrounding tellurium atoms; it prefers a simpler bonding environment.”

Instead of binding evenly with all four surrounding tellurium atoms, like copper does, silver preferentially (but randomly) moves closer to two of the four. These bonding electrons are what pull the silver atom off-center, initiating the twisting, shrinking, and vibrational changes that ultimately reduce the thermal conductivity of AgGaTe2.

Northwestern’s Mercouri Kanadzidis stated, “We’ve discovered a novel way for reducing the thermal conductivity of lattices.” “Perhaps this technique can be used to engineer or search for other novel materials with this type of behaviour for future high-performance thermoelectrics.”

Further information: Hongyao Xie et al, Hidden Local Symmetry Breaking in Silver Diamondoid Compounds is Root Cause of Ultralow Thermal Conductivity, Advanced Materials (2022). DOI: 10.1002/adma.202202255

Journal information: Advanced Materials

Source: Brookhaven National Laboratory