Hidden distortions trigger promising thermoelectric property
Date:
May 9, 2022
Source:
DOE/Brookhaven National Laboratory
Summary:
A study describes a new mechanism for lowering thermal conductivity
to aid the search for materials that convert heat to electricity
or electricity to heat. Scientists describe the previously hidden
sub- nanoscale origins of exceptional thermoelectric properties
in silver gallium telluride. The discovery reveals a quantum
mechanical twist on what drives the emergence of these properties
-- and opens up a completely new direction for searching for new
high-performance thermoelectrics.
FULL STORY ==========================================================================
In a world of materials that normally expand upon heating, one
that shrinks along one 3D axis while expanding along another stands
out. That's especially true when the unusual shrinkage is linked to a
property important 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 hidden sub-nanoscale origins of both the unusual shrinkage and the exceptional thermoelectric properties in this material, silver gallium telluride
(AgGaTe2). The discovery reveals a quantum mechanical twist on what
drives the emergence of these properties -- and opens up a completely
new direction for searching for new high-performance thermoelectrics.
"Thermoelectric materials will be transformational in green and
sustainable energy technologies for heat energy harvesting and cooling
-- but only if their performance can be improved," said Hongyao Xie,
a postdoctoral researcher at Northwestern and first author on the
paper. "We want to find the underlying 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 Mars rover, where heat released by the radioactive
decay of plutonium is converted into electricity. Future applications
might include materials controlled by voltage to achieve very stable temperatures critical for operation of high-tech optical detectors
and lasers.
The main barrier to wider adoption is the need for materials with just
the right cocktail of properties, including good electrical conductivity
but resistance to the flow of heat.
"The trouble is, these desirable properties tend to compete,"
said Mercouri Kanadzidis, the Northwestern professor who initiated
this study. "In most materials, electronic conductivity and thermal conductivity are coupled and both are either high or low. Very few
materials have the special high-low combination." Under certain
conditions, silver gallium telluride appears to have just the right
stuff -- highly mobile conducting electrons and ultra-low thermal
conductivity. In fact, its thermal conductivity is significantly lower
than theoretical calculations and comparisons with similar materials
such as copper gallium telluride would suggest.
==========================================================================
The Northwestern scientists turned to colleagues and tools at Brookhaven
Lab to find out why.
"It took a meticulous x-ray examination at Brookhaven's National
Synchrotron Light Source II (NSLS-II) to reveal a previously hidden sub-nanoscale distortion in the positions of the silver atoms in this material," said Brookhaven Lab physicist Emil Bozin, leader of the
structural analysis.
Computational modeling revealed how those distortions trigger the
one-axis crystal shrinkage -- and how that structural shift scatters
atomic vibrations, thus blocking the propagation of heat in the material.
But even with that understanding, there was no clear explanation of what
was driving the sub-nanoscale distortions. Complementary computational
modeling by Christopher Wolverton, a professor at Northwestern, indicated
a novel and subtle quantum mechanical origin for the effect.
Together the findings point to a new mechanism for turning down thermal conductivity and a new guiding principle in the search for better thermoelectric materials.
========================================================================== Mapping atomic positions The team used x-rays at NSLS-II's Pair
Distribution Function (PDF) beamline to map out the "large" scale
arrangement of atoms in both copper gallium telluride and silver gallium telluride over a range of temperatures to see if they could discover
why these two materials behave differently.
"A stream of hot air heats the sample with degree-by-degree
precision," said Milinda Abeykoon, who is the lead scientist of the
PDF beamline. "At each temperature, as the x-rays bounce off the atoms,
they produce patterns that can be translated into high spatial resolution measurements of the distances between each atom and its neighbors (each
pair). Computers then assemble the measurements into the most likely 3D arrangements of the atoms." The team also did additional measurements
over a wider range of temperatures but at lower resolution using the
light source at the Deutsches Elektronen- Synchrotron (DESY) in Hamburg, Germany. And they extrapolated their results down to a temperature of
absolute zero, the coldest anything can get.
The data show that both materials have a diamond-like tetragonal
structure of corner-connected tetrahedra, one with a single copper atom
and the other with silver at the center of the 3D object's tetrahedral
cavity. Describing what happened as these diamondlike crystals were
heated, Bozin said, "Immediately we saw a big difference between the
silver and copper versions of the material." The crystal with copper
at its core expanded in every direction, but the one containing silver
expanded along one axis while shrinking along another.
"This strange behavior turned out to have its origin in the silver atoms
in this material having very large amplitude and disorderly vibrations
within structural layers," said Simon Billinge, a professor at Columbia University with a joint appointment as a physicist at Brookhaven. "Those vibrations cause the linked tetrahedra to jiggle and jump with large amplitude," he said.
This was a clue that the symmetry -- the regular arrangement of atoms --
might be "broken" or disrupted at a more "local" (smaller) scale.
The team turned to computational modeling to see how various local
symmetry distortions of the silver atoms would match with their data.
"The one that worked the best showed that the silver atom goes off center
in the tetrahedron in one of four directions, toward the edge of the
crystal formed by two of the tellurium atoms," Bozin said. On average,
the random, off- center shifts cancel out, so the overall tetragonal
symmetry is retained.
"But we know the larger scale structure changes too, by shrinking in
one direction," he noted. "As it turns out the local and larger scale distortions are linked." Twisting tetrahedrons "The local distortions
are not completely random," Bozin explained. "They are correlated
among adjacent silver atoms -- those connected to the same tellurium
atom. These local distortions cause adjacent tetrahedra to rotate with
respect to one another, and that twisting causes the crystal lattice
to shrink in one direction." As the shifting silver atoms twist the
crystal, they also scatter certain wavelike vibrations, called phonons,
that allow heat to propagate through the lattice. Scattering AgGaTe2's energy-carrying phonons keeps heat from propagating, dramatically lowering
the material's thermal conductivity.
But why do the silver atoms shift in the first place? The Brookhaven scientists had seen similar behavior a decade earlier, in a rock-salt
like lead-telluride material. In that case, as the material was heated,
"lone pairs" of electrons formed, generating tiny areas of split electric charge, called dipoles. Those dipoles pulled centrally located lead
atoms off center and scattered phonons.
"But in silver gallium telluride there are no lone pairs. So, there must
be something else in this material -- and probably other 'diamondoid' structures as well," Bozin said.
Bending bonding behavior Christopher Wolverton's calculations
at Northwestern revealed that "something else" to be the bonding characteristics of the electrons orbiting the silver atoms.
"Those calculations compared the silver and copper atoms and found that
there is a difference in the arrangement of electrons in the orbitals
such that silver has a tendency to form weaker bonds than copper,"
said Northwestern's Xie. "Silver wants to bond with fewer neighboring
tellurium atoms; it wants a simpler bonding environment." So instead
of binding equally with all four surrounding tellurium atoms, as copper
does, silver tends to preferentially (but randomly) move closer to two
of the four. Those bonding electrons are what pull the silver atom off
center, triggering the twisting, shrinkage, and vibrational changes that ultimately lower thermal conductivity in AgGaTe2.
"We've stumbled upon a new mechanism by which lattice thermal conductivity
can be reduced," Northwestern's Mercouri Kanadzidis said. "Perhaps
this mechanism can be used to engineer, or look for, other new
materials that have this type of behavior for future high-performance thermoelectrics." This research was primarily supported by the DOE
Office of Science. NSLS-II is a DOE Office of Science user facility.
========================================================================== Story Source: Materials provided by
DOE/Brookhaven_National_Laboratory. Note: Content may be edited for
style and length.
========================================================================== Journal Reference:
1. Hongyao Xie, Emil S. Bozin, Zhi Li, Milinda Abeykoon, Soham
Banerjee,
James P. Male, G. Jeffrey Snyder, Christopher Wolverton, Simon J. L.
Billinge, Mercouri G. Kanatzidis. Hidden Local Symmetry Breaking
in Silver Diamondoid Compounds is Root Cause of Ultralow Thermal
Conductivity. Advanced Materials, 2022; 2202255 DOI: 10.1002/
adma.202202255 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2022/05/220509191551.htm
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