• Hidden distortions trigger promising the

    From ScienceDaily@1:317/3 to All on Mon May 9 22:30:42 2022
    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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