MSGID: 1:317/3 6279ea9a   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    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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