Hidden distortions trigger promising thermoelectric property —

In a world of supplies that usually increase upon heating, one which shrinks alongside one 3D axis whereas increasing alongside one other stands out. That is very true when the weird shrinkage is linked to a property essential for thermoelectric units, which convert warmth to electrical energy or electrical energy to warmth.

In a paper simply revealed within the journal Superior Supplies, a staff of scientists from Northwestern College and the U.S. Division of Vitality’s Brookhaven Nationwide Laboratory describe the beforehand hidden sub-nanoscale origins of each the weird shrinkage and the distinctive thermoelectric properties on this materials, silver gallium telluride (AgGaTe2). The invention reveals a quantum mechanical twist on what drives the emergence of those properties — and opens up a totally new course for looking for new high-performance thermoelectrics.

“Thermoelectric supplies shall be transformational in inexperienced and sustainable vitality applied sciences for warmth vitality harvesting and cooling — however provided that their efficiency might be improved,” stated Hongyao Xie, a postdoctoral researcher at Northwestern and first writer on the paper. “We wish to discover the underlying design ideas that can permit us to optimize the efficiency of those supplies,” Xie stated.

Thermoelectric units are presently utilized in restricted, area of interest purposes, together with NASA’s Mars rover, the place warmth launched by the radioactive decay of plutonium is transformed into electrical energy. Future purposes would possibly embrace supplies managed by voltage to realize very steady temperatures vital for operation of high-tech optical detectors and lasers.

The principle barrier to wider adoption is the necessity for supplies with simply the fitting cocktail of properties, together with good electrical conductivity however resistance to the move of warmth.

“The difficulty is, these fascinating properties are likely to compete,” stated Mercouri Kanadzidis, the Northwestern professor who initiated this examine. “In most supplies, digital conductivity and thermal conductivity are coupled and each are both excessive or low. Only a few supplies have the particular high-low mixture.”

Below sure situations, silver gallium telluride seems to have simply the fitting stuff — extremely cell conducting electrons and ultra-low thermal conductivity. In actual fact, its thermal conductivity is considerably decrease than theoretical calculations and comparisons with comparable supplies corresponding to copper gallium telluride would recommend.

The Northwestern scientists turned to colleagues and instruments at Brookhaven Lab to search out out why.

“It took a meticulous x-ray examination at Brookhaven’s Nationwide Synchrotron Gentle Supply II (NSLS-II) to disclose a beforehand hidden sub-nanoscale distortion within the positions of the silver atoms on this materials,” stated Brookhaven Lab physicist Emil Bozin, chief of the structural evaluation.

Computational modeling revealed how these distortions set off the one-axis crystal shrinkage — and the way that structural shift scatters atomic vibrations, thus blocking the propagation of warmth within the materials.

However even with that understanding, there was no clear clarification of what was driving the sub-nanoscale distortions. Complementary computational modeling by Christopher Wolverton, a professor at Northwestern, indicated a novel and refined quantum mechanical origin for the impact.

Collectively the findings level to a brand new mechanism for turning down thermal conductivity and a brand new tenet within the seek for higher thermoelectric supplies.

Mapping atomic positions

The staff used x-rays at NSLS-II’s Pair Distribution Perform (PDF) beamline to map out the “giant” scale association of atoms in each copper gallium telluride and silver gallium telluride over a variety of temperatures to see if they might uncover why these two supplies behave otherwise.

“A stream of scorching air heats the pattern with degree-by-degree precision,” stated Milinda Abeykoon, who’s the lead scientist of the PDF beamline. “At every temperature, because the x-rays bounce off the atoms, they produce patterns that may be translated into excessive spatial decision measurements of the distances between every atom and its neighbors (every pair). Computer systems then assemble the measurements into the most probably 3D preparations of the atoms.”

The staff additionally did extra measurements over a wider vary of temperatures however at decrease decision utilizing the sunshine supply on the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany. They usually extrapolated their outcomes all the way down to a temperature of absolute zero, the coldest something can get.

The information present that each supplies have a diamond-like tetragonal construction of corner-connected tetrahedra, one with a single copper atom and the opposite with silver on the heart of the 3D object’s tetrahedral cavity. Describing what occurred as these diamondlike crystals have been heated, Bozin stated, “Instantly we noticed a giant distinction between the silver and copper variations of the fabric.”

The crystal with copper at its core expanded in each course, however the one containing silver expanded alongside one axis whereas shrinking alongside one other.

“This unusual conduct turned out to have its origin within the silver atoms on this materials having very giant amplitude and disorderly vibrations inside structural layers,” stated Simon Billinge, a professor at Columbia College with a joint appointment as a physicist at Brookhaven. “These vibrations trigger the linked tetrahedra to jiggle and leap with giant amplitude,” he stated.

This was a clue that the symmetry — the common association of atoms — could be “damaged” or disrupted at a extra “native” (smaller) scale.

The staff turned to computational modeling to see how numerous native symmetry distortions of the silver atoms would match with their information.

“The one which labored the very best confirmed that the silver atom goes off heart within the tetrahedron in considered one of 4 instructions, towards the sting of the crystal shaped by two of the tellurium atoms,” Bozin stated. On common, the random, off-center shifts cancel out, so the general tetragonal symmetry is retained.

“However we all know the bigger scale construction adjustments too, by shrinking in a single course,” he famous. “Because it seems the native and bigger scale distortions are linked.”

Twisting tetrahedrons

“The native distortions usually are not utterly random,” Bozin defined. “They’re correlated amongst adjoining silver atoms — these linked to the identical tellurium atom. These native distortions trigger adjoining tetrahedra to rotate with respect to at least one one other, and that twisting causes the crystal lattice to shrink in a single course.”

Because the shifting silver atoms twist the crystal, additionally they scatter sure wavelike vibrations, known as phonons, that permit warmth to propagate by way of the lattice. Scattering AgGaTe2‘s energy-carrying phonons retains warmth from propagating, dramatically decreasing the fabric’s thermal conductivity.

However why do the silver atoms shift within the first place?

The Brookhaven scientists had seen comparable conduct a decade earlier, in a rock-salt like lead-telluride materials. In that case, as the fabric was heated, “lone pairs” of electrons shaped, producing tiny areas of break up electrical cost, known as dipoles. These dipoles pulled centrally situated lead atoms off heart and scattered phonons.

“However in silver gallium telluride there aren’t any lone pairs. So, there should be one thing else on this materials — and doubtless different ‘diamondoid’ constructions as nicely,” Bozin stated.

Bending bonding conduct

Christopher Wolverton’s calculations at Northwestern revealed that “one thing else” to be the bonding traits of the electrons orbiting the silver atoms.

“These calculations in contrast the silver and copper atoms and located that there’s a distinction within the association of electrons within the orbitals such that silver tends to type weaker bonds than copper,” stated Northwestern’s Xie. “Silver needs to bond with fewer neighboring tellurium atoms; it needs an easier bonding surroundings.”

So as a substitute of binding equally with all 4 surrounding tellurium atoms, as copper does, silver tends to preferentially (however randomly) transfer nearer to 2 of the 4. These bonding electrons are what pull the silver atom off heart, triggering the twisting, shrinkage, and vibrational adjustments that in the end decrease thermal conductivity in AgGaTe2.

“We have stumbled upon a brand new mechanism by which lattice thermal conductivity might be lowered,” Northwestern’s Mercouri Kanadzidis stated. “Maybe this mechanism can be utilized to engineer, or search for, different new supplies which have this sort of conduct for future high-performance thermoelectrics.”

This analysis was primarily supported by the DOE Workplace of Science. NSLS-II is a DOE Workplace of Science consumer facility.