The ability to store and convert thermal energy in various ways makes phase-change materials very handy for everything from heating pads to medical applications to heating and cooling equipment, spacecraft, and even buildings. Their convenient qualities are the result of a predictable phase change at a particular temperature, sometimes involving a solid-solid change in the crystal lattice of a material.
One example is germanium telluride (GeTe), a IV-VI semiconductor which also has potential for thermoelectric and phase-change memory applications. Researchers from Cornell University, Virginia Tech, and Argonne National Laboratory used GeTe as a research model to gain deeper insight into phonon dynamics and thermal conductivity in phase-change materials. Their work appeared in Nature Communications.
GeTe was chosen for the study because of an intriguing characteristic that seems to defy the general idea that the lattice thermal conductivity of a crystalline material tends to decrease with rising temperatures. At room temperature, GeTe mostly exists in a rhombohedral structure, undergoing a phase transition to a cubic structure at about 650 Kelvin. Multiple experiments by various research groups, however, consistently demonstrate that the cubic form of GeTe shows a marked increase in lattice thermal conductivity with increasing temperatures.
The present research team used this interesting and unexpected quality to probe further into the behavior of phase-change materials, and also to uncover more details on this specific cubic GeTe characteristic. They combined machine learning (ML)-assisted first-principles calculations with inelastic X-ray scattering (IXS) studies at the 30-ID beamline of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory.
The X-ray diffraction pattern confirms the complete phase transition in the experimental sample from rhombohedral to cubic at 693 K. Using ML algorithms with advanced linear regression to extract third- and fourth-order interatomic force constants (IFCs) gives a picture of the acoustic and optical phonon frequencies in good agreement with the IXS data. The calculated phonon lifetimes match much better with the experimental lifetimes from IXS when four-phonon processes are included.
The researchers next calculated the thermal conductivity of the cubic GeTe lattice as a function of temperature based on the IFCs. The calculations show a marked rise in thermal conductivity from 750 K through at least 850 K. This represents the first time this phenomenon has been reproduced computationally, and also confirms the results of the current and other previous experimental work.
To further investigate the mechanisms behind the increasing thermal conductivity, the experimenters focused on the temperature dependence of the atomic bonding within the cubic GeTe lattice. They found that although no such dependence is demonstrated between nearest-neighbor Ge and Te pairs, bonding strength between second-nearest like atom pairs increases considerably with rising temperature. The Ge-Ge bond strength increased by 8.3% and Te-Te strength by 103% from 693 K to 850 K. The research group proposes that this this marked strengthening of second-nearest neighbor bonds is the cause of the temperature-dependent increase in thermal conductivity.
The investigators also note that similar increased thermal conductivity is also observed in other IV-VI semiconductors such as tin telluride (SnTe) and tin selenide (SnSe), which raises the intriguing possibility of using the techniques demonstrated in this work to model and study high-temperature phase transitions and thermal conductivity in other candidate materials for thermoelectric and phase change applications. A better and more comprehensive understanding of the thermal mechanisms at work in these phase change materials will lead to more effective solutions for their design and use in practical applications. – Mark Wolverton
See: S. Kielar1, C. Li1,2, H. Huang1, R. Hu1, C. Slebodnick3, A. Alatas4, Z. Tian1, “Anomalous lattice thermal conductivity increase with temperature in cubic GeTe correlated with strengthening of second-nearest neighbor bonds,” Nat Commun 15 6981 (2024)
Author affiliations: 1Cornell University; 2Chongqing University; 3Virginia Tech; 4Argonne National Laboratory
This work was funded by Z.T.’s NSF CAREER Award (CBET1839384). This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the D.O.E. Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation Grant ACI-1053575. This work used Expanse at San Diego Supercomputer Center (SDSC) through allocation CTS150063 from the Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) program, which is supported by National Science Foundation grants #2138259, #2138286, #2138307, #2137603, and #2138296.
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