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In the Focus

An article was published on Nature Materials by JQ He Group

Jan 17, 2018 Research

On January 15th, Professor Jiaqing’s group from the Department of Physics of SUSTech published a paper entitled Liquid-like thermal conduction in intercalated layered crystalline solids in the scientific journal Nature Materials with himself as one of the corresponding authors, and Dr. Di Wu who also largely participated in the study. The paper is mainly completed by the Japan Atomic Energy Agency, the University of Los Angeles (Irvine), Southern University of Science and Technology (SUSTech) and the Northwestern University in the United States. The work has also been greatly assisted by a number of universities and research institutions such as Iowa State University, Juelich Research Center, Hong Kong University, Japan Synchrotron Radiation Research Institute and the Neutron Technology Center in Japan.

The process of heat transport in an object is achieved by the microscopic collision of the constituent particle units. In condensed-matter physics, people are used to characterizing this process by the scattering of phonons. In general, the heat conduction in the solid involves both transverse and longitudinal phonons, whereas that in the liquid is only achieved by axial longitudinal phonons due to the lack of tangential force. Therefore, under normal circumstances, the solid has a better thermal conductivity than the liquid. In this research, the author reported that the heat conduction in AgCrSe2 crystals with peculiar sandwich structure is liquid-like and therefore has very low thermal conductivity. AgCrSe2 is a hexagonal structure where the Ag atomic layer and the CrSe6 octahedral layer are stacked along the c direction, in which Ag ions occupy two kinds of positions. At low temperatures, the Ag ions completely occupy Position I, and as the temperature increases, some of the Ag ions have sufficient energy to diffuse to Position II to complete an order-disorder phase transition at about 450 degrees, i.e., after this temperature point, the Ag ions have 50% of probability to occupy Position I and Position II respectively, and the crystal structure changes from R3m to R m. The phonon spectra measured by the variable temperature neutron /X-ray diffraction show that the transverse acoustic (TA) branch in the AgCrSe2 crystal is completely suppressed due to the ordered-disorder phase transition of the Ag ions between the CrSe6 octahedral layers, while the longitudinal-wave acoustic (LA) branch phonon is also strongly scattered. That is, the acoustic branch that dominates the heat transport has a strong liquid characteristic. Meanwhile, the calculation of density functional-perturbation theory also shows that the inherent disturbance caused by the order-disorder occupancy of the transverse-wave acoustic branch (TA) and Ag ion reflecting the vibration of Ag ions has a competitive relationship, as the temperature increases, the TA branch gradually decreases until it is completely suppressed after the phase change is completed.

The research work also reveals that this intriguing phenomenon of TA suppression (liquid-like heat transport of the solid) is also common in a series of compounds with similar lamellar structures, i.e., there are compounds of heavy element intercalation crystal structure in the van der Waals gap. This will most likely reshape people's basic knowledge of heat transport in substance and will provide good ideas and opportunities for the enhancement of electroacoustic transport properties of thermoelectric materials.

This work at SUSTech was supported by the Natural Science Foundation of Guangdong Province and the Leading Talent of "Pearl River Talents Project" of Guangdong Province.

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Submitted by: Department of Physics

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