One of the most fundamental properties of matter is thermal transport – the transfer of heat though microscopic collisions of constituent particles. Thermal transport is known to differ between solids and liquids. Indeed, solids conduct heat though both transverse and longitudinal acoustic (TA and LA) phonons, while liquids are limited to longitudinal vibrations.

Mercouri G. Kanatzidis, Northwestern University, Evanston, IL, USA, and colleagues have discovered a new type of heat transfer in the intercalated layered crystalline solid AgCrSe₂ – the liquid-like thermal conduction – in which only longitudinal acoustic phonons are transferred. The team conducted a comprehensive study combining both neutron/X-ray scattering and density-functional-perturbation-theory (DFPT) calculations on the chalcogenide AgCrSe₂.

Above a critical temperature of ~450 K, an order-to-disorder transition takes place within the layered compound, in which the Ag ions located in the van der Waals gap migrate to new sites due to weaker coupling. This atomic fluctuation results in a disorder-phonon coupling where the TA phonons are completely suppressed, and the LA phonons are strongly scattered but survive.

The team expects that this scenario is applicable to other layered compounds with heavy intercalants such as AgCrS₂ and AgNiSe₂ and anticipates liquid-like thermal conduction to be a promising strategy for the suppression of thermal conductivity of solids for applications such as thermal insulation and thermoelectric energy conversion.

• Liquid-like thermal conduction in intercalated layered crystalline solids,
  B. Li, H. Wang, Y. Kawakita, Q. Zhang, M. Feygenson, H. L. Yu, D. Wu, K. Ohara, T. Kikuchi, K. Shibata, T. Yamada, X. K. Ning, Y. Chen, J. Q. He, D. Vaknin, R. Q. Wu, K. Nakajima, M. G. Kanatzidis,
  Nature Mater. 2018.
  https://doi.org/10.1038/s41563-017-0004-2