Phonon Symphony of Stacked Multilayers and Weak Bonds Lowers Lattice Thermal Conductivity.

Controlling lattice vibrations to obtain intrinsic low thermal conductivity play a critical role in thermal management of electronic and photonic devices, energy converters, and thermal insulation, which necessitates exploring new compounds and a thorough understanding of their chemical structure, bonding, and lattice dynamics. Herein, a new chalcogenide, Ga 6 Cr 5 Se 16 , shows intrinsic low lattice thermal conductivity κ lat , which crystallizes in the monoclinic phase (C2/m) with the stacked inverse GaSe 4 layers (g'), close-packed Cr 3+ Se 6 layers (c), GaSe 4 layers (g) and loosely-stacked Cr 2+ Se 6 layers (c') along the c-axis. In this structure, a wide variety of chemical bonding is arranged in each layer, such as covalent Ga-Se, covalent Cr 3+ -Se, and weaker Cr 2+ -Se bonding, which endow it with a large phonon symphony by strong coupling of soft acoustic and low-lying optical phonons. As a result, Ga 6 Cr 5 Se 16 realizes an intrinsic low κ lat of 0.79 W m - 1  K - 1 at 323 K, which is almost four times, or twice lower than that of Cr 3 Se 4 (2.95 W m - 1  K - 1 ), or Cr 2 Se 3 (1.56 W m - 1  K - 1 ), Ga 2 Se 3 (1.36 W m - 1 K - 1 ) at 323 K, respectively. These insights will offer comprehensive understanding of the phonon propagation in complex layered chalcogenides, and also shed useful light on future design of low-κ lat solids.