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, Ga₆ Cr₅ Se₁₆ , shows intrinsic low lattice thermal conductivity κ_lat , which crystallizes in the monoclinic phase (C2/m) with the stacked inverse GaSe₄ layers (g'), close-packed Cr³⁺ Se₆ layers (c), GaSe₄ layers (g) and loosely-stacked Cr²⁺ Se₆ 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 Cr³⁺ -Se, and weaker Cr²⁺ -Se bonding, which endow it with a large phonon symphony by strong coupling of soft acoustic and low-lying optical phonons. As a result, Ga₆ Cr₅ Se₁₆ 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 Cr₃ Se₄ (2.95 W m^{- 1}  K^{- 1} ), or Cr₂ Se₃ (1.56 W m^{- 1}  K^{- 1} ), Ga₂ Se₃ (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.