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, Ga6Cr5Se16, shows intrinsic low lattice thermal conductivity kappa(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, Ga6Cr5Se16 realizes an intrinsic low kappa(lat) of 0.79 W m(-)(1) K--(1) at 323 K, which is almost four times, or twice lower than that of Cr3Se4 (2.95 W m(-)(1) K--(1)), or Cr2Se3 (1.56 W m(-)(1) K--(1)), Ga2Se3 (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-kappa(lat) solids.