Screw symmetry induced low-frequency optical phonons in heat-insulating CsCu2I3

The existence of low-lying local vibrational modes is critical for increasing phonon scattering channels, thus leading to a significant enhancement in heat insulation of crystals. Proven mechanisms include rattling vibrations in clathrates and skutterudites, and ferroelectric instability in chalcogenides, of which the low-frequency optical phonons are generated by characteristic vibrations of either an individual atom or an atom pair. Here, we report a mechanism for forming low-lying vibrational modes, in which the screw symmetry of the lattice triggers the low-frequency vibrations of certain atom groups. As typified in CsCu₂I₃, the screw-operated iodine atoms at the crystallographic 8g-site induce two sets of low-frequency optical modes, which eventually results in a low-lying vibrational mode of 0.4 THz. The resultant intrinsic thermal conductivity of ∼0.35 W/m-K in single-crystalline CsCu₂I₃ is further reduced to 0.22 W/m-K in poly-crystalline counterpart at room temperature. This is in consistent with the computational glass-like thermal conductivity obtained in previous studies focusing on the dimensional confinement effects and Cu ion migration confinement effects. Since the screw operations commonly exist in many materials, this work might offer new opportunities for advancing heat insulators.