Flapping vibrations contributed to heat insulation in Cs₂ZnI₄

Shear modulus is commonly lower than its elastic counterpart in solids, leading to our experience that in the elastic region, it is usually much easier to shear than to compress/stretch a solid. Scientifically, this implies an easier change in molecular bonding angle than bonding length, further corresponding to the normally observed phenomenon that the propagation of longitudinal lattice vibrations is faster than that of shear ones. Populating the easier vibrations of flapping bond angles fundamentally opens an opportunity to insulate heat conduction in solids, since this type of vibration ensures slow propagation and low energy/frequency, leading to a low thermal conductivity. Here we demonstrate that single-crystalline orthorhombic Cs₂ZnI₄, with two sets of extremely low-frequency flapping vibrations in the crystallographic planes of ac (0.4 THz) and bc (0.5 THz), respectively, exhibits extraordinarily low thermal conductivities of 0.11 W m^-1 K^-1 along the c direction and of 0.16 W m^-1 K^-1 along the b direction at room temperature, as compared to ∼0.2 W m^-1 K^-1 in existing heat insulators in the dense form. The strategy and material developed in this work are believed to enrich the advancements of heat insulation technology.