Structural chemistry-guided revelation of superior thermally insulative TeI4

As the cornerstone of structural chemistry, the elemental compositions and spatial arrangements of atoms determine the functionalities of compounds. This principle is fully epitomized by 'magic' angle materials, where the lattice twisting extends the periodicity of the Moiré superlattice, revealing many unexpected properties. Here, we investigate how the extended lattice periodicity affects the properties of lattice dynamics, with a primary focus on thermal conductivity. Through the modulation of bond length and angle, the lattice periodicities of binary iodides CsI, BaI₂, BiI₃ and TeI₄ are extended as the cationic valences increase from monovalent to tetravalent states, leading to a substantial decrease in thermal conductivity. It is revealed that even in a simple binary compound like TeI₄, an extremely low thermal conductivity of 0.17 W m^-1 K^-1 at room temperature can be achieved. Compared to CsI, BaI₂ and BiI₃, the superior heat insulation of TeI₄ is found to stem from the large extended periodicity of the atomic arrangement enabled by having nearly an order of magnitude more atoms in the primitive cell.