# Structural chemistry-guided revelation of superior thermally insulative TeI4

## TL;DR

This paper shows how changing the cation valence in iodides can create materials with very low thermal conductivity, like TeI4, which is great for thermal insulation.

## Contribution

The study reveals that extended lattice periodicity in TeI4 leads to exceptionally low thermal conductivity through cation valence manipulation.

## Key findings

- TeI4 exhibits a thermal conductivity of 0.17 W m⁻¹ K⁻¹ at room temperature, the lowest among the studied iodides.
- Extended lattice periodicity in TeI4 is due to a larger number of atoms in the primitive cell compared to other iodides.
- Cation valence increases lead to extended lattice periodicity and reduced thermal conductivity in binary iodides.

## Abstract

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.

By manipulating the cation valences in binary iodides, this work achieves an extremely low thermal conductivity in TeI4, revealing how extended lattice periodicity lead to superior thermal insulation.