Realizing Intrinsically Glass-like Thermal Transport via Weakening the Ag-Ag Bonds in Ag$_{6}$ Octahedra

TL;DR

This study demonstrates that weakening specific chemical bonds in Ag$_6$ octahedra can induce glass-like, ultralow thermal conductivity in compounds, validated through theoretical predictions and experimental measurements, advancing thermal transport engineering.

Contribution

We introduce a chemical bonding strategy to weaken Ag-Ag bonds, leading to glass-like thermal transport in Ag$_6$-based compounds, confirmed by theoretical design and experimental validation.

Findings

Identified five compounds with ultralow thermal conductivity (0.3-0.6 W/mK)

Confirmed experimental $_{ m L}$ of NaAg$_3$S$_2$ as 0.45 W/mK from 200-550 K

Weak chemical bonds significantly influence glass-like thermal transport

Abstract

Crystals exhibiting glass-like and low lattice thermal conductivity ($\kappa_{\rm L}$) are not only scientifically intriguing but also practically valuable in various applications, including thermal barrier coatings, thermoelectric energy conversion, and thermal management. However, such unusual $\kappa_{\rm L}$ are typically observed only in compounds containing heavy elements, with large unit cells, or at high temperatures, primarily due to significant anharmonicity. In this study, we utilize chemical bonding principles to weaken the Ag-Ag bonds within the Ag$_6$ octahedron by introducing a ligand in the bridge position. Additionally, the weak Ag-chalcogen bonds, arising from fully filled $p$-$d$ antibonding orbitals, provide an avenue to further enhance lattice anharmonicity. We propose the incorporation of a chalcogen anion as a bridge ligand to promote phonon rattling in Ag$_6$-octahedron-based compounds. Guided by this design strategy, we theoretically identified five Ag$_6$ octahedron-based compounds, $A$Ag$_3X_2$ ($A$ = Li, Na, and K; $X$ = S and Se), which are characterized by low average atomic masses and exhibit exceptionally strong four-phonon scattering. Consequently, these compounds demonstrate ultralow thermal conductivities (0.3 $\sim$ 0.6 Wm$^{-1}$K$^{-1}$) with minimal temperature dependence (T$^{-0.1}$) across a wide temperature range. Experimental validation confirmed that the $\kappa_{\rm L}$ of NaAg$_3$S$_2$ is 0.45 Wm$^{-1}$K$^{-1}$ within the temperature range of 200 to 550 K. Our results clearly demonstrate that weak chemical bonding plays a crucial role in designing compounds with glass-like $\kappa_{\rm L}$, highlighting the effectiveness of chemical bonding engineering in achieving desired thermal transport properties.