Realizing Intrinsically Ultralow and Glass‐Like Thermal Transport via Chemical Bonding Engineering

Crystals exhibiting glass-like and low lattice thermal conductivity ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>κ</mml:mi> <mml:mi>L</mml:mi></mml:msub> <mml:annotation>$\kappa_{\rm L}$</mml:annotation></mml:semantics> </mml:math> ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>κ</mml:mi> <mml:mi>L</mml:mi></mml:msub> <mml:annotation>$\kappa _{\rm L}$</mml:annotation></mml:semantics> </mml:math> are typically observed only in compounds containing heavy elements, with large unit cells, or at high temperatures. In this study, chemical bonding principles are utilized to weaken the Ag-Ag bonds and enhance lattice anharmonicity. The incorporation of a chalcogen anion as a bridge ligand is proposed to facilitate phonon rattling in Ag₆-octahedron-based compounds. Guided by this design strategy, five Ag₆ octahedron-based compounds, AAg₃X₂ (A = Li, Na, and K; X = S and Se), which are characterized by low average atomic masses and exhibit exceptionally strong four-phonon scattering, are theoretically identified. Consequently, these compounds demonstrate ultralow thermal conductivities (0.3-0.6 W m^-1 K^-1) with minimal temperature dependence (T^-0.1) across a wide temperature range. Experimental validation confirms that the κ_L of NaAg₃S₂ is 0.45 W m^-1 K^-1 within the temperature range of 200-550 K. The results clearly demonstrate that weak chemical bonding plays a crucial role in designing compounds with glass-like κ_L, highlighting the effectiveness of chemical bonding engineering in achieving desired thermal transport properties.