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arXiv · 2610.35360

Simultaneous suppression and temperature-driven crossover of particle-like and wave-like thermal transport in trigonal CsAg$_3$S$_2$

Vivek Gusain·B K Mani
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Abstract

The simultaneous suppression of particle-like ($κ_p$ ) and wave-like ($κ_c$) thermal transport channels in crystalline materials remains a challenging problem, as reducing one channel can enhance the other. Here, we investigate the thermal transport in trigonal $P\Bar{3}m1$ CsAg$_3$S$_2$ using self-consistent phonon theory with cubic and quartic anharmonicity and the unified theory of thermal transport. Our calculations predict an ultralow lattice thermal conductivity of 0.222 W m$^{-1}$ K$^{-1}$ at 300 K, featuring simultaneously low $κ_p$ and $κ_c$ with a weak temperature dependence of $\sim$T$^{-0.42}$, which deviates significantly from the conventional $\sim$T$^{-1}$ behavior. This simultaneous suppression arises from localized Cs modes, which enhance anharmonic scattering, and the softer Ag-S framework, which reduces phonon group velocities, together suppressing $κ_p$, in conjunction with sparse, well-separated high-frequency S-derived optical modes that suppress $κ_c$ by limiting the coherent coupling between the phonon modes. Furthermore, the dominant transport channel exhibits a temperature-dependent crossover, with $κ_p$ contributing $\sim$74 \% of total lattice thermal conductivity at 300~K and $κ_c$ contributing $\sim$63 \% at 700~K. Combining the ultralow thermal conductivity with electrical transport calculated within the momentum relaxation time approximation (MRTA), we obtain a $ZT$ of 1.28 for $n$-type doping at 700 K. The present work not only clarifies the microscopic mechanism of ultralow thermal conductivity in trigonal CsAg$_3$S$_2$ but also identifies the covalent M-Ch framework modification within the AM$_3$Ch$_2$ family of ternary chalcogenides as an additional tuning parameter for further reducing $κ_p$ within the bonding-hierarchy design principle.

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Vivek Gusain, B K Mani. 2026-10-22. Simultaneous suppression and temperature-driven crossover of particle-like and wave-like thermal transport in trigonal CsAg$_3$S$_2$. https://arxiv.org/abs/2610.35360

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