銅基與銀基的黃銀礦超離子導體晶體在超離子相變溫度附近展現出類似非晶體的超低晶格熱導率特性，引起了學術界的廣泛關注。這些材料因其獨特的熱電性能，被視為具有潛力的熱電材料候選。然而，對於其晶格熱導率的量化分析以及驅動其顯著熱電性能的微觀機制仍不充分了解。
在本研究中，我們通過結合實驗和理論分析的方法，揭示了在銅基黃銀礦-Cu7PS6中，低能量光學聲子的存在與其超低晶格熱導率之間的關聯。研究指出，這些低能量的聲頻支透過減少聲學-光學聲子間的能量交叉，引發了強烈的聲子散射現象，從而實現了類似非晶體的超低熱導率。透過統一的熱傳導理論對熱導率進行分析，本研究成功地再現了實驗觀察到的超低晶格熱導率範圍，從0.43到0.58 W mˉ¹ Kˉ¹，在100至400 K的溫度範圍內。
進一步的分析顯示，Cu7PS6中觀察到的類非晶超低熱導率主要源自於一種顯著的波動導熱機制。模擬結果揭示，這種波動式熱傳輸主要由於Cu相關的低能量重疊光學聲子的貢獻。本研究的發現突顯了低能量和重疊光學模式在促進類非晶超低熱傳導過程中的關鍵作用，為深入理解黃銀礦材料複雜的動力學過程提供了全面的洞察。
透過本研究的成果，我們不僅對Cu和Ag基黃銀礦超離子導體材料的低熱導率現象有了更深入的理解，同時也為開發新型高效熱電材料提供了重要的理論基礎和實驗證據。
Due to their amorphous-like ultralow lattice thermal conductivity both below and above the superionic phase transition, crystalline Cu- and Ag-based superionic argyrodites have garnered widespread attention as promising thermoelectric materials. However, despite their intriguing properties, quantifying their lattice thermal conductivities and a comprehensive understanding of the microscopic dynamics that drive these extraordinary properties are still lacking. Here, an integrated experimental and theoretical approach is adopted to reveal the presence of Cu-dominated low-energy optical phonons in the Cu-based argyrodite Cu7PS6. These phonons yield strong acoustic-optical phonon scattering through avoided crossing, enabling ultralow lattice thermal conductivity. The Unified Theory of thermal transport is employed to analyze heat conduction and successfully reproduce the experimental amorphous-like ultralow lattice thermal conductivities, ranging from 0.43 to 0.58 W mˉ¹ Kˉ¹, in the temperature range of 100–400 K. The study reveals that the amorphous-like ultralow thermal conductivity of Cu7PS6 stems from a significantly dominant wave-like conduction mechanism. Moreover, the simulations elucidate the wave-like thermal transport mainly results from the contribution of Cu-associated low-energy overlapping optical phonons. This study highlights the crucial role of low-energy and overlapping optical modes in facilitating amorphous-like ultralow thermal transport, providing a thorough understanding of the underlying complex dynamics of argyrodites.
(a) Raman spectrum and b) the calculated PDOS for Cu7PS6 at 100 K. The phonon energy of the calculated PDOS is multiplied by a factor of 1.06 for comparison. (c) The fitting profile of the Raman spectrum at 100 K. The black cross symbols represent experimental values; the blue solid line illustrates the fitted peaks, while the red solid line indicates the sum of the fitted peaks. (d) The contour temperature-dependent Raman spectrum is from 100 to 300 K, where hollow circles mark the energy positions of the fitted Raman peaks.
