層狀快離子晶體是一種獨特的材料，既有長程的液相離子擴散率，又有固態的亞晶格。由於其獨特的固液二元性，它們在燃料電池、固體充電電池電解質、高效熱電材料等領域有著廣闊的應用前景。在熱電轉換中，晶格熱導率和載流子熱導率直接影響zt值。因此，如何抑制聲子的傳播是提高zt的關鍵因素。層狀快離子晶體AgCrSe2是高性能熱電材料的潛在候選者。儘管人們提出了不同的聲子散射框架來解釋液體的導熱係數，但由於缺乏基於模式的非簡諧聲子計算和相應的動量分辨非彈性中子散射測量，詳細的微觀機制尚不完全清楚。
來自香港大學機械工程系的Yue Chen團隊提出了一種有效的混合方案來代替三聲子散射機制，用從頭算分子動力學模擬中的非諧聲子準粒子，對於定義良好的聲子準粒子，這種方法已經被證明是有效的。基於從頭計算的分子動力學模擬表明，主要與Ag貢獻有關的聲學聲子是選擇性散射的，而縱向光學聲子在快離子狀態下保持良好。此外，本研究結果表明，流體靜水壓可以抵消熱誘導類液體Ag流的擴散散射，這可能為進一步的聲子工程和新型熱電材料設計提供了一種新的選擇和思路。
該文近期發表於npj Computational Materials 6: 26 (2020)，英文標題與摘要如下，點擊https://www.nature.com/articles/s41524-020-0295-8可以自由獲取論文PDF。
Highly selective phonon diffusive scattering in superionic layered AgCrSe2
Chen Wang and Yue Chen
Superionic materials that exhibit coexistence of rigid crystalline lattices and liquid-likefluctuating substructures have emerged as promising thermoelectric materials. The inadequate understanding of the phonon behavior in the superionic state, however, still prevents further revealing of the underlying correlation between the thermally induced liquid-like atomic dynamics and anomalous thermal transport properties. Herein, by adopting a hybrid scheme to directly characterize anharmonic phonon quasiparticles from ab-initio molecular dynamics, we manifest that low-energy transverse phonons dominated by Ag atoms totally collapse, whereas longitudinal optical phonons remain largely intact during the superionic transition. The ultralow thermal conductivity originates from the atomic level structural heterogeneity can be ultimately attributed to diffusive phonon dynamics. Our study also reveals that the extremely large selective phonon diffusive scattering can be counteracted by hydrostatic pressure induced deactivation of the liquid-like flow of Ag atoms. These results demonstrate the decisive role of ion superionicity in phonon scattering across superionic transition and may pave the way for new phonon engineering strategies in related superionic materials.
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