以第一原理計算探討鋰鍺合金之液相與非晶相之動力學與結構性質

Abstract

本研究利用第一原理計算與分子動力學模擬探討液態與非晶相鋰鍺合金在不同成分下的動力學、結構等性質，LixGe合金成分介於x = 0.45-4.81，藉由適當的程序產生液態和非晶相結構，並且與鋰矽合金系統進行相關性質的比較，此篇論文主要分成兩個部分： 第一部分的研究主要針對液態鋰鍺合金相關性質進行分析，鋰鍺合金內原子的擴散係數強烈地受成分濃度所影響，在我們計算的濃度範圍中，Li1.00Ge原子的擴散係數呈現極小，當合金內鋰濃度上升，原子的擴散系數轉而提高，另外在低鋰濃度Li0.45Ge原子的擴散能力也會較Li1.00Ge大幅提升。但鋰矽系統不同，原子的擴散系數只隨著鋰濃度提高而上升。比較兩系統，低鋰濃度時，鋰鍺合金原子的擴散系數大於鋰矽合金系統，但濃度接近x = 1.00時，鋰鍺系統原子擴散系數會迅速地降低，兩系統出現反轉，鋰矽合金內原子的擴散系數會大於鋰鍺合金。我們藉由速度自相關函數(velocity autocorrelation function)的方法來分析系統，發現在Li1.00Ge內鋰和鍺原子間的交互作用力極強，導致原子的擴散能力大幅的降低，但Li1.00Si中無此現象。在結構性質的分析中顯示Li1.00Ge因為極強的鋰和鍺原子間的交互作用力導致特殊的原子分布關係，另外我們還觀察到鋰鍺合金在低鍺濃度時鍺原子隨著溫度升高而聚集的現象。最後我們分析了鋰鍺合金的能量態密度，合金呈現金屬材料的特性，且鋰鍺濃度接近八隅體組成時(Li3.57Ge)，在費米能階附近呈現極小，代表其電子導電率低於它組濃度，具最弱的金屬特性。 在本研究的第二個部分，我們將前述的液態合金進行適當的降溫程序，再進行結構最適化處理後得到非晶相鋰鍺合金固體。對各濃度非晶相鋰鍺合金進行結構與電子性質分析，結果顯示，非晶相合金與液態合金有相似的短程局域結構，結構並不會隨著相變化的過程有大幅度的轉變，在態密度的計算上也與液態合金具有相似的結果。

This study discusses two main topics. First part, we investigated the dynamic, structural and electronic properties of the liquid Li-Ge alloys. We also compared the Li-Ge system with Li-Si system which had been studied. Second, we studied their amorphous solid. By using first-principles calculations and molecular dynamics simulations, we generate realistic structural models of the liquid and amorphous alloys within various compositions. Diffusivities of Li and Ge atoms appear to be largely dependent on the chemical composition of alloys. Initially, the diffusivities of Li and Ge decrease rapidly with the increase of Li, when the composition reaches Li1.00Ge, the diffusivities of the atoms exhibit the minimum values. Nevertheless, as the Li concentration continued to rise, the diffusivities of both elements go up instead. On the other hand, the trending in diffusivity of the Li-Si system is different, it increases with the concentration of Li only. In addition, we found that the diffusivity of the atoms in Li-Ge alloy are larger than atoms in Li-Si alloy in early stage of lithiation, as previous studies had shown. But when the concentration of Li reaches a certain level, the diffusivities of the atoms in Li-Si alloy will be greater than atoms in Li-Ge alloy. The interactions between atoms in the alloy can be examined via the velocity autocorrelation function. Results showed that severe decline of diffusivities in Li¬1.00Ge is caused by the strong interactions between Li and Ge atoms, while this phenomenon can’t be found in Li-Si system. This special interactions between Li and Ge atoms can be proved by structural property analysis. Finally, we calculated the DOS of LixGe and the results showed the poor metallic behavior of LixGe near the octet composition. Also, amorphous LixGe alloy share similar local bonding structure and electronic property with liquid LixGe alloy.