在大氣壓力下液態鎵結構的分子動力模擬之研究

Abstract

在環境壓力下液態鎵靜態結構因子的異常是為人所知的，其異常性是在第一個尖峰右邊(高波數側)有一凸起的肩膀結構。自從近半個世紀前液態鎵的實驗觀察以來，這個凸起肩膀的靜態結構因子的成因一直是液態金屬研究領域的長期問題。在本論文中，將藉由古典與第一原理分子動力模擬，探討在環境壓力下液態鎵的結構。在兩種分子動力模擬所產生的液態中，皆發現具有低配位數的鬆散結構。 為了探討模擬液態中的局部結構，發展出用局域方向鍵結(LBOO)參數所定義的叢集方法。在兩種分子動力模擬方式所產生的液態中，皆找到兩種不同類型的局域方向鍵結叢集。其中一種局域方向鍵結叢集具有六重方向對稱性，其叢集靜態結構因子與硬球模型液體的靜態結構因子相類似。另一種局域方向鍵結叢集具有四重方向對稱性，其叢集靜態結構因子則有非對稱的主峰，甚至出現凸起肩膀的結構因子，若叢集是在高度的局域方向鍵結狀態。因此，這兩種局域方向鍵結叢集對液態鎵的靜態結構因子出現凸起的肩膀結構，形成了對比的作用。具有低配位數粒子的局部結構，偏好形成高度的局域方向鍵結狀態，所以液態鎵的異常靜態結構因子與低配位數粒子所造成的局部結構有關是肯定的。另一方面，檢驗了局域方向鍵結叢集的局部結構與多個鎵固體的結構之間的相似性。在第一原理分子動力模擬所產生的液態中，有四重方向對稱性的局域方向鍵結叢集結構，具有與β-鎵的固態結構相似的趨勢。

Liquid Ga at ambient pressure (AP) is well known for its anomalous static structure factor (SSF), which displays a shoulder on the high-q side of its first peak. Since the experimental observation for liquid Ga nearly half a century ago, the physical cause for the high-q shoulder in the SSF has been a long-standing issue in liquid metal research. In this thesis, the structure of liquid Ga at AP were studied by using classical and ab initio molecular dynamics (MD) simulations. Loose pack structures with low coordination numbers were found in liquids generated by both MD simulations. To explore the local structures of the simulated liquids, a cluster approach defined by using the local bond-orinetational order (LBOO) parameters was developed. With common analyzed results, two types of LBOO clusters were found in liquids obtained by both MD simulations. One type of the clusters is characterized by sixfold orientational symmetry and their cluster SSFs are akin to that of a hard-sphere fluid. The other type of the clusters is typified by fourfold orientational symmetry and their cluster SSFs generally display an asymmetric first peak, and even a high-q shoulder as the clusters are in highly LBOO. Therefore, the two types of LBOO clusters play contrasting roles for the appearance of the high-q shoulder in the SSF of liquid Ga. The local structures of particles with low coordination numbers are favorable to LBOO with fourfold orientational symmetry, so it is confirmed that the anomalous SSF of liquid Ga is related to the structures caused by particles with low coordination numbers. On the other hand, the similarity between the local structures of the LBOO clusters and the structures of multiple Ga solids were examined. According to the ab initio MD simulations, the cluster structures with fourfold orientational symmetry have a tendency to resemble more to the structure of β-Ga.