Bonding nature of the amorphous structure studied by a combination of cutoff and electronic localization function

Abstract

The analysis of the local structure of covalent glass is one of the major challenges for analyzing the amorphous structure. Usually, people use a cutoff distance to determine the coordinated atoms and relevant structural information, such as coordination number and bond angles. Recently, the electron localization function (ELF) has been used to analyze the local structure of amorphous Ge2Sb2Te5. But how to determine the EFL threshold and cutoff distance has not been reported. Here, according to the ab-initio calculations, we systematically investigate the relationship between the bond number and the ELF threshold, and also the cutoff distance in amorphous GeTe. The reasonable value of the ELF threshold and the cutoff distance are determined according to the inflection point and slope change of the bond number with ELF value respectively. Furthermore, the minimal ELF value distributions of Ge-Ge, Ge-Te and Te-Te bonds are presented. The comparison shows that the majority of removed bonds in structural analysis are weak Ge-Te bonds due to the low localization degree of electron. In contrast, the stronger Ge-Ge bonds are almost unchanged when changing the ELF threshold value from 0.58 to 0.63 because of the high localization degree of electron. The average minimal ELF value of Ge-Te bonds in crystalline GeTe is calculated, and it is close to the ELF threshold that is determined by the inflection point. t is easy to find that the Ge-Te bonds which are removed by increasing the ELF threshold are relatively weak. Therefore, these weaker bonds should be removed in structure analysis, which also means that the ELF threshold determined by the inflection point are reasonable value. Finally, based on the EFL threshold value, the coordination number and bond angle distribution of Ge in amorphous GeTe are obtained. The analysis of the coordination number of the Ge atoms shows that as the ELF threshold increases from 0.58 to 0.63, the 5- fold Ge atoms almost disappear because they are against the (8-N) rule. Furthermore, when the ELF threshold value is 0.58, the bond angle distribution analysis of Ge atoms shows that the local structure is a configuration that is mainly defectively octahedral (3-fold Ge) and distorted tetrahedral (4-fold Ge), but it remains unchanged when the threshold value increases to 0.63. It further demonstrates that all the removed chemical bonds are weaker ones as the ELF threshold increases. This approach is useful to improve the accuracy of amorphous structure analysis by obtaining the more reasonable inter-atomic bonding information. And it should be applied to the structural analyses of other systems generally.