Theoretical Study On Crystal Structure And Properties Of Gold - Silicon Alloy

This paper took an in-depth study on Au-Si system by selecting 12 special compositions’ Au-Si alloys according to the research status and calculating conditions: 12.5%, 25.0%, 33.3%, 37.5%, 50.0%, 57.1%, 60.0%, 62.5%, 66.7%, 75.0%, 80.0% and 87.5%. Based on the powerful calculating ability of contemporary computer, the method of GA combined with DFT calculations was used in systematic global crystal structure searches for these 12 compositions’ Au-Si alloys in this paper. In view of the complexity of Au-Si system, an idea of indifference to treat was taken in order to make sure the comprehensiveness of the searches. The global structure optimizations were performed without any assumptions on the Bravais lattice type, atom basis, crystal shapes, or unit cell dimensions, the initial atomic positions in the unit cells were randomly generated and the chemical compositions were the only given information. The low energy structures of all 12 compositions’ Au-Si system were obtained after a total search more than 100 thousands structures, and the convex hull of the formation energies of the whole Au-Si system was draw out. Afterwards, a detailed systematic analysis of the whole convex hull was performed and 3 important conclusions obtained:1. Most of the Au-Si alloys are difficult to exist stably;2. When Au content is 50%, the most stable Au-Si alloy can be formed;3. Au-Rich Au-Si alloys are much easier to be stable than Si-rich ones.Based on the calculation results and the 3 above important conclusions, 3 compositions that can exist stably were filtrated with a negative formation energy and locating a minimum point in convex hull: 50.0%, 66.7% and 80.0%. Afterwards, their low energy structures were analyzed in detail and the stabilities of these low energy structures were investigated by calculating formation energies, phonon bands and elastic constants. The results show #63 CMCM Au8Si8, #20 C2221 Au16Si8 and #121 I-42 M Au8Si2 3 structures meet thermodynamic stability, dynamic stability and mechanical stability. Therefore, the low energy structures of 50.0%, 66.7% and 80.0% 3 compositions may exist stably. Properties of Au-rich Au-Si alloys prefer Au properties was verified by calculating the electronic properties of these 3 structures. Finally, the mechanism of Au can be used to grow Si nanowires in the experiment was explained qualitatively based on the convex hull of formation energies of whole Au-Si system. This paper provides valuable theoretical information for the application of Au-Si alloys in practical applications.