摘要/Abstract

氧化铝纳米团簇在众多技术应用中日益受到重视, 找到其最优结构对进一步的研究非常重要. 本工作提出了一种定向Monte Carlo格点搜索算法用于搜索不同氧化铝晶体(α, θ和δ)内的不同尺寸的氧化铝纳米团簇的结构, 并对结构进行了分析比较. 通过定向移动策略, 定向Monte Carlo格点搜索中每一步都是“有效”移动, 极大地增加了搜索效率. 研究结果发现α氧化铝团簇形成一种多层结构, θ和δ氧化铝团簇形成一种单层薄膜结构. θ和δ氧化铝团簇的二阶能量差分存在奇偶震荡, 偶数尺寸的氧化铝团簇具有相对更高的稳定性. 通过相对能量比较发现相同尺寸下θ和δ氧化铝团簇薄膜结构比α氧化铝团簇结构更稳定, 在对这种薄膜进行第一性原理计算后进一步验证这种薄膜具有良好的稳定性和抗氧化性.
关键词: 氧化铝团簇, 定向Monte Carlo, 结构搜索, 稳定性, 单层薄膜结构
Alumina nanoclusters have attracted increasing attention in many technical applications due to their excellent properties in optics, electricity, thermodynamics, chemical reactions and other aspects. Therefore, it is very important to find the optimal structure for further research. In this paper, we use the directional Monte Carlo lattice search algorithm combined with Woodley's potential function to investigate the stable structures of different sizes of alumina nanoclusters in different crystal forms (α, θ and δ). In our algorithm, the lattice was chosen from supercells which the primitive lattice cell from the inorganic crystal structure database (ICSD) was duplicated to generate large supercells of α, θ and δ phases. The initial random structure generated in the lattice, then the selective probability for all chosen and empty atoms is calculated based on the Boltzmann distribution of energy. In the directional Monte Carlo exchange, at elevated temperatures, the system has enough energy to cross energy barriers and find the basins, and at low temperatures, it converges to around the global minimum. After structure searching, we analyze and compare the stable structures of different crystal forms. The results show that the α-alumina clusters consist of multiple (Al₂O₃)₁ and form a multilayer structure, as the size increased, hexagonal aluminum began to form inside the cluster. The θ and δ alumina clusters form a stable monolayer thin film structure, with the increase of size, the monolayer film structure can still be maintained. To further study the relative stability of the cluster structure, we introduced a second-order energy difference. The second-order energy difference of θ and δ alumina clusters has obvious odd-even oscillation, and the even size alumina clusters have relatively higher stability due to better symmetry. α-Alumina is the most stable crystalline phase in the bulk phase, so we take the α-alumina clusters as the benchmark to compare the relative energy of alumina clusters with different crystal types. Through the comparison of relative energy, we found that the thin film structure of θ and δ alumina clusters under the same size is more stable than that of the α-alumina clusters. Therefore, we further carried out first-principles calculations on this film structure, and we found that this film structure has good kinetic and thermodynamic stability and good oxidation resistance.
Key words: alumina cluster, directional Monte Carlo, structure search, stabilization, monolayer thin film structure


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