Order-disorder transition in Co-Pt nanoparticles: Coexistence, transition states, and finite-size effects
Order-disorder transition in Co-Pt nanoparticles: Coexistence, transition states, and finite-size effects
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DOI：
10.1103/physrevb.84.035409
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发表时间：
2011-07-20
影响因子：
3.7
通讯作者：
Mottet, C.
中科院分区：
文献类型：
--
作者：
Calvo, F.;Mottet, C.
通过蒙特卡罗模拟重新研究了Co-Pt纳米合金和块状样品中的有序到无序转变，使用平行回火策略来缓解缓慢收敛和规避滞后行为。该方法可以有效地实现有序相和无序相的共存，这一点可以从块状样品和800原子纳米颗粒的双峰分布中得到证明。这反过来又提供了一些精确的转变温度的评估。通过在伞式抽样选择的配置空间的相关区域附近执行提交者分析，我们检查了几个顺序参数如何允许表征过渡状态。除了混合指数和键取向有序参数外，形状变形参数（由纳米颗粒的球形度偏差和块体的四方畸变定义）最能令人满意地分离有序相和无序相。当无序相从局部表面缺陷形成核时，需要显著的晶格变形才能完全进行。
The order to disorder transition in stoichiometric Co-Pt nanoalloys and in a bulk sample has been revisited by Monte Carlo simulations, using the parallel tempering strategy to alleviate slow convergence and circumvent hysteretic behavior. The method is efficient enough to produce coexistence between ordered and disordered phases, as evidenced by the bimodal distributions found for the bulk sample and the 800-atom nanoparticle. This in turn provides some precise evaluation of the transition temperature. By performing committor analyses near relevant regions of configuration space selected by umbrella sampling, we examine how several order parameters allow to characterize the transition state. In addition to a mixing index and a bond-orientational order parameter, a shape-deformation parameter, defined from the deviations from sphericity in the nanoparticles and from the tetragonal distortion in the bulk, is found to separate the ordered and disordered phases most satisfactorily in both cases. While the disordered phase nucleates from local surface defects, it requires significant lattice deformations to proceed completely.