A crystal phase is a key factor to determine the physical and chemical properties of crystalline materials. As a new class of nanoscale structures, heterophase nanoparticles, which assemble conventional and unconventional phases, exhibit exceptional properties in comparison with their single-phase counterparts. In this work, we explored the thermodynamic stability of Au, Co, and AuCo heterophase nanoparticles with fcc and hcp phases by using molecular dynamics simulations. These heterostructured nanoparticles were continuously heated to examine their thermally activated structural evolutions. Au and Co single-phase nanoparticles were also considered for comparison. The results show that the phase transition between fcc and hcp is absent in these heterophase nanoparticles despite the existence of an unconventional phase. Although the melting of Au and Co heterophase nanoparticles is homogeneous, AuCo heterophase nanoparticles show heterogeneous melting, i.e., the Au fcc domain firstly melts, followed by the melting of the Co hcp domain, exhibiting a typical two-stage melting characteristic and resulting in the existence of a solid-core/liquid-shell structure within a considerable temperature region. Furthermore, the mutual diffusion of atoms between fcc and hcp domains is observed in the Au and Co heterophase nanoparticles. However, the unidirectional diffusion from the Au domain to the Co domain is found in the AuCo heterophase nanoparticles prior to their overall melting. This study deepens the fundamental understanding of the thermodynamic evolution of metallic heterogeneous nanoparticles and provides mechanistic and quantitative guidance for the rational design and applications of nanoscale multiphase heterostructures.

中文翻译：

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金属异相纳米粒子的热激活微结构演化：分子动力学模拟的见解

晶相是决定结晶材料物理和化学性质的关键因素。作为一类新的纳米级结构，异相纳米粒子由常规相和非常规相组装而成，与单相对应物相比表现出优异的性能。在这项工作中，我们通过分子动力学模拟探索了具有 fcc 和 hcp 相的 Au、Co 和 AuCo 异相纳米粒子的热力学稳定性。这些异质结构的纳米粒子被连续加热以检查它们的热激活结构演变。Au和Co单相纳米颗粒也被考虑用于比较。结果表明，尽管存在非常规相，但这些异相纳米颗粒中不存在 fcc 和 hcp 之间的相变。即Au fcc域首先熔化，然后Co hcp域熔化，表现出典型的两阶段熔化特性，导致在相当大的温度区域内存在固核/液壳结构。此外，在 Au 和 Co 异相纳米颗粒中观察到 fcc 和 hcp 域之间原子的相互扩散。然而，从 Au 域到 Co 域的单向扩散是在 AuCo 异相纳米粒子中发现的，在它们的整体熔化之前。本研究加深了对金属异质纳米粒子热力学演化的基本认识，为纳米级多相异质结构的合理设计和应用提供了机理和定量指导。