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arXiv 1106.4765cond-mat.softcond-mat.mtrl-scicond-mat.stat-mechphysics.chem-ph
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硬四面体的Phase Diagram

Phase Diagram of Hard Tetrahedra

Amir Haji-Akbari, Michael Engel, Sharon C. Glotzer

摘要:

多面纳米粒子和胶体合成的进展激发了人们对多面体形状相行为的兴趣。正四面体吸引了特别的注意,因为它们更喜欢与周期性不相容的局部对称性。最近报道了正四面体的两个致密相。在三角双锥(二聚体)晶体中实现了密度已知的四面体堆积,堆积密度为4000/4671= 85.63%。在模拟中观察到了一个十二方准晶,它的近似体,具有周期性的平铺结构(3.4.3^2.4),可以被压缩到85.03%的填充率。在这里,我们表明,准晶近似是更稳定的比二聚体晶体的堆积密度低于84%,使用Monte Carlo计算机模拟和自由能计算。为了进行自由能的计算,我们使用的各向异性形状和热力学积分的弗伦克尔-拉德方法的变化。近似的增强的稳定性可以归因于网络子结构,其最大化颗粒可用的自由体积(以及因此的“摆动空间”),并促进颗粒的相关运动,这进一步有助于熵并导致填充密度低于65%的扩散。结构上不同的相位之间的固体-固体过渡的存在不相关的对称性破缺-近似和二聚体晶体-是不寻常的硬粒子系统。

英文摘要:

Advancements in the synthesis of faceted nanoparticles and colloids have spurred interest in the phase behavior of polyhedral shapes. Regular tetrahedra have attracted particular attention because they prefer local symmetries that are incompatible with periodicity. Two dense phases of regular tetrahedra have been reported recently. The densest known tetrahedron packing is achieved in a crystal of triangular bipyramids (dimers) with packing density 4000/4671=85.63%. In simulation a dodecagonal quasicrystal is observed; its approximant, with periodic tiling (3.4.3^2.4), can be compressed to a packing fraction of 85.03%. Here, we show that the quasicrystal approximant is more stable than the dimer crystal for packing densities below 84% using Monte Carlo computer simulations and free energy calculations. To carry out the free energy calculations, we use a variation of the Frenkel-Ladd method for anisotropic shapes and thermodynamic integration. The enhanced stability of the approximant can be attributed to a network substructure, which maximizes the free volume (and hence the 'wiggle room') available to the particles and facilitates correlated motion of particles, which further contributes to entropy and leads to diffusion for packing densities below 65%. The existence of a solid-solid transition between structurally distinct phases not related by symmetry breaking -- the approximant and the dimer crystal-- is unusual for hard particle systems.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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