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arXiv 1907.02508cond-mat.mtrl-sciphysics.chem-phphysics.comp-ph
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从第一性原理理解金属-有机体系中的声子性质

Understanding Phonon Properties in Isoreticular Metal-Organic Frameworks from First Principles

Tomas Kamencek, Natalia Bedoya-Martínez, Egbert Zojer

摘要:

金属-有机骨架(M0F)是由金属中心和有机连接体组成的晶体材料,形成开放和多孔结构。由于利用其大量的内表面积的各种可能的应用,它们已被广泛研究。然而,尽管它们与导热性和导电性、热膨胀和机械性质相关,但MOF的声子性质仍然在很大程度上未被探索。在这里,我们使用量子力学模拟提供了一个深入的分析的声子性质的isoreticular MOFs。我们考虑声子能带结构,模式的空间限制,投影态密度,和群速度分布。我们发现,更复杂的链接器移动的声子态密度的频谱权重向更高的频率,而增加节点中的金属原子的质量具有相反的效果。由于高孔隙率的MOFs,我们观察到一个特别明显的偏振依赖性的色散的声学声子具有相当高的群速度的纵向声学模式。有趣的是,也为几个光学声子模式的群速度达到几千米/秒。对于像MOFs这样的异质系统,相关的群速度和模式的位移是特别相关的。在这里,我们发现,高群速度通常与离域振动,而逆相关并不一定成立。为了量化非谐性,我们计算模式Grüneisen参数,我们发现只有频率低于3太赫兹的声子是显着的。所提出的结果提供了深入了解的MOF的振动特性的基础,因此,铺平了道路,为未来的合理设计系统具有明确的声子性质。

英文摘要:

Metal-organic frameworks (MOFs) are crystalline materials consisting of metal centers and organic linkers forming open and porous structures. They have been extensively studied due to various possible applications exploiting their large amount of internal surface area. Phonon properties of MOFs are, however, still largely unexplored, despite their relevance for thermal and electrical conductivities, thermal expansion, and mechanical properties. Here, we use quantum-mechanical simulations to provide an in-depth analysis of the phonon properties of isoreticular MOFs. We consider phonon band structures, spatial confinements of modes, projected densities of states, and group velocity distributions. We find that more complex linkers shift the spectral weight of the phonon density of states towards higher frequencies, while increasing the mass of the metal atoms in the nodes has the opposite effect. Due to the high porosity of MOFs, we observe a particularly pronounced polarization dependence of the dispersion of acoustic phonons with rather high group velocities for longitudinal acoustic modes. Interestingly, also for several optical phonon modes group velocities amounting to several thousand m/s are obtained. For heterogeneous systems like MOFs, correlating group velocities and the displacement of modes is particularly relevant. Here we find that high group velocities are generally associated with delocalized vibrations, while the inverse correlation does not necessarily hold. To quantify anharmonicities, we calculate mode Grüneisen parameters, which we find to be significant only for phonons with frequencies below ~3 THz. The presented results provide the foundations for an in-depth understanding of the vibrational properties of MOF, and, thus, pave the way for a future rational design of systems with well-defined phonon properties.

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