Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai 400005, India.
J Phys Condens Matter. 2011 Aug 17;23(32):325401. doi: 10.1088/0953-8984/23/32/325401. Epub 2011 Jul 25.
We report a study of the relative effects of dimensional and kinetic constraints on the stabilization of metastable, polytypic forms of metallic silver. We show that the hexagonal 4H polytype (hitherto observed only in size-constrained systems) can be produced in the form of bulk thin films by suitably slowing down the growth kinetics. Further, using extremely slow growth conditions, we have been successful in depositing a novel, two-dimensional, metastable polytype (2H) of silver, which is highly reactive (easily oxidized) and has a density 23% lower than normal silver. First-principles calculations based on density functional theory confirm that the 4H structure is relatively stable. However, local stability analysis via a determination of the phonon dispersion of the 2H structure reveals that it is only marginally stable with an energy surface that is rather flat or weakly varying with respect to many of the modes. This makes a large contribution to the configurational entropy and is probably the reason for the metastability of the observed 2H polytype with an unusually large lattice constant along the c-direction.
我们研究了维度和动力学约束对亚稳多型金属银稳定化的相对影响。我们表明，六方 4H 多型（迄今仅在尺寸受限系统中观察到）可以通过适当减缓生长动力学来形成体薄膜形式。此外，通过使用极其缓慢的生长条件，我们成功地沉积了一种新型的二维亚稳多型（2H）银，它具有很高的反应性（易氧化），密度比普通银低 23%。基于密度泛函理论的第一性原理计算证实 4H 结构相对稳定。然而，通过确定 2H 结构的声子色散进行局部稳定性分析表明，它仅略微稳定，其能面相对于许多模式相当平坦或变化很小。这对构象熵有很大贡献，可能是观察到的 2H 多型具有异常大的 c 方向晶格常数的亚稳性的原因。
