A novel approach to study pressure-induced phase transitions of nano-structured materials: The barium giant dipole resonance

本项目的目的是通过x射线散射技术研究和理解具有内嵌钡原子的纳米结构硅包合物高压诱导相变的机理。插入的钡原子在NIV，V -阈值之上显示出所谓的“巨偶极子共振”。这种共振的整体形状和开始受到电子结构和嵌入原子局部环境变化的强烈影响。因此，谐振是一个非常敏感的探头，同样根据压力引起的相变变化。这种变化将首先研究不同类型的Ba/Si化合物，其中钡嵌入到各种硅环境中，包括硅纳米笼和通道结构。在高达25 GPa的压力范围内，研究包合物Ba8Si46和Ba24Si100的高压性能，以详细了解相应相变的机理。它们在偏离中心位置、硅空位形成和电子重构方面在文献中有争议。实验将使用非共振非弹性x射线散射（NRIXS）来完成，它可以测量硬x射线的低能激发。这是一种在极端压力条件下研究钡巨共振的独特方法，在这种条件下，不使用软x射线和电子作为探针。所有的实验研究都将伴随着测量光谱的计算，通过实空间多重散射方法在时间依赖的局部密度近似内。使用NRIXS进行这种高压巨共振研究具有巨大的潜力，可以作为一种独特的探针，用于同时研究广泛的纳米结构材料的局部结构和电子性质。

The aim of this project is to study and understand the mechanism of high pressure induced phase transitions for nano-structured silicon clathrates with endohedrally intercalated barium atoms by x-ray scattering techniques. The intercalated barium atoms show a so-called “giant dipole resonance” above the NIV,V -threshold. The overall shape and onset of this resonance is strongly affected by changes of both the electronic structure and the local environment of the embedded atom. Thus, the resonance is a very sensitive probe for likewise changes according to pressure induced phase transitions. Such changes will be investigated first for different types of Ba/Si compounds in which barium is embedded into various silicon surroundings including silicon nano-cages and channel structures. The high pressure properties of the clathrates Ba8Si46 and Ba24Si100 will also be studied within a pressure range up to 25 GPa in order to understand in detail the mechanism of the corresponding phase transitions. They are controversially discussed in the literature in terms of off-center positions, silicon vacancy formation and electronic reconfiguration. The experiments will be accomplished using non-resonant inelastic x-ray scattering (NRIXS) which enables the measurement of low energy excitations with hard x-rays. This is a unique way to study barium giant resonances under extreme pressure conditions where the use of soft x-rays and electrons as probes is excluded. All experimental investigations will be accompanied by computations of measured spectra by means of a realspace multiple scattering approach within the time dependent local density approximation. Such high pressure studies of giant resonances using NRIXS have great potential to serve as a unique probe for the simultaneous study of local structures and electronic properties in general for a wide class of nano-structured materials.