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

本计画旨在利用x光散射技术研究奈米结构矽笼合物在高压下诱发相变的机制。插入的钡原子显示出高于NIV，V -阈值的所谓的“巨偶极共振”。这种共振的整体形状和发生强烈影响的电子结构和嵌入原子的局部环境的变化。因此，共振是一个非常敏感的探针，同样根据压力引起的相变的变化。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。包合物Ba 8 Si 46和Ba 24 Si 100的高压性能也将在高达25 GPa的压力范围内进行研究，以详细了解相应的相变机制。在文献中，它们在偏离中心位置、硅空位形成和电子重构方面进行了有争议的讨论。这些实验将使用非共振非弹性X射线散射（NRIXS）完成，该散射可以测量硬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，其中排除了使用软X射线和电子作为探针。所有的实验研究将伴随着测量光谱的计算，通过一个realspace多重散射的方法内的时间相关的局部密度近似。使用NRIXS的巨共振的这种高压研究具有很大的潜力，作为一个独特的探针，同时研究的局部结构和电子性质一般为广泛的一类纳米结构材料。

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.