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

点击翻译按钮获取中文摘要

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.