Temperature and pressure induced changes in the local atomic and electronic structure of complex materials

Abstract

Changes in the local atomic and electronic structure of complex materials under high tempera- ture and high pressure conditions are explored by means of x-ray absorption spectroscopy and x-ray Raman scattering. Three consecutive studies on the formation of well-defined germanium (Ge) nanocrystals in oxide matrices are presented. The temperature induced phase separation and Ge nanocrystal formation in amorphous GeO is inverstigated using x-ray absorption spectroscopy and x-ray diffraction. Size control is achieved via a multilayer approach. Ge oxide free Ge nanocrystals in silicate matrices from ternary multilayers are reported on. The complex local environment of the Ba atoms and interactions between Ba guest and Si host atoms of Ba intercalated Si clathrates are studied using x-ray Raman scattering. The Ba giant resonance is investigated for several complex Ba intercalated Si clathrates and compounds and is shown to be dependent on the local environment of the Ba atoms. The nature of the peculiar pressure induced phase transitions in the clathrate Ba_8 Si_46 is investigated by measurements of the Ba multiplet features in the vicinity of the Ba N_4,5 absorption edge. First direct proof of the electronic topological nature of these isostructural phase transitions is presented. An x-ray Raman scattering study of water under high pressure and high temperature con- ditions offers new insight on the local atomic and electronic structure of water under such extreme conditions. In corroboration with molecular dynamic simulations, the results suggest supercritical water to consist of a highly disordered and disrupted but homogeneous hydrogen bond network. First measurements on geologically relevant silicate melts show the feasibility of such studies and implications for future experiments are given.