Photochemical Decomposition Of Germanium Tetraiodide Under High Pressure And Fluorescence Of Europium/Silver Co-doped Yttrium Disilicates

As an emerging but important research area, High pressure science focuses on re-searching the physical and chemical properties of matter under high pressure. The great reduction of the distance between atoms or molecules, brought out by high pressure, changes the electronic structure of materials, and results in the rearrangement of atomic positions, electronic energy levels and spin states of condensed matters. These modifi-cations contribute to the electronic phase transition and structural transition. Previously, a great deal of research enthusiasm has been poured into the influence of temperature, electromagnetic field or chemical constituents. However more and more research re-ports demonstrate that, like temperature and chemical compositions, pressure is an ir-replaceable thermodynamic parameter to which should be paid more attention. On one hand, by studying the performance of materials under high pressure we can find or even develop novel functional materials. On the other hand, new physical and chemical phenomena observed in materials under high pressure can lead us on the way to test ex-isting theories or put forward new ones. In the future, high pressure science is expected to make a greater contribution in helping us to solve some basic problems in physics, to understand the origin of life, to realize structures and developments of celestial bodies and so forth.This dissertation includes two major works. The first one is about pressure-induced reverse reaction of the photochemical decomposition of germanium tetraiodide (GeI4) molecular crystal; the second one involves the research on temperature induced crystal-lization of Y2Si2O7and the white light emission of Eu3+/Ag co-doped Y2Si2O7. The dissertation is composed of four chapters.In chapter1, we provide an overview of high pressure science, the technology to achieve high pressure and to calibrate pressure, and the means of measuring physical properties of materials under high pressure. The diamond anvil cell (DAC) technique, the method of applying the shift of ruby fluorescence to calibrate pressure, and the characterization methods of high pressure Raman and high pressure fluorescence spec-troscopy have been discussed emphatically.In chapter2, the concept of van der Waals force and molecular crystal have been introduced. By presenting some basic information about solid helium, solid hydrogen and solid CO2, some fundamental properties and possible applications of molecular crystals are reviewed. We also summarized the performance of molecular crystals under high pressure by listing some concrete examples, such as the insulator-metal transition of sulfur, the amorphization of ice, the superconducting transition of solid oxygen, the molecular dissociation of nitrogen, and the pressure induced chemical reaction of benzene.In chapter3, high pressure research on GeI4molecular crystal is presented. GeI4molecular crystal and its solution in cyclohexane were irradiated by lasers of different wavelengths to investigate the critical wavelength for photochemical decomposition of GeI4. We have observed that632.8nm laser can photochemically decompose GeI4, exceeding the previously reported wavelength limit of514nm. XPS spectra indicate that GeI4is photochemically decomposed into Ge2I6and I2; unlike GeBr4, Ge2+(GeI2) cannot be found in the photochemical reaction products. Raman spectra measurement of GeI4under high pressure up to24GPa show that Raman signals of Ge2I6and I2vanish at0.5-1.7GPa. This finding clearly shows that high pressure can effectively reverse the photochemical decomposition of GeI4and influence the direction of the solid state reaction, which is usually found on gas phase reactions.In chapter4, we discuss the spectroscopic properties of Eu3+and Ag co-doped Y2Si2O7. The Eu3+/Ag co-doped rare earth disilicate Y2Si2O7microcrystal was syn-thesized by a sol-gel method. Through controlling the thermal treatment process of Y2Si2O7:Eu3+/Ag precursor, various phases (amorphous, α,β,γ,δ) were prepared. White light emission was observed under UV light excitation in the samples heavily doped with Ag. The white light was realized by combining the intense red emission of Eu3+, the green emission attributed to the very small molecule-like, non-plasmonic Ag particles (ML-Ag-particles), and the blue emission due to Ag ions. Results demonstrate that Eu3+/Ag co-doped Y2Si2O7microcrystal could be potentially applied as white light emission phosphors for UV LED chips.