High Pressure Studies Of Metal-Hydrogen Systems

Study on metal-hydrogen systems (including Metal hydrides) is an important scientific research field and is also a hot topic due to its hydrogen storage applications. Hydride is a group of compounds, which formed by hydrogen and other elements. Metal hydride is the main part of this compound, making up more than 80%. High pressure study on metal hydride is somehow unique, for it is advantage to form metal hydride under high pressure. Sometimes high pressure is the only way to synthesis hydride which is not formed under ambient pressure. There are some important and meaningful topics in high pressure studies of metal-hydrogen systems (metal hydrides): Firstly, basic studies on the synthesis of metal hydride, studies on pressure and temperature effects on the formation of hydride which finally construct the phase diagram. Also taking the advantage of high pressure, we can synthesis new hydrogen-rich compound like high pressure phase Xe(H₂)7 which is good for hydrogen storage research; Secondly, High pressure study on the stability of metal hydride and its phase transitions; Thirdly, study on hydrogen molecular properties in metal hydride. Hydrogen in metal hydride is under precompression, it will save a big range of pressure (^Mbar) for scientist to squeeze hydrogen within metal hydrides. This study may help scientist for further understanding of metallic hydrogen. Fourthly, application studies on the hydrogen storage in metal hydride under high pressure and searching for high hydrogen content materials for hydrogen storage. Finally, study superconductivity properties in metal hydrides and searching for high Tc superconductor in this compound. In this thesis, we select five typical metal-hydrogen systems (metal hydrides) to study. We have performed room temperature and low temperature high pressure synchrotron X-ray diffraction on them, and also magnetic susceptibility and electrical resistance measurement on two of them. Based on our results, we come to new conclusions and further extending our understanding of these systems under high pressure. It will also make a contribution to the hydrogen-rich compound study and also hydrogen storage research field. Our research work is made up of four parts in this thesis:For Rhodium-hydrogen system, we carried out both low temperature and room temperature experiments on this system; electrical resistance measurement is also performed to check the superconductivity property under high pressure for all three phases in this system. Furthermore we collaborate with theoretical calculations on this system for detail information. Our results showed that under high pressure rhodium-hydrogen system formed RhH and RhH₂ with increasing pressure. When releasing pressure the phase transitions are reversible and with a little sluggish. Low temperature (>5K) resistance experiment showed that no superconductivity was observed. From low temperature X-ray diffraction experiment we successfully recovered the high hydrogen content RhH₂ phase to ambient pressure. With increasing temperature, RhH₂ firstly broke down to RhH and finally to pure Rh phase. According to this property with hydrogen going into rhodium and coming out when heating, Rhodium-hydrogen system (RhH₂) is a good candidate of hydrogen storage materials with high hydrogen volumetric density 163.7g/l which is 2.3 times larger than liquid hydrogen.For Yttrium-hydrogen system (YH₃), we have carried out low temperature and high pressure x-ray diffraction experiments and measured the magnetic susceptibility and electrical resistance under low temperature and high pressure on this system; the results show that under low temperature YH₃ undertake the same phase transitions from hexagonal to face center cubic phase as room temperature, and there is also the intermediate phase between phase transitions. From magnetic susceptibility measurement we found that there is no superconductivity in this system to 40GPa within 5K to room temperature. Moreover, the x-ray exposure experiment combined with electrical resistance measurement results showed that the resistance of Yttrium-hydrogen system (YH₃) is very sensitive to the exposure of x-ray, which showed the typical photoconductivity effect.For Calcium-hydrogen system (sample CaH₂), after our earlier research on this sample at room temperature, we performed low temperature high pressure X-ray diffraction experiment. We found that the high pressure phase transition of Calcium hydride is the same as the one at room temperature, which is from orthorhombic with space group pnma to hexagonal with space group p63/mmc. We also found the low temperature effect on this system.At last, for Vanadium and Chromium with hydrogen systems. We performed room temperature high pressure X ray diffraction experiments, unfortunately our results showed that with experiment pressure range there are no phase transitions for both Vanadium-hydrogen system and Chromium system. However, we get the equation of state of Vanadium and Chromium under good hydrostatic conditions and extent our understanding for both systems for reaction properties under high hydrogen pressure conditions.