Studies On The Phase Transitions Of Rare Earth Vanadate And Rare Earth Oxide Submicro-and Nano-Crystals Under High Pressure

High Pressure Physics is an important subject that investigates the properties of matters under high pressure. High pressure can effectively reduce the distance between atoms or molecules, change the crystal structure and electronic structure of matters, and lead to amazing physical changes of materials. The studies of structural phase transition in condensed matters have generally assumed that pressure, temperature and composi-tion are the only important variables in determinating the stable states of materials. However, the studies of the properties of nanomaterials with finite scale have shown that the properties of materials depend intensively the physical scale of the materials, which indicate that the physical scale can also be an important variable in determining phase transition.The study of the size dependence of a solid-solid phase transition, not only from a theoretical point of view but also from a experimental point of view, can reveals the novel physical and chemical properties. Corresponding the bulk materials, understand-ing the influences of finite size in the structural stability of nanomaterials is of funda-mental interest. Variation in stability with size could provide a method for control of the properies of materials.The mechanical properties and phase stability of nanomaterials such as nanoparti-cles, nanowires, and nanobelts strongly depend on their grain size, shape and structure. The mechanical properties and transformations of nanomaterials under high pressure have generated significant interest. The size effects in nanomaterials can be reflected in phase transition pressure, and even phase transition routines. We investigates a va-riety of rare earth vanadate and rare earth oxide nanocrystals, led them to changes in structures under high pressure, and compared the differences from those of the corre-sponding bulk materials. This dissertation is composed of seven chapters.In Chapter1, we introduced the high pressure physics and the history of the devel-opment of the high pressure technique, described the diamond anvil anvil cell technique and the related technique with emphasis, and summarized the author’s work. In addi-tion, the fields of the application of high pressure were reviewed, and the studies of nanomaterials under high pressure were emphatically discussed.In Chapter2, GdVO4:Eu3+submicrocrystals and EuVO4submicrocrystals and nanocrystals were prepared. The high pressure Raman and luminescence spectra of GdVO4:Eu3+microcrystals and EuVO4submicrocrystals and nanocrystals had been measured at room temperature by using a diamond anvil cell. The discontinuities on Raman mode and luminescence intensity and the appearance of new Raman bands and luminescence peaks have provided strong evidence for a phase transition from zircon-type to scheelite-type structure, The scheelite-type phase is retained after re-lease of pressure. For GdVO4/Eu34submicrocrystals, the phase transition pressure is7.4GPa. For EuVO4submicrocrystals, the transition pressure is5.6GPa, and for EuVO4nanocrystals, the phase transition pressure is6.9GPa. The high pressure study of EuVO4submicrocrystals and nanocrystals indicated that a decreased of particle size results in an increase of the transition pressure.In Chapter3, the monoclinic GdOOH:Eu3+, EuOOH and YOOH:Eu3+nanorods were prepared, and their luminescence spectra were measured in a diamond anvil cell at room temperature under high pressure. The changes of luminescence spectra indicated that pressure-induced phase transitions from the low pressure monoclinic phase to the high pressure and temperature tetragonal phase occurred at room temperature under10.7GPa for GdOOH:Eu3+,9.0GPa for EuOOH, and13.4GPa for YOOH:Eu3+. Af-ter releasing the pressure to ambient, the luminescence spectra revealed that the phase transitions for GdOOH:Eu3+, EuOOH and YOOH:Eu3+nanorods are irreversible. Dif-ferent from the preparing tetragonal phase (RE)OOH by high pressure and high tem-perature, only ultrahigh pressure technology can obtain the tetragonal materials at room temperature.In Chapter4, The cubic Gd2O3:Eu3+and Y2O3:Eu3+nanorods were synthesized by a hydrothermal method. The SEM image indicated the Gd2O3:Eu3+nanorods with diameter of30-35nm and length of200-500nm. The structural stability of Gd2O3:Eu3+nanorods was investigated by in situ high pressure luminescence and Raman spectra up to18.9GPa at room temperature. The results revealed a pressure-induced phase transi-tion from cubic to hexagonal structure at about11.3GPa. After releasing pressure, the hexagonal structure transfers to monoclinic phase. The comparison of the high pres-sure behavior between bulk Gd2O3and Gd2O3nanorods indicated that the transition pressure of Gd2O3nanorods is higher than that for bulk Gd2O3. The different in the pressure-induced phase transition between Gd2O3nanorods and bulk Gd2O3may be due to the unique morphology of Gd2O3nanorods. The high pressure luminescence study of Y2O3:Eu3+nanorods also indicated that the cubic phase transformed into the hexagonal structure at13.4GPa. After releasing pressure, Y2O3:Eu3+nanorods then change to the monoclinic phase. In Chapter5, the Raman spectra of CeO2nanorods with an average diameter of9nm and the length of50-200nm were measured at high pressures up to33.3GPa. The phase transition from the fluorite structure to orthorhombic PbCl2structure for CeO2nanorods is not found under33.3GPa. The result is quite different from the pressure-induced phase transition for bulk CeO2at31GPa and for nanocrystalline CeO2in the region of22.3-26.5GPa. The experiment indicates the fluorite structure of CeO2nanorods is steady under high pressures. The novel high pressure behavior of CeO2nanorods may be attributed to the unique morphology.In Chapter6, the hexagonal MoO3prepared by a hydrothermal method is in mor-phology of microrod with diameter of0.8-1.2μm and length of2.0-4.3μm. It’s struc-tural stability was investigated by an in situ Raman scattering method in a diamond anvil cell up to28.7GPa at room temperature. The new Raman peak around1000cm-1implies that a phase transition from hexagonal to amorphous starts at5.6GPa, and the evolution of the Raman spectra indicates that the structural transition is completed at about13.2GPa. After releasing pressure to ambient condition, the Raman spectrum pattern of the high pressure phase was retained, revealing that the phase transition is irreversible.In Chapter7, the contents of the studies in the dissertation were systemetically outlined.