High Pressure Study Of Organic Molecular Crystals And Nd:YVO₄ Crystal

High pressure technique plays an important role in modern science and technology, with impressive development and broad applications in many fields like physics, chemistry, materials and earth science. As another effect other than temperature, pressure exhibits its strength on changing properties of materials to a large extent. The distinct effect it brings is to reduce the distance between molecules or atoms of the materials, which results in the variation of its structure and composition (crystal structure, molecular structure, the alignment of atoms), and further a series of changes happen, such as the energy band structure, the combination, orbital configuration and density of states of electrons. Meanwhile, high pressure (or together with high temperature) can be a highly effective method of synthesizing materials towards various purpose. For those reasons above, investigations of physical properties under high pressure is of great importance and thus highlighted. In this thesis, we present our work as follows. The pressure effects on physical properties of several kinds of organic aromatic conjugated compounds included Acetophenone azine, Trans,trans-Dibenzylideneacetone, Benzalazine and the inorganic material Nd:YVO₄ crystals are studied. The thesis consists of seven chapters.In Chapter One, we present a brief introduction to the development of high pressure techniques and methods and emphasis on technique of diamond anvil cell and the relative pressure transmitting medium and pressure calibrations.The second Chapter is an introduction to organic molecules and rare earth complex. We mainly summarize studies of physical properties under high pressure for organic molecules and rare earth complex. Finally we explained the outline of this thesis.In Chapter Three,we reported the high pressure study of acetophenone azine (APA). High pressure Raman spectra of acetophenone azine have been measured up to 17.7 GPa with a diamond anvil cell. Two crystalline-to-crystalline phase transformations are found at pressures about 3.6 and 5.8 GPa. A disappearance of external modes and the C-H vibration at pressures higher than 8.7 GPa suggests that the sample undergoes a phase transition to amorphous or orientationally disordered (plastic) state, and the amorphization was completed at about 12.1 GPa. The disordered state is unstable and, then, a polymerization transformation reaction occurs with a further pressure increase. After the pressure has been released, the polymerization state can remain at the ambient condition, indicating that the virgin crystalline state is not recovered. The results show that the phenomenon underlying the pressure induced phase transition of APA may involve profound changes in the coordination environments of the symmetric aromatic azine.In Chapter Four, the trans,trans-Dibenzylideneacetone molecular crystal has been studied with high pressure Raman, fluorescence spectroscopy and EDXRD at room temperature. A crystalline to crystalline phase transition is found at pressure about 1.0-1.3 GPa; meanwhile, a pressure induced chemical reaction occurs and the reaction is completed at pressure about 6.5 GPa.The EDXRD spectra show that a new covalent bond may be produced in this chemical reaction process. The possible chemical reaction paths are suggested as the openning of C=C bond of the molecule and then combination between adjacent molecules. Another crystalline to crystalline phase transition may occur at pressure about 11 GPa. After the pressure has been released, the newly formed material can remain stable at ambient condition.In Chapter Five, Pressure-induced changes of the fluorescence spectra of Nd:YVO₄ crystals have been studied up to 10.88 GPa using a diamond anvil cell at ambient temperature. The changes of spectra indicate that pressure has remarkably influenced both the fluorescence intensity and the energy levels of Nd:YVO₄ crystals. With pressure increasing up to 7.5 GPa, the intensity of the spectra decreases and the peak positions shift red gradually. For pressure above 7.5 GPa, the fluorescence intensity decreases quickly and the shift rates of the peaks is slowed down. This suggests that 7.5 GPa is a critical point of the pressure at which the crystal may have a pressure-induced phase transition and the crystal-field changes. The new characters of the fluorescence spectra remain stable as the pressure is released to ambient pressure.In Chapter Six, we reported the high pressure study of benzalazine. High pressure Raman spectra of benzalazine have been measured up to 17.8 GPa and EDXRD up to 15.1 GPa with a diamond anvil cell at room temperature. Two crystalline-to-crystalline phase transformations are found at pressures about 2.2 and 6.5 GPa. A disappearance of external modes and the C-H vibration at pressures higher than 12 GPa suggests that the sample undergoes a phase transition to amorphous or orientationally disordered (plastic) state. The disordered state is unstable and, then, a polymerization transformation reaction occurs with pressure increasing above 12 GPa. After the pressure has been released, the polymerization state can remain at the ambient condition, indicating that the virgin crystalline state is not recovered. The possible chemical reaction paths are suggested as the openning of C=C bond of the molecule and then combination between adjacent molecules.In Chapter Seven, we summarize the contents of six chapters above.