The Experimental And Theoretical Studies Of Crystal Structures And Phase Transitions In Typical Binary Compounds Under High Pressure

The types of materials will increase dramatically under high pressure, accompanying with many novel chemical and physical properties. Especially, high pressure can effectively overcome the dynamic barrier in the reactions, and allow us to obtain a lot of new functional materials which don't exist at normal conditions. The crystal structures of the materials are the most important to understand their physical and chemical properties. And to ascertain and to determine their structures are priority for the other researches. In this thesis, we use our own developed crystal structure prediction code-Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) to explore and predict the crystal structure in some typical systems. We use theory to guide experiment and make a simple preliminary estimation, that avoids the blindness in the experiment. However, experiments are also necessary in high pressure research. Experimental data is more authentic, witch can be used to validate and improve the theory. In turn, the theory can make up experimental data, analyze and explain novel experimental phenomena. Here, we combine theory and experiment to extensively explore the high-pressure phase transitions and crystal structures in some simple binary inorganic compounds. According to the theoretical prediction, we synthesize niobium polyhydrides at high pressure and temperature, and make clear their crystal structures.We investigated the high pressure phases of CdF2 by a joint theoretical and experimental study. The structural and electronic properties of CdF2 were extensively explored to high pressure by ab initio calculations based on the density functional theory. According to our calculation, a structural phase transition from the fluorite-type (Fm-3m, Z=4) structure to the cotunnite-type (Pnma, Z= 4) structure was estimated below 8 GPa, and this phase transition was examined by the high pressure experiments up to 35 GPa at room temperature in diamond anvil cell. Both high pressure angle dispersive X-ray diffraction and Raman spectroscopy experiments provided convincing evidence to verify the phase transition, and their results are in agreement with theory. During the decomposition, we found that it is a reversible first order phase transition with an obvious relaxation. Our work makes clear pressure-induced phase transition and structural information of CdF2 under high pressure. This work represents a significant step in understanding the high pressure phase diagram of CdF2 and enriches the information of divalent metal fluoride under high pressure.We report a theoretical and experimental research on the high-pressure structures of bismuth selenide (Bi2Se3) up to 50 GPa. Our first-principles structure prediction via calypso methodology combined the high-pressure X-ray diffraction experiments. We established that the ambient-pressure rhombohedral phase transformed to a monoclinic C2lm structure at 9.8 GPa, and then to a monoclinic C2/c structure at 12.4 GPa. Above 22.1 GPa, we were able to identify that Bi2Se3 developed into a novel 9/10-fold structure, which was not taken by its other family members Bi2Te3 and Sb2Te3. The large differences in atomic core and electronegativity of Bi and Se are suggested to be the physical origin of the stabilization of this 9/10-fold structure. Our research work allows us to reveal a rich chemistry of Bi in the formation of 6,7,8, and 9/10-fold covalent bond with Se at elevated pressures. This work represents a significant step forward in understanding the high-pressure behaviors of Bi2Se3. We hope that our study will contribute to other research on high-pressure structures of A2B3-type materials.Hydrogen rich compounds have attracted great attentions since they are proposed to become potential high-temperature superconductive materials. Here, we report a systematic study on niobium hydrides system by both experiment and theory up to one megabar. NbH2, NbH2.5 and NbH3 were successfully synthesized from niobium and hydrogen under high pressure with the aid of laser heating and were measured by in-situ synchrotron X-ray diffraction. Heating under different pressures would cause different products. NbH2 was found to undergo a phase transition from the fluorite-type phase to a distorted hexagonal phase with space group Pnma above 39 GPa. Interestingly, a new double hexagonal close-packed NbH2.5 was observed after laser heating at about 46 GPa. With further compression, cubic NbH3 was formed at 56 GPa and kept stable up to 102 GPa. These experimental results were supported by our further theoretical calculations. Our findings can provide new insights into the understanding of the formation of metal polyhydride under high pressure.