High-Pressure Investigation Of Cesium Halides On Phase Transition And Physical Property

High pressure can effectively change the interaction between two adjacent atoms, and it will induce forming a series of new high pressure phases with novel physical and chemical properties. Pressure plays an important role in searching newly type functional material and it provide a new way to develop new idea or principle. Experimentally, diamond-anvil-cell devices combined with other techniques, such as X-ray diffraction and neutron diffraction, have been used to determine high-pressure structures. However, due to the small size of the samples the diffracted X-ray beam is usually weak. Moreover, the pressure range accessible to diamond anvil cells is limited. It happens frequently that experiments fail to determine the high-pressure structures. As the developing of theoretical prediction method, the accuracy rate is continuous improving. Our research group has developed a structure prediction software CALYPSO based on the particle-swarm optimization(PSO) technique. The remarkable feature of this methodology is the capability of predicting the stable structure by structural evolution and energy calculation with only the knowledge of the chemical composition at given external conditions(for example, pressure). CALYPSO method has been successful in correctly predicting structures for various systems, including elements and binary and ternary and clusters and surface and interface materials. Thus, it is significant for developing theoretical prediction method. Carrying out high-pressure research for typical material is remarkable for searching its most stable structures, ant it also can broad concerning on the synthesis and the fundamental structural properties of materials with implications for an entire family of similar materials. Such work can represents the frontier problem in the field of condensed matter physics.Cesium halides are the simplest and most representative ionic solids. The cesium chloride(Cs Cl), cesium bromide(Cs Br) and cesium iodide(Cs I) with high-symmetry cubic structure represent a series of AB-type compounds. To explore high-pressure research of cesium halides is remarkable for its structural sequence and potential physical properties, and it provide important implications for other AB-type compounds. We carried out a structural search of Cs Cl, Cs Br and Cs In using CALYPSO methodology which is based on density functional theory of first-principles calculation. The main results of the thesis are as follows:1. At ambient pressure, the high-symmetry structure of Cs Cl and Cs Br with Pm-3m space group is one of the prototypical AB-type compounds. Carrying out high-pressure research of cesium halides is remarkable for understanding the structural sequence and potential physical properties, and it also provide important implications for other AB-type compounds. In the previous experimental work, a tetragonal phase of Cs Cl was observed at a pressure of 65±5GPa, and the detailed crystal information is unsettled. While Cs Br was found to undergo a second-order phase transition to the tetragonal Cu Au–I(P4/mmm) structure at 53 GPa. Our work shows that Cs Cl undergo complicated transitions from Pm-3m structure to orthorhombic Pmma phase at 73.5 GPa, and then transforms to Pbam phase at 123.6 GPa. It is not the previous work raised that Cs Cl transformed from cubic phase to tetragonal phase at high pressure. Moreover, the structure sequence of Cs Br is Pm-3m â†' Pmma. Our result show that the current Pmma phase of Cs Br is energetically much superior to the previously proposed P4/mmm structure. Further electronic calculations indicate that Cs Cl and Cs Br similar with Cs I will become metallic via band-gap closure at strong compression(> 150 GPa). The present work establishes the comprehensive understanding of the high-pressure evolution of the structural properties of Cs Cl and Cs Br. Our findings represent a significant step toward the understanding of the behavior of AB-type compounds under extreme conditions.2. Iodine is a fascinating and attracting element. Iodine compounds have always been the subject of extensive studies because of their significant properties such as conduction characteristic, optical property, catalytic performance and medical application, etc. And numerous hypervalent iodine compounds reveal vital value of applications in organic synthesis. Investigation of the synthesis and application of new type of hypervalent iodine compound has extremely significant meaning. Here, we present systematic structure searches of Cs In(n = 2-5) up to 200 GPa using the developed CALYPSO method. Strikingly, Cs I3 with space group of Pm-3n is thermodynamic stability under high pressure. Within the cubic Pm-3n Cs I3 phase, iodine atoms arranging into several endless linear chains show an attractive hypervalence phenomenon under high pressure, which is in sharp contrast to the conventional understanding of 3c-4e theory. We further discovered that Pm-3n Cs I3 is a metallic phase with several energy bands crossing Fermi-surface, and the pressure creates a peculiar reverse electron donation from iodine to cesium. The electron-phonon coupling calculations have proposed superconductive potential of the metallic Pm-3n Cs I3 at 10 GPa which is much lower than that of Cs I(180 GPa). With the pressure increasing, the superconductivity of Cs I3 disappears at 150 GPa. This work has wide implications for other inorganic compounds that likely harbor similar high-pressure behavior, and the significance for synthetic chemistry is highlighted.