Theoretical Study Of Yttrium Hexaboride And Cesium Iodide At High Pressures

Exposing a material to high pressures can alter its physical properties, which shows unique physical phenomenon that wouldn't appear at ambient pressure. Therefore, high pressure study has become an important branch in the research area of Condensed Matter Physics. The distances between atoms in materials reduced effectively, as well as the overlap of electron orbits between two adjacent atoms changed at high pressures, which result in the change of bonding behavior significantly and induce structural phase transitions and electronic transformations. Accordingly, new physical phenomena and effects would appear. This dissertation is devoted to the determination of new crystal structures and the exploration of new physical phenomena (such as superconductivity) at high pressures.With the progress in theory and its numerical methods, density functional theory (DFT) based first-principles calculation has become an important method for the research of condensed matter theory, quantum chemistry and material science. This method has many advantages such as the moderately computation, the widely application of systems, the highly accuracy and etc. The high-pressure physical properties have been studied systemically based on this method according to different problems on given materials in present study.The discovery of superconductivity at ~40 K in MgB2 has stimulated a renewed interest in the physical properties of other borides in pursuing high temperature superconductors. Yttrium hexaboride (YB6) is a typical representative for its second-highest superconducting critical temperature of ~7.1 K in the borides at ambient condition. At present, there is no consensus on the mechanism of superconductivity in YB6. Moreover, the behavior in physics under high pressures is still far from being clear. Therefore, we have performed detailed studies of the electronic structure, lattice dynamics, thermodynamic, and superconducting properties of YB6 within the DFT, obtaining original results in below:(1) The changes of electronic band structure and Fermi surface along theΓ-M symmetry line indicate an electronic topological transition under compression. The frequencies at R25 and M2 show a pressure-induced phonon softening behavior, thus a possible phase transition under pressure in YB6 is predicted by this theory.(2) The pressure and temperature dependence of thermodynamic quantities of interest have been derived within the quasi-harmonic approximation for the first time. The zero pressure temperature dependences of linear thermal expansion coefficient, heat capacity, and overall Grüneisen parameter are found to be in a good agreement with the experimental results. The present calculations successfully explain the experimental anomaly around 50 K in linear thermal expansion coefficient because of large volume dependence in some low frequency modes at ambient pressure. The zero pressure specific heat at room temperature is very small compared with the Dulong-Petit value, indicating that the thermal energy at 300 K is still too low to excite all the spectral modes. The anomaly in overall Grüneisen parameter at 0 GPa is well understood in that it is heavily weighted by the heat capacity at constant volume at low temperatures and is less weighted at higher temperatures.(3) The pressure dependence of superconductivity has been systematically studied using both the linear response theory and the rigid muffin-tin approximation methods for YB6. The calculated electron-phonon coupling of YB6 with both methods suggested that pairing electrons are mainly mediated by the Y low-lying phonon vibrations, which is responsible for the high superconducting critical temperature observed experimentally. These results agree well with the experimental measurements, but in apparently contrast to the previous theoretical calculations. The decreasing of EPC is also discovered by both methods which attributes to the hardening of Y low-lying phonon frequency with pressure.Cesium iodide (CsI) has long been a prototype system for high pressure investigations due to the wide variety of phenomena it exhibits on compressions. CsI is also interesting because it is isoelectronic to rare-gas solid xenon (Xe). However, there is no consensus yet with regard to the phase transition sequence in CsI and the high-pressure crystal structure of CsI has not been determined, as well as a full understanding of the close similarity in x-ray diffraction (XRD) and equation of states (EOS) between CsI and Xe observed experimentally at high pressures (> 100 GPa) is still not possible. Furthermore, the mechanism of pressure induced metallization and superconductivity in CsI is still far from being clear and well established. Thus, we present detailed studies of the crystal structures, electronic and electron-phonon coupling of CsI based on the DFT. The obtained original results are as follows:First, the evolutionary methodology is employed to predict the high-pressure crystallography of CsI. A novel high pressure orthorhombic Pnma structure has been uncovered. The transition sequence of CsCl→Pmma→ Pnma with pressure is revealed from enthalpy calculations with the transition pressures of 39.5 GPa and 42 GPa, respectively. This indicates that Pnma structure becomes favorable above 42 GPa and remains to be stable at higher pressure range studied here. These two phase transitions can be both characterized as first order but with very small volume reduction of 0.7 % and 1.3 % from EOS calculations, respectively. This explains the nearly continuous experimental EOS data.Second, the close similarity between Pnma structure of CsI and hcp phase of Xe in stacking order of atoms successfully explains the convergence of XRD and EOS between CsI and Xe at high pressures observed experimentally.Third, the pressure dependence of the electronic properties indicates that the metallization of Pnma CsI occurs via indirect band-gap closure along the Z-? and ?-Y directions, in apparently contrasted to the results of direct band-gap closure in previous theoretical works. This shows the importance of choosing appropriate structures in the research of material properties. The peculiar reverse electron donation from I-?Cs+ is responsible for the superconductivity in CsI at 180 GPa.Finally, the electron-phonon coupling with pressure for Pnma structure of CsI has been extensively studied. It is found that a strong localized behavior at high pressures. The experimental observation of superconducting transition temperature reduction with pressure is well understood based on these calculations.