Structures, bonding and transport properties of high pressure solids
The objective of this investigation is to study the distinct physical and electronic properties of high-pressure solids, through state-of-the-art first-principles numerical computations. This thesis is composed of four distinct research topics. The superconducting properties of several high-pressure solids were investigated based on the Migdal-Eliashberg theory within the framework of the BCS model. The possibility of pressure-induced superconductivity was investigated for selected materials, including dense Li, Xe, and Group IV hydrides. The pressure-induced phase transition FCC → cI16 in Li and the superconducting properties in the FCC and cI16 phases were investigated. Noble gas Xe is predicted being a superconductor under pressure with a comparatively low Tc. Two Group IV hydrides, SiH4 and SnH4, were predicted to be good superconductors under high pressure. The Bader's AIM analysis, IR and Raman spectroscopes were used as diagnostic tools to differentiate among candidate structural models for solid H2, O2, and SiH4. For solid H2, IR and Raman spectra are used to examine two recently proposed competing structures of the high-pressure phase III; the Cmcm and C2/c structures. For solid O2, the experiment observed structure, IR and Raman spectra of the recently solved C2/m structure of the high-pressure ε phase were well produced. Using Bader's AIM method and from the analysis of the electron charge density, the preference on the formation of (O2)4 clusters in the C2/m structure and the nature of the interactions between O2 molecules is explained. For SiH4, IR and Raman spectra were calculated for our predicted P42/nmc structure and the agreement with available experiment results is very good. On theoretical aspect, typical approaches for predicting/determining unknown high-pressure crystal structures usually involve dynamical processes. An alternate approach based on a recently proposed genetic algorithm was explored in this thesis. The focus is to predict stable and meta-stable structures at high pressure without any preference on initial structures. The high-pressure structures of Ca were investigated and two new stable structures that might explain the diffraction pattern of the Ca-IV and Ca-V phases were predicted. The high-pressure phase II and phase III of AlH3 were also investigated, and structures were successfully predicted for each phase. Another example presented is the prediction of a metastable single-bonded phase of nitrogen. A first-principles approach was developed for the calculation of XAS within the framework of the DFT. The PAW method was used to reconstruct the core orbitals. These orbitals are essential for the calculation of the transition matrix elements. This approach provides a straightforward framework for the investigation of single particle core hole and electron screening effects, which have been demonstrated to be significant for all investigated materials. To test the implementation, the C, Si, and O K-edge XAS were calculated for diamond, fullerene C60, α-quartz and water molecule. In all cases, the calculated XAS agree very well with experiments. For water molecule, the quality of the calculated XAS sensitively depends on the delicate theoretical treatment of core hole potential and electron screening. The overall agreement between the calculated XAS and experiment is reasonable.
Bonding, Superconductivity, High Pressure Physics, Crystallography
Doctor of Philosophy (Ph.D.)
Physics and Engineering Physics
Physics and Engineering Physics