THEORETICAL STUDY OF STRUCTURAL TRANSFORMATIONS AND PROPERTIES OF SELECTED MATERIALS AT EXTREME CONDITIONS

Abstract

There are several objectives that have been addressed in this thesis. Under a broader heading, the methods that have been explored and applied are density functional theory (DFT), ab initio metadynamics and ab initio molecular dynamics (AIMD). These methods have been employed to analyze structural phase transitions, electronic, vibrational and transport properties of selected materials at high pressure. All the materials that have been considered in this thesis have been studied experimentally by various research groups. Using theoretical methods and the sophisticated computational tools mentioned above, the aim of this thesis is to predict as well explain the experimental observations, thus bridging the gap between experiment and theory. The thesis has been divided as follows.

The first project that has been discussed is on the structural phase transition of aluminium triiodide (AlI3). Experimentally, no structural phase transition was reported for crystalline AlI3 at high pressure in spite of getting certain subtle results which hinted at a first order phase transition. Thus, in our study, we employed ab initio metadyanamics to scan the potential energy surface (PES) and find the energetically most stable configuration. Indeed, we found first order structural phase transition at approximately 1.3 GPa which was verified by the Raman spectra as well.

The next project was to explore the structure of the superconducting phase of hydrogen sulfide (H2S) which was experimentally observed to have a high superconducting critical temperature of 203 K. However, the crystal structure of the superconducting phase has been ambiguous and has been proposed to be a metastable phase. Therefore, in our study, we performed ab initio metadyanamics to search for metastable phases. At 80 GPa and 80 K, a metastable structure was found. This metastable structure on further ab initio molecular dynamics (AIMD) at 200 GPa and 200 K resulted in a modulated structure whose X-Ray diffraction (XRD) pattern matched excellently with that obtained experimentally. Analysis of the electron-phonon interactions on this modulated structure gave superconducting critical temperatures close to the value obtained experimentally.

The third project is based on the electron-phonon interaction and subsequent calculation of superconducting properties of an experimentally synthesized polyhydride of iron, FeH5. The structure was found to have hydrogen in the atomic form, which has long since been proposed to be a criterion for high temperature superconductivity. First principles theoretical calculations revealed FeH5 to be a superconductor at high pressure albeit with a low critical temperature of 51 K at 130 GPa, confirming a hypothesis that the superconductivity of any material is sensitive to several factors that have been discussed in the chapter.

The final project deals with the study of structural, electronic and transport properties of glass and molten basalt (igneous rock). This material is amorphous and abundant in the Earth’s mantle. Although several experimental and theoretical studies have been performed on materials that mimic basalt, there is still a lot to be unravelled regarding its structural and transport properties at the mantle conditions. A clear understanding of the structure and transport properties of basalt can explain in depth about the thermochemical evolution of the Earth and origin of life. In the study reported in this thesis, ab initio molecular dynamics simulations were performed on an amorphous model basalt structure (containing the most abundant chemical species, Si, Al, Ca, Mg and O) at the mantle conditions over a range of high pressures. The results that have been reported here are in very good agreement with earlier experimental and theoretical results, confirming that the model basalt considered is indeed a good approximation and can be further improved by considering the minor occurring elements (Na, K, etc.) for future research.