LITHIUM AND OXYGEN UNDER HIGH PRESSURE: FINITE-T PHASE STABILITY AND MELTING
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Understanding the phase diagram of materials as a function of pressure and temperature is one of the most fundamental problems in condensed matter physics. When materials are compressed, modifications of their electronic structure lead to dramatic and often counterintuitive changes of their physical and chemical properties. The study of matter at extreme conditions is not only of fundamental interest but is also important for planetary science and has practical applications. Theoretical high-pressure physics plays an important role in predicting the properties of materials where no experimental measurements are available, or explaining observations where measurements are insufficient. This thesis reports results from first-principles density functional theory (DFT) calculations on the phase diagrams of compressed lithium and oxygen. The work determines a large part of the lithium finite-temperature phase diagram and elucidates the physical mechanism responsible for the observed phase stability. It is shown that the complex oC88 phase is stabilized by lattice phonon free energies at finite temperature. The significance of quantum ion dynamics for the melting behavior of lithium is determined for the pressure range of 40 to 60 GPa, and estimates for its contribution to solid and liquid free energies at higher pressures are obtained. Finally, the melting curve of lithium is predicted for pressures up to 150 GPa. For oxygen, we investigate the zero- and finite-temperature stability of its molecular solid phases up to 150 GPa. A long-standing inconsistency between theory and experiment regarding the stability of the ε(O8)-phase is resolved, and the thermodynamic stability of the η’ (O2)-phase above 550 K at 50 GPa is established. Furthermore, a new metallic structure is predicted for the ζ-phase at pressures above 100 GPa. Finally, the newly predicted finite-temperature solid structures are used to start calculations of the melting curve of oxygen up to 150 GPa.