Ab Initio Structure Determination and Property Characterization of High-Pressure Ca-O Compounds and Li2(OH)Br Crystals

Theoretical modeling and computational simulations play an important role in materials research. In this thesis, we report on the study of two material systems using various computational methods. The first material system studied here are Cax-Oy compounds under high pressure. Calcium and oxygen are amongst the most abundant elements on Earth, and they form the compound calcium oxide (CaO) that is widely used and very stable at ambient pressures. Although the crystal structure and chemical composition of CaO seems to be simple and well understood, metastable or stable Ca-O compounds with unconventional stoichiometry may exist at high pressures. In this work, first-principles density functional theory calculations and ab initio evolutionary simulations have been used to predict high pressure Ca-O structures. We show that under increasing pressure, the stability of the Ca-O system changes and new materials emerge with different stable or metastable structures. In addition to CaO, we systematically examined structures for Ca, Ca2O, Ca2O3, Ca3O, Ca3O2, Ca7O, CaO2, CaO3, CaO7, and O at high pressures. The high-pressure phase diagram for these compounds is determined along with the electronic density of states and stoichiometry plots. Our energetic analysis identified three new stable Ca-O compounds, namely Ca2O3 and CaO2 that are thermodynamically stable above 40 GPa, and CaO3 becomes thermodynamically stable at 150 GPa. The second material system studied in this work is the compound Li2(OH)Br. This compound has been shown by experiment to be a promising solid electrolyte for advanced solid state ionic battery applications. However, the crystal structure of this compound is highly complex and has not been determined accurately. This has impeded its further investigation and potential applications. We modelled the structure effectively using the density functional theory via VASP software. Having successfully determined the correct structure within our model and confirming it by comparison to experimental XRD results, we then performed other calculations to ascertain the characteristics of this compound. The electronic density of states was determined, along with the phonon density of states and the volume vs pressure curve. From the information found within the volume vs pressure plot we then determined the bulk modulus of the material