| High pressure can effectively change the interatomic distances and the electrons over-lap between two adjacent atoms. As a result, the crystal and electronic structures will bechanged forming a series of new high pressure phases with novel physical and chemicalproperties. Moreover, high pressure can also lower the kinetic energy thus promoting theformation of many new compounds that cannot be formed at normal conditions. The chem-ical and physical properties of metarials depend on crystal structures, thus knowledge ofcrystallography is essential if the properties of materials are to be understood and exploited.Experimentally, diamond-anvil-cell devices combined with other techniques, such as X-raydiffraction and neutron diffraction, have been used to determine high-pressure structures.However, due to the small size of the samples the diffracted X-ray beam is usually weak.It happens frequently that experiments fail to determine the high-pressure structures. More-over, the pressure range accessible to diamond anvil cells is limited to several millions atmo-spheric pressure. Extreme high-pressure conditions relevant to the interiors of giant planetsare beyond the current experimental reach. Unbiased theoretical prediction of high-pressurestructures with the only given information of chemical compositions of materials has to berelied on and become a topic of considerable interest in recent years. Recently, We have de-velopedasturcturepredictionsoftwareCALYPSObasedontheparticle-swarmoptimization(PSO) technique. In this thesis, we applied this method to the prediction of various systemunder high pressure in materials and planetary sciences. The main results of the thesis areas follows: 1. Being a best known thermoelectric material and a topological insulator at ambientcondition, magic bismuth telluride (Bi2Te3) under pressure transforms into several super-conducting phases, whose structures remain unsolved for decades. Here, we have solved thetwo long-puzzling low high-pressure phases as seven-and eightfold monoclinic structures,respectively, through particle-swarm optimization technique on crystal structure prediction.Above14.4GPa, we experimentally discovered that Bi2Te3unexpectedly develops into aBi-Te substitutional alloy by adopting a body-centered cubic disordered structure stable atleast up to52.1GPa. The continuously monoclinic distortion leads to the ultimate forma-tion of the Bi-Te alloy, which is attributed to the Bi Te charge transfer under pressure.Our research provides a route to find alloys made of nonmetallic elements for a variety ofapplications.2. Oxygen is in many ways a unique element: It is the only known diatomic molecularmagnet, and it exhibits an unusual O8cluster in its high-pressure solid phase. Pressure-induced molecular disso-ciation as one of the fundamental problems in physical scienceshas been reported from theoretical or experimental studies of diatomic solids H2, N2, F2,Cl2, Br2, and I2but remains elusive for molecular oxygen. We report here the predictionof the dissociation of molecular oxygen into a polymeric spiral chain O4structure (spacegroup I acd, θ-O4)above1.92-TPapressureusingtheparticle-swarmsearchmethod. Theθ-O4phase has a similar structure as the high-pressure phase III of sulfur. The molecularbondingintheinsulatingε-O8phaseortheisostructuralsuperconductingζ-O8phaseremainsremarkably stable over a large pressure range of0.008–1.92TPa. The pressure-inducedsoftening of a transverse acoustic phonon mode at the zone boundary V point of O8phasemight be the ultimate driving force for the formation of θ-O4. Stabilization of θ-O4turnsoxygen from a superconductor into an insulator by opening a wide band gap (approximately5.9eV) that originates from the sp3-like hybridized orbitals of oxygen and the localizationof valence electrons.3. Studies of the Earth’s atmosphere have shown that more than90%of xenon (Xe) isdepleted compared with its abundance in chondritic meteorites. This long-standing missingXe paradox has become the subject of considerable interest and several models for a Xereservoir have been proposed. Whether the missing Xe is hiding in the Earth’s inner core has remained as a long unanswered question. The key to address this issue lies in the reactivityof Xe with iron and nickel (Fe and Ni, the main constituents of the Earth’s core), whichhas been denied by earlier studies. Here we report on the first prediction of the chemicalreaction of Xe with Fe and Ni at the conditions of the Earth’s core through first-principlescalculations and unbiased structure searching techniques. We find that Xe and Fe/Ni canform inter-metallic compounds of XeFe3/XeNi3energetically most stable at the conditionsof the Earth’s core. As the result of a Xe Fe/Ni charge transfer, Xe loses its chemicalinertness by opening up the filled5p electron shell and functioning as a5p-like element,whilst Fe/Ni is unusually negatively charged, acting as an oxidant rather than a reductant.Our work establishes that the Earth’s inner core is a natural reservoir for Xe storage, andpossibly provides the key to unlocking the missing Xe paradox.4. The composition and state of the Earth’s inner core, located deeper than5,100kmfrom the surface, is subject to ultrahigh P-T (pressure and temperature) conditions exceed-ing360GPa and6,000K. The composition and state of the core remain uncertain to a largeextent, in part because static experiments performed on candidate compositions at such ex-treme conditions have been technically challenging. Oxygen is also a strong candidate asa light alloy in the core, in part because it can be incorporated in large amounts as a con-sequence of coremantle chemical reaction. It is believed that FeO is the most Fe-rich solidcompound in the Fe-O system. Here, we report that the unexpected stoichiometries of ironand oxygen (Fe3O) can stable in the Earth’s iron-rich core. Our calculations indicate largecharge transfer between ions in these oxides for these systems. The present findings willshed light on the exploitation the properties of the Earth’s core. |
PDF Full Text Request