High Pressure Structures And Properties Of Typical Oxygen-rich Compounds

Under extreme conditions of high pressure,the distance between atoms and molecules becomes smaller,and the atoms in the crystal structure will rearrange,tending to form a denser stacking pattern,showing completely different structures and properties under normal pressure.These new structures provide more possibilities for finding new materials(such as superhard materials,superconducting materials and thermoelectric materials,etc.).Therefore,high-pressure research has received more and more attention and aroused widespread research interest.Oxygen,as a highly reactive non-metallic oxidant,can easily form oxides with most elements and other compounds.Normally,oxygen is in the-2 oxidation state in the form of O2-,but it can also form other ions,such as peroxide(O22-),superoxide(O2-)and ozonide(O3-).When an element reacts with oxygen,oxygen ions with different valences can form a variety of possible molecular compounds.As we all know,oxygen is one of the most abundant and widely distributed elements in the earth’s interior.It can form different types of compounds with elements of groups Ⅰ and Ⅱ in the periodic table,such as H2O,NaO3,MgO,CaO,etc.These oxides can be used as strong oxidants because of their strong oxidizing properties and are the most basic materials for industrial science.In addition,these oxides are also important components of giant planets.Therefore,exploring the existence of hydrogen and alkaline(earth)metal oxides under high-pressure conditions plays a vital role in understanding the evolution of the planet’s composition and promoting the development of industry.In this paper,we mainly use the CALYPSO structure prediction method that independently developed by our research group and the high-pressure experimental technology to systematically and deeply study the structure and properties of a series of hydroxides,alkali metals and alkaline earth metal oxides under extreme conditions.1.From the poles of inner planets and comets to the moons of the gas giants and distant dwarf planets,water/ice is an eternal companion of the entire solar system.As one of the most abundant compounds in the solar system,H2O exhibits a large number of complex and novel stable or metastable structures under different pressure and temperature conditions.However,studies have found that in addition to the original H2O stoichiometry,other proportions of H-O compounds can also exist stably,and these new hydroxide compounds are very likely to exist in giant planets such as Saturn and Jupiter,thereby affecting the physical and chemical properties of the planets themselves.In this thesis,CALYPSO structure prediction method combined with first principles is used to make systematic high-pressure theoretical prediction of H-O system on the O-rich side.The study found that under high-pressure oxygen-enriched environment,the orthorhombic phase of H2O2 is thermodynamically stable in the pressure range of～423-600 GPa,and its structure is formed by stacking interesting planar H2O2 molecules.Particularly,unlike H2O that forms hydrogen bond symmetrically under high pressure conditions,H2O2 still exists in the form of molecular crystals under ultra high-pressure conditions.This research has important guiding significance for the experimental synthesis of hydrogen and oxygen compounds and the exploration of their properties.2.Peroxides are compounds containing oxygen-oxygen single bonds(O-O),and the oxidation state of each oxygen atom is-1.Simple peroxides are formed by hydrogen or metal ions(such as alkali metals and alkaline earth metal elements)and peroxide anions(O22-).Due to their special reactivity and oxidizing ability,peroxide compounds are widely used in various fields such as chemistry,energy storage,materials science and medicine.At present,a lot of research work has revealed the structure and properties of alkaline earth metal peroxides under various conditions.Barium peroxide(BaO2),as one of the earlier experimentally synthesized peroxides,and the research on its high-pressure behavior is not perfect.This research work explores the high-pressure structure and properties of barium peroxide through a combination of theory and experiment.Using the CALYPSO structure prediction method independently developed by the research group,it is predicted that BaO2 will undergo a phase transition from an orthorhombic structure(Cmmm)to a monoclinic structure(C2/m)under the pressure of 110 GPa.Unlike the previous conjecture that high pressure may induce a polymerization of O2 units in BaO2,leading to an AlB2-type structure with graphene-like O sheets,the new C2/m phase retains the peroxide O2 groups.The calculation and analysis of phonons found that the phase transition can be explained by soft mode theory,that is,the high-pressure phase C2/m structure can be obtained by phonon mode softening of the low-pressure Cmmm structure.Further,we carried out the BaO2 high-pressure X-ray diffraction pattern and Raman experiment.The high-pressure X-ray diffraction pattern of BaO2 up to 130 GPa can be indexed with the new monoclinic C2/m,and the Raman frequency of the stretching peroxide mode shows a anomaly supporting the theoretically predicted phase transition.3.Alkali metal ozonides have thermodynamic instability at room temperature,so experimental synthesis is extremely difficult.So far,the information about the crystal structure and physical properties of alkali metal ozonides still needs to be further improved.The study of the high-pressure phase evolution of NaO3 is of great significance for understanding its stable existence environment and can further understanding the properties of inorganic ozonides.In this thesis,the CALYPSO structure prediction method combined with first-principles calculations has carried out an in-depth study on the crystal structure and properties of the typical alkali metal ozonide NaO3,and successfully predicted a new structure with orthorhombic symmetry Immm.In terms of thermodynamics and kinetics,this structure has obvious advantages over the earlier proposed Im2m structure,and the original ozone unit O3-anion in the Im2m structure disappeared,forming a brand new O2 ion group.Besides,in this structure,the transformation from suppressed magnetic to non-magnetic is found.