Structural Design On Several Unusual Stoichiometric Compounds Under High Pressure

It is well known that external high pressure can efficiently change the atoms to a denser packing mode, tune structural transitions, tune the band gap and sometimes can also synthesis some non-conventional stoichiometric compounds which cannot exist in the natural environment. Therefore, high-pressure is a very good way for us to searching new novel structures and new properties. Structure is the most basic and important information of the material, it is closely related to physical and chemical properties of the material. Thus knowledge of crystallography 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-ray diffraction, neutron diffraction and high-resolution electron microscopy have been used to determine high-pressure structures. However, diffraction signals intensity and the purity of the samples will effect experiment results and always fail to determine the high-pressure structures. Moreover, the pressure range accessible to diamond anvil cells is limited to several millions atmospheric pressure and experiments could not achieve extreme high-pressure conditions. Thus, structure prediction of high-pressure structures with the only given information of chemical compositions of materials has become the focus research in the field of materials, physics, and chemistry. The purpose of this paper is to explore some of the new high-pressure structure and unconventional stoichiometric compound in specific systems (Y/La-C, Te-H and Ti-O system) with novel properties. The main results of the thesis are as follows:1. Carbon(C) is able to form various bonding patterns, including graphene sheets, chains and dimers, but stable bare six-membered C6 hexagonal rings, which are the fundamental structural motifs of graphite and graphene have long been missing. Here we have solved long-puzzling stabilization of such bare C6 rings under high pressures in the charge-transfer systems of binary sesquicarbides Y2C3 and La2C3 as predicted by firstprinciples swarm structure searching simulations. We found that the external pressure can be used to efficiently tune structural transitions in the sesquicarbides from the ambient-pressure cubic phases into high-pressure orthorhombic phases, accompanied by significant C-C bonding modification from C-C dimers to bare C6 rings and polymerized graphene-like double C6 sheets. The bare C6 rings are stabilized in Y2C3 and La2C3 at pressures above 32 and 13GPa, respectively, which are readily accessible to experiments. Chemical bonding analysis reveals that the bare C6 rings feature a benzene-like sp2 C-C bonding pattern with a delocalized Ï€ system. Y /Laâ†'C charge transfer and the need for denser structure packing are found to be part of the underlying mechanisms behind the stabilization of the bare C6 rings. Our research provides a route to synthesis long-puzzling stabilization of such bare C6 rings and be of great significance in experimental synthesis and exploration of its properties.2. Observation of high-temperature superconductivity in compressed sulfur hydrides has generated an irresistible wave of searches for new hydrogen-containing superconductors. We herein report the prediction of high-Tc superconductivity in tellurium hydrides stabilized at megabar pressures identified by firstprinciples calculations in combination with a swarm structure search. Although tellurium is isoelectronic to sulfur or selenium, its heavier atomic mass and weaker electronegativity makes tellurium hydrides fundamentally distinct from sulfur or selenium hydrides in stoichiometries, structures, and chemical bondings. We identify three metallic stoichiometries of H4Te, H5Te2, and HTe3, which are not predicted or known stable structures for sulfur or selenium hydrides. H4Te is so far the most H-rich stoichiometry reported in the family of chalcogen hydrides. The two hydrogen-rich H4Te and H5Te2 phases are primarily ionic and contain exotic quasimolecular H2 and linear H3 units, respectively. Their high-Tc (eg,104K for H4Te at 170GPa) superconductivity originates from the strong electron-phonon couplings associated with intermediate-frequency H-derived wagging and bending modes, a superconducting mechanism which differs substantially with those in sulfur or selenium hydrides where the high-frequency H-stretching vibrations make considerable contributions. Our work summarizes and improves the scientific research of the high temperature superconductivity of chalcogen hydrides.3. Titanium dioxide (TiO2) has a wide range of industrial applications (e.g., in photocatalysis and solar cells). Pressure causes profound structural and electronic changes in TiO2, leading to the fundamental modification of its physical properties. We discovered the metallization of TiO2 at high pressures through first-principles swarm structure searching calculations. Metallization accompanies the stabilization of a body-centered tetragonal CaC2-type structure (space group I4/mmm), which is more stable than the Fe2P-type structure above 690GPa. This phase adopts a ten-fold coordinated structure consisting of face-sharing "TiO10" dodecahedrons, the highest coordination number among all TiO2 phases known so far. For the titanium-oxygen system, we discovered a non-conventional stoichiometric compound TiO3 and a new high-pressure phase of TiO. For TiO3, each Ti atoms and 12 O atoms around it form an icosahedron. Phase transition of TiO from ambient-pressure a-phase (Space group A2/m) into high-pressure y-phase (NaCl-type), and then transforms to our new predicted cubic phase at 72GPa with eight-fold coordination. Our work shows a new potential energy map of titanium oxides at high pressures, and enables the opportunity to understand the structure and electronic properties of TiO2 at high pressures.