Theoretical Design On Three Different Types Of Unusual Chemical Reactions At High Pressure

High pressure can be used to synthesize new chemical compositions,crystal structures, and even lead to superconductivity. Therefore, the research on the high-pressure chemical reactions of condensed matters has been a fascinating subject in solid state physics, geophysics, chemistry, materials science, et al for a long time. Pressure is independent of the temperature and chemical composition. High pressure will shorten the interatomic distance with the volume compression of materials, change the chemical valence state of an atom, and promote the reactions which cannot happen at ambient condition by reducing the activation energy. It seems that high pressure has become an important means of synthesizing new materials. In this paper, by using of the in-house developed CALYPSO structure design method combined with the first-principles calculations, we explored three typical reaction systems of hydrogen(H2) and selenium(Se), cesium(Cs) and xenon(Xe), lithium(Li) and silicon(Si), and performed systematically high-pressure chemical reaction research. We obtained the following innovative results:1. The discovery of high-temperature superconductivity(Tc = 200 K) in sulfur hydrides at megabar pressures breaks the traditional belief on the Tc limit of 40 K for conventional superconductors, and opens up the doors in searching new high-temperature superconductors in compounds composed of light elements. Selenium is a sister and isoelectronic element of sulfur, whereas it has a larger atomic core and a weaker electronegativity. Whether selenium hydrides share the similar high-temperature superconductivity remains elusive, but it is a subject of considerable interest. First-principles swarm structure predictions are performed in an effort to achieve the chemical reactions between elemental hydrogen and selenium. We find that the phase diagram of selenium hydrides is rather different from its sulfur analogy, which is indicated by the emergence of new phases and the change of relative stabilities. Three energetically stable and metallic species with novel stoichiometries of HSe2, HSe, and H3 Se are designed above ~120 GPa and they all exhibit superconductive behaviors, of which the hydrogen-rich H3 Se phase is isostructural to H3 S, showing a high Tc up to 110 K. Our simulations established the high-temperature superconductive nature of selenium hydrides and provided useful route for experimental verification.2. Xenon(Xe) is a member of the inert gases. Because of its stable electron configuration and larger ionization energy, it is quite chemical inert, and is difficult to participate in chemical reactions. Recent theoretical investigations identified that Xe becomes more reactive under compression. The discovery of chemical reactivity of the noble-gas Xe at high pressure has reignited great interest in Xe-containing compounds. By comparison with Xe, Cs has one more electron and relatively low electronegativity. At elevated pressures, when mixing with Xe, Cs(Xe) can lose(obtain) a partial electron and possess a nearly identical electron state with Xe(Cs). To understand the high-pressure chemical reactions of Cs-Xe system and the competing relationship between Cs and Xe over an electron at high pressure, this paper studied chemical reactions between elemental Cs and Xe solids. Our results showed that Cs can form a series of energetically stable intermetallics(Cs Xe4,Cs Xe3,Cs Xe2,Cs Xe,Cs2 Xe,Cs3Xe, and Cs4Xe) with Xe under high pressure. All of them are mainly characterized by the close-packed arrangement of Cs and Xe atoms. Partial electrons transfer from Cs to Xe atoms. Research extends the new cognition of the chemical reaction of Xe.3. The bandgap and optical property of diamond silicon cannot satisfy the requirement of many advanced applications(e.g., thin-film photovoltaic device and light-emitting diode). As a consequence, constant effort is made to search new Si allotropes with desired bandgap and optical property. Recently, novel Si allotrope with a desirable bandgap of ~1.3 e V is obtained by leaching the sodium from Na Si6 which was synthesized under high pressure. This finding opens up the doors in finding new Si allotropes. This paper investigated the chemical reactions between elemental Li and Si solids at high pressures, where seven novel compositions(Li Si4, Li Si3, Li Si2, Li2Si3, Li2 Si, Li3 Si, and Li4Si) become stable above 8.4 GPa. All of these compounds exhibit metallic feature. The Si–Si bonding patterns in these compounds evolve from frameworks in turn to layers, linear chains, and eventually isolated Si ions with increasing Li content. Notably, nearest-neighbor Si atoms, in silicon-rich Li Si4, form covalent open channels hosting one-dimensional Li atom chains, which has the similar structural feature as the observed Na atom chains in Na Si6. It is expected that the Li Si4, acting as a precursor, can be used to obtain novel Si allotrope.