Structures And Properties Of Be-h And Cl-H Compounds Under High Pressure

As the simplest element, hydrogen atom contains only one electron. However,hydrogen and its compounds contains quite complicated varieties of physical andchemical properties. Pressures can change the distances between atoms, induce chargetransfer and facilitate chemical reactions. Hydrogen rich compounds may besynthesised under high pressure through the doping of excess hydrogens. Thesehydrogen rich compounds are not only good materials for hydrogen storage andenergy storing devices, but also can be metalized under pressure and thus becomepromising high temperature superconductivities. Furthermore, due to the simplicity ofits electron structure and the complexity of the stoichiometry, hydrogen richcompounds tend to display rich H species distinguished from the well-known H-ion inhydrides at traditional stoichiometric ratios. Thus, many scientific works in the fieldof hydrogen rich compounds under pressure are motivated.Here, we present a series of investigation about the phase diagram, crystalstructures and electronic properties of hydrogen rich compounds by employing the CALYPSO methodology developed by our group for crystal structure prediction in awide pressure range.We come to the innovative results as followed:1. Superconductive materials have zero resistance and are completelydiamagnetic under certain temperature, therefore had a extended application.Searching for high temperature superconductors has received a widespreadattention. The cuprate superconductors and Fe-based superconductors haverather high transition temperature Tc, however, the mechanism of theirsuperconductivity is still in the veil. Therefore, it is important to research inthe tradition superconductors in which the mechanism is already revealed.Lately, studies on the tradition superconductors have made a breakthroughwhen Mikhail Eremets discovered the high transition temperature of H2S of150K under200GPa and a even higher Tc of190K at room temperature,which is suggest to be a hydrogen rich sulfide. To explore thepressure-induced metallization and potential superconductivity of BeH2, weinvestigate the highpressure structures by employing the CALYPSOmethodology developed by us for crystal structure prediction in a widepressure range of0–300GPa. We predicted that the α phase would at firsttransform to a1T structure (space group: P-3m1). The1T is a stereotypestructure for transition metal dichalcogenides. Owing to its pseudotwo-dimensional nature, strongly bonded slabs of AB2stacked along thedirection perpendicular to the layers by virtue of weakly bonded van derWaals gaps, the1T structure and its intercalation varieties are tied withremarkable physical properties. Upon further compression, the R-3m structure is predicted to undergo a reconstructive phase transition to a Cmcmstructure, which has a metallic ground state. Linear response electron-phononcoupling calculations indicated that once metallized, BeH2can potentiallyreach a superconducting state with a high Tcof45K at300GPa.2. The hydrides of group I A and IIA elements formed from the reaction of themetal and hydrogen molecules have been the most studied. Most of thestructures and structural trends can be explained from the simple concept ofelectron transfer from the metal to the hydrogen due to the largeelectronegativity differences between the alkali and alkaline elements andhydrogen molecules. The predicted compounds display rich H speciesdistinguished from the well-known H-ion in hydrides at traditionalstoichiometric ratios. Perhaps one of the most exciting predictions is theemergence of symmetric and linear H3-at high pressures as observed in denseCsH3and BaH6. Moreover, the formation pressures of these compounds ofjust a few tens of GPa are accessible by experiments. In comparison, thebonding pattern is quite different for group IVA and transition elements. Forexample, a Van der Waals solid with such molecular H2units was foundexperimentally in SiH4at low pressure. At higher pressure, the atoms ofgroup IVA elements tend to aggregate to form a2D layered structuredecorated with molecular like H2species as predicted for SiH4and SnH4. Incomparison, the high pressure chemistry of hydrogen with electron-richgroup VII A halogens has not been investigated. In this paper, we presentresults on a study of the crystal structures and phase stabilities ofhydrogen-rich HCl–H2system. A major finding is the stabilization of cationic (H3)+(H2) species in H5Cl. The geometry of H3+becomes almost anequilateral triangle under very high pressure. The observation of a triatomichydrogen cation H3+in the solid state is new and significant. The isolatedmolecule is important in various branches of science, such as physics,chemistry and astronomy. For example, it is known that the H3+ion with atriangular configuration is stable in the interstellar medium thanks to the lowtemperature and low density of the interstellar space and the H3molecule iscommonly formed from the neutralization reaction of H3+and an electron,and rather evanescent as a result of the repulsive nature of its ground state.Furthermore, the multicenter (H3)+(H2) bond in group VIIA Cl compoundssignifies a deviation in the nature of chemical bonding from the chargetransfer interactions in group IA and IIA hydrides and covalent bonding ingroup IVA hydrides.