| Pressure is a basic thermodynamic variable.Pressure can reduce the atomic distance between substances,causing atoms to rearrange and change the crystalline and electronic structure of matter.New physical and chemical phenomena and new structures are formed.Therefore,it is necessary to obtain materials with excellent properties that cannot be synthesized under normal pressure by controlling the pressure.Many functional materials with excellent properties have been synthesized experimentally using static and dynamic high-pressure techniques,such as the discovery of high-pressure new phases of materials and the synthesis of superconductors.This paper uses the CALYPSO first-principles calculation software package based on the particle swarm optimization algorithm to predict the crystal structure of the high-pressure phase of alkaline-earth metal compounds and ternary hydrogen-rich compounds.The main work is as follows:(1)The experimental and theoretical work on the high-pressure phase of the previous alkaline-earth-metal sulfide has found that the research pressure does not exceed 150 GPa.We know that the experimental high pressure level can reach about 500 GPa,and because the material will get new material configuration or other strange phenomena under high pressure,it will help us to understand the nature of the material.Therefore,we want to see if new structures appear for alkaline-earth metal compounds at higher pressures(greater than 150 GPa).We used CALYPSO software to predict the structure of alkaline-earth metal high-pressure phases and obtained four new high-pressure structures.At higher pressures,we found that BeS will transform from B8 structure to Cmca structure;CaS and SrS have the same phase transition sequence from B2 structure to ?-NbP-type structure,while MgS will become from B28 structure to The ?-NbP-type structure;however,the B2 structure of BaS will turn into an Imma structure.The four new phase structures are stable in thermodynamics and dynamics within the corresponding pressure range.The calculated energy bands and density of states indicate that most of these high-pressure new phases are metallic.(2)To date,superconductivity studies for hydrides have focused on binary hydrides.Through the analysis of the results of binary hydride studies,it has been found that the highly symmetric structures of hydrogen-rich compounds at high pressures all exhibit very high superconducting transition temperatures,especially cubic structures such as bodycentered cubic Im-3m-H3 S and Im-3m-MH6(M=Mg,Ca,Y)at high pressure,the Tc reaches 200 K,while the cubic structures Fm-3m-LaH10 and Im-3m-YH10 break 250 K under high pressure;In the transition hydride binary hydride,the contribution of superconductivity mainly comes from the light-mass hydrogen element;due to the pre-compression effect,the doped non-hydrogen element helps to reduce the hydrogen metallization pressure,and the hydride of the heavier element With low metallization pressure and high superconducting transition temperature.Based on the above factors,we will explore whether rare earth metal X(X = Y and La)doped H3 S ternary hydride XSH6 can be a candidate material for high temperature superconductors.The high-pressure structures P42/mmc-YSH6 and CmcmLaSH6 of the ternary hydride YSH6 and LaSH6 were predicted by the CALYPSO crystal structure prediction software based on the particle swarm optimization algorithm,and the superconductivity of the two structures at 300 GPa was predicted.The transition temperature Tc is 61 K and 35 K,respectively.Our results indicate that the new ternary hydrides superconductors can be obtained by doping heavy elements in superconducting binary hydrides of high Tc and can be synthesized by strong compression on YS(LaS)+ H2 or H3 S + LaH3,which provides guidance for further exploration of high Tc ternary hydrides. |
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