Theoretical Studies On Structures And Properties Of Alkali Metal Silicides Under High Pressure

Silicon element?Si?is one of the most essential elements due to its abundance in nature,high chemical stability,low cost,and non toxicity,etc.However,pure Si is an indirect semiconductor at ambient pressure,which significantly limits the efficiency of conversion for Si-based solar batteries.Finding new silicon materials with ideal direct band gap have become the focus of attention.Recently,high pressure technology had been used to synthesize Si-rich alkali metal silicides,and then alkali metal was removed from the precursor by chemical process.A new Si allotrope of silicon was formed,which possesses a quasi direct band gap.Therefore,our work dedicated to systematically explore the crystal structure and properties of ASi_x?A=Li,Rb,Cs;x=4–8?via first-principles swarm-intelligence structure searches coupled with density-functional theory under high pressure,then obtain the new silicon by removing the alkali metals.The main results are as follows:1.First principles swarm-intelligence structure searches coupled with density-functional theory were performed to explore the stable high-pressure phases of silicon-rich lithium compounds,LiSi_x?x=4-8?.The LiSi₄ stoichiometry was predicted to be stable within the pressure range of 0 to 50 GPa,and it was found to be the following space group of P4/mnc,Cmmm and C2/m that were found to be stable between 8.6 and 33.8 GPa respectively.By removing the Li atoms from these compounds,three silicon allotropes were obtained that were metastable at ambient pressures.I4/mmm-Si is an indirect band gap semiconductor,with a band gap of 0.5eV?HSE?,Both Cmmm-Si of orthogonal structure and C2/m-Si of monoclinic structure are metallic.2.Using CALYPSO structure prediction method,we systematically explored the crystal structures of RbSi_x?x=4-8?within the pressure range of 0-50 GPa.We found that RbSi₄,RbSi₅,RbSi₆ stoichiometry was predicted to be stable.For RbSi₄,anorthorhombic Cmcm phaseat ambient pressure,and that phase undergoes to an cubic I4/mcm phase at¹2 GPa,transforms to a monoclinic C2/m phase at²6.8GPa;then transforms to a monoclinic P2/m phase at³0.2 GPa and P2/m phase are stabilized by increased pressures below 50 GPa;For RbSi₅,the stable phases is Imma phase;For RbSi_6,new cubic Im-3m phaseat ambient pressure,and that phase undergoes to anorthorhombic Cmcm phase at³.3 GPa,then transforms to a monoclinic C2/m phase at?41.6 GPa.these RbSi_xare metallic under pressure.By removing the Rb atoms from these compounds,eight silicon allotropes were obtained that were metastable at ambient pressures:One direct bandgap semiconductor Cmcm-Si,two indirect bandgap semiconductors I4/mcm-Si and Im-3m-Si,and five metallic silicon elements;3.We report someinteresting crystal structures of CsSi₆ using a global structure search at the range of 0-30 GPa.We identified a new cubic Im-3m phase of CsSi₆ at ambient pressure,and that phase undergoes to anorthorhombic Cmcm phase at?8.2GPa,then transforms to a monoclinic C2/m phase at?20.8 GPa.Moreover,these newly predicted structures are all dynamically stable.By removing the Cs atoms from these compounds,three silicon allotropes were obtained which were metal stable.Im-3m-Si is a?cage indirect band gap semiconductor,with a band gap of 1.167 eV?HSE?.Our workprovide systematical insights for the high-pressure structures and physica properties of alkali metal silicides,and theoreical guidance for synthesizing novel Si materials in experiment.