The Structural Stability And Properties Of Alkali Metal Sulfides Under High Pressure

Recently,the alkali metal compounds have revived the interest of the scientific community to explore its novel structures and characters upon compression.Different Na-Cl compounds such as Na₃Cl,Na₂Cl,Na₃Cl₂,NaCl₃,and NaCl₇ are thermodynamically stable at nonambient pressure conditions which modify the familiar rules of chemistry.The compound of helium and sodium Na₂He reported recently has changed the hitherto bare field of helium chemistry,provided new twists to the chemistry of noble gases.Antimetallization is predicted in dense lithium-rich compound CLi₄ which means this phenomenon is expected in not only alkali metals Li and Na but also multiple compounds.Interestingly,the superconductivity is predicted in alkali metal compounds Li-S system,T_c has given rise to 80 K in highly compressed Li₃S.K₂S has been extensively investigated by theoretical and experimental methods since it was first synthesized by Zintl et al.It is reported that K₂S can crystallize in the antifluorite structure（Fm-3m）,at ambient pressure,then transforms to an orthorhombic crystal with the space group of Pnma at about 2.7GPa.Theoretical prediction points out that Pnma crystal can transform to a distorted P6₃/mmc at 4.4 GPa.Nonetheless,the Pnma phase is not observed by experiment to date.In this work,four stable phases of P6₃/mmc,Cmcm,P6/mmm and P-3m1 have been found at different pressures through ab initio ELocR code.The low-pressure phase of Pmma proposed by an experimental analysis is found unstable by our theoretical calculations.Based on the reported X-ray diffraction and our theoretical analysis,we update the low-pressure phase with a hexagonal crystal of P6₃/mmc.Our calculations reveal the insulating character of the two phases of P6₃/mmc and Cmcm,and the Raman spectra are subsequently analyzed for the experimental convenience.Electronic band structure and the partial density of states show the metallic properties of P6/mmm and P-3m1 at pressures.As shown in the chart of electronic localization functions（ELF）,the delocalization of electrons around S atoms in the phases of P6/mmm and P-3m1denotes the improvement of conductivity of sulfur in Li₂S battery under pressure.In this work,the various stoichiometries structures of K_xS（x=1-4）system are widely investigated up to 100 GPa.We found six thermodynamically stable structures with the stoichiometries of KS,K₃S,and K₄S under these conditions and proposed phase diagram on the K-rich side firstly.KS the totally counterintuitive chemical regimes with extraordinary bonding and electronic properties is predicted stable at atmospheric pressure in K_xS compounds.The mechanism of superconductivity in Pm-3m of K₃S is deeply investigated comparing with the reported Pm-3m Li₃S.The low T_c is attributed to weak interaction of electron-phonon coupling in K₃S compound instead of interstitial charge localization in reported Li₃S compound.In addition,we have performed structural searches on Na-S compounds for various Na_mS_n（m=1-4 and n=1,or n=1 and m=1-4）compositions at 0 K and selected pressures of 0,50,and 100 GPa.Besides,we predicted the stabilization of several new compounds NaS₄,NaS₃,Na₃S,and Na₄S that were not previously reported,and mainly explored the structure and properties of Na₂S and Na₃S.