Study On The Alkaline-earth Metal/transition-metal Chromium Sulfides Under High Pressure

High pressure can significantly shorten the distance between atoms,alter the arrangement of atoms,and tune the bonding patterns,consequently inducing the phase transition and emergence of intriguing physical-chemical properties.On the other hand,high pressure can also stabilize the compounds with unconventional stoichiometries and make the constituent elements exhibit the exotic configuration units,oxidation states and properties that are not available under ambient conditions,broadening the classical theories.Thus it can be seen that the high pressure plays an important role in promoting scientific development and discovering new materials.Sulfur has various valence states and configurations in which the Sn units with negative valence states constituted with muti-S atoms attract the most attention.Metal elements usually have strong metallicity properties and their valence electrons can easily transfer to the S atoms in the process of forming sulfur-bearing compounds,which influence the bonding properties,structural configuration and electrical properties significantly.From the view of investigating the effect of the metal with different valence electrons on the properties of Sn and the role the Sn plays in the compounds,we systemically study the structure and properties of the metal sulfide using the CALYPSO method combined with first-principles calculations and high pressure experiment technique.The following results have been achieved:1.As an alkaline earth metal,calcium,having a strong metallic property will 1:1reactive with sulfur,forming a stable,shell-closed compound with a direct band gap.To explore the Ca-S compounds with unconventional stoichiometry,we systemically performed the structural prediction under the pressure of 0,10,30 and 50 GPa via the CALYPSO method,discovering the Ca-rich compound: Ca3S2,Ca4S3 and Ca2 S,and Srich compound Ca S2 and Ca S3.For Ca S2 and Ca S3 containing the S22-and S32-units respectively,we performed the high pressure high temperature experiments using the DAC combined with laser-heating technique to synthesis the two compounds,and the XRD is used to characterize the predicted Ca S2 and Ca S3 compound,clarifying the structural identity between the synthesised and predicted phases.We also simulated the electrical properties and constructed the high pressure high temperature phase diagram,which is consistent with the experimental result.Our works have elaborated the abundant calcium polysulfides and the knowledge of the Sn unit under high pressure.2.Besides the Ca element,we also systemically investigated the novel structures and properties of three other alkaline-earth metal sulfides.Two Ba S3 structures containing S32-units with degenerated enthalpies were proposed and the role the S32-plays in affecting the structure and electrical properties were thrown light on by the electronic structure simulation.Moreover,we studied the stabilities of the Ba S3 and Ba S2 compounds using the ab initio molecular dynamics.For magnesium sulfide,we discovered a metallic Mg4 S compound in which the electrons confined in the interstitial space are extension in the x-y plane two dimensionally.For Sr-S system,the SrS₃ isostructed with Ba S3 was found in the prediction.Our findings expand the understanding of the effect of the Sn₂-units existed in sulfur-bearing compounds and extend the family of the alkaline-earth metal sulfide.3.Compared with the alkaline-earth metal,the transition metal with electrons from d-orbital could provide more electrons to modify the Sn unit.Chromium,the element in the same group with molybdenum,was chosen in the structure searching to discover the novel structure of its sulfide under high pressure.Two new phases of Cr S2 and Cr S3 were found,respectively.Cr S2 is isostruct with the marcasiten at the pressure range from 21 to 59 GPa,and Cr S3 becomes energetically preferable when the pressure is above 37 GPa.In these two S-rich compounds,no Sn units appear and the Cr S6 and novel Cr S8 unites with Cr atoms are preferable,respectively.These findings provide promising knowledge for the studying of the Cr-S system under high pressure theoretically and experimentally.