Theoretical Study On The Crystal Structure And Properties Of MCI(M=Br,I) Under High Pressure

As a kind of chemically active substance,halogen compounds are still researched in experiments and theoretical calculations under ambient pressure.The crystal structure and electronic properties of halogen compounds under high pressure are rarely studied.This article is dedicated to exploring the ambient and high pressure structures of bromine chloride and iodine chloride MCl（M=Br,I）,and use software packages such as VASP（Vienna Ab-initio Simulation Package）to explore its electronic properties and lattice dynamics.In theoretical calculations and analysis,we used CALYPSO（Crystal structure Ana LYsis by Particle Swarm Optimization）structure prediction method combined with First-Principles calculations,and successfully found the new high pressure phase of BrCl and ICl crystals.The study of the high pressure crystal structure and related physical properties of MCl（M=Br,I）in this paper will help to deepen the understanding of binary interhalogen compounds and lay the foundation for further theoretical and experimental research.The structure and phase transition of BrCl have been studied in depth in theory.The geometric optimization results show that BrCl has undergone the following crystal structure phase transition:Cmcm→C2/m→Fddd→P4/mmm.These phase transitions occurred at 75 GPa,94 GPa and 140 GPa,respectively.At the same time,the calculation of the equation of state shows that these phase transitions are first-order phase transitions.According to the calculation result that there is no virtual phonon mode in the entire Brillouin zone,we can say that all four phases are dynamically stable.According to the formula that the elastic constant satisfies the criterion of mechanical stability,it is proved that these four phases are also mechanically stable.According to the structure diagrams of Cmcm,C2/m,Fddd and P4/mmm,we find that the number of bonds between Br and Cl increases as the pressure increases.Within the pressure range we studied,BrCl finally transformed into a high pressure P4/mmm phase composed of eight-coordinated Br atoms.which was predicted for the first time in the BrCl compound.Subsequently,in the function relationship between the bandgap value of the Cmcm phase and the pressure,it was found that the bandgap closure of the Cmcm phase at about 50 GPa resulted in the occurrence of pressure-induced metallization.The phase transition sequence of ICl predicted by the above method is:P2₁/c→Imma→P4/mmm.These phase transitions occur at 14 GPa and 46 GPa,respectively.The discontinuity of the volume change indicates that the entire phase change process is a first-order phase change.Through the calculation of phonon spectrum and elastic constant,it is proved that all three phases are mechanically and dynamically stable.As the pressure increases,the number of bonds between I and adjacent Cl changes from six（Imma）to eight（P4/mmm）.In addition,calculations of the electronic energy bands of the P2₁/c,Imma,and P4/mmm phases at 10 GPa,30 GPa,and 100 GPa pressures show that the P2₁/c phase is a semiconductor,and the Imma and P4/mmm phases are metals.In order to find the pressure point of semiconductor→metal,we fitted the relationship between the band gap value of the Imma phase and the pressure,and finally found that the bandgap closure occurs when the pressure is 28 GPa.Finally,the pressure range in which the stable structure we predict is within the reachable pressure range of today’s high pressure experimental science,which is expected to provide a theoretical basis for the synthesis of halogen compounds and the exploration of electronic properties in subsequent related experiments.