Theoretical Study On Crystal Structure And Properties Of Carbon Group Elements(Ge,Sn) Under High Pressure

High pressure can change the atomic spacing in the material,redistribute the charge,change the crystal structure to form a new high-pressure phase,and some physical and chemical properties will also change,providing a new and effective way to study the structure and properties of materials.Carbon group elements have become one of the hotspots of current research because of their various allotropes with excellent properties.This paper takes the carbon group elements Ge and Sn as the research objects,and explores the crystal structures and properties of the two elements under high pressure.Firstly,the ground state or metastable structures of Ge and Sn were predicted by the crystal structure prediction software package CALYPSO（Crystal structure Ana LYsis by Particle Swarm Optimization）;secondly,the predicted new structures were optimized and the properties were calculated by VASP software.For Ge,we predict two new high-pressure metastable phases（Fmmm（I）and Cmcm）around 75 GPa and two new high-pressure metastable phases（Fmmm（II）and Fmm2）around 130 GPa.The four new high pressure phases are all metallic phases.Combining the crystal structure diagram and the phonon dispersion curve,we speculate that the predicted phases Cmcm and Fmmm（II）may be incommensurate modulation structures.For Sn,a new Sn structure—P2₁/m was discovered for the first time.The energy of the P2₁/m structure is close to that of the I4/mmm structure.According to the calculation results of the energy band and the density of states,it can be seen that the structure is a metallic phase.The ELF map shows that there is weak hybridization between adjacent atoms of the P2₁/m structure.The results of elastic constants and phonon dispersion curves indicate that the P2₁/m structure is mechanically and dynamically stable.The research in this paper has important guiding significance for exploring other structures and properties of Ge and Sn and other metastable structures of other carbon elements（such as C and Si）under high pressure conditions.