The Application Of Crystal Structure Prediction In Several High-pressure And Low Dimensional Materials

With the continuous development of computer technology,the ability of crystal structure prediction method based on density functional theory is also improving.The crystal structure prediction method can not only search the crystal structure under ambient conditions and three dimensional crystal structure,but also predict special structure under high pressure and low dimensional crystal structure.In the structure searching,we can judge the stable and meta-stable structure under certain conditions and calculate its physical properties using only the information of chemical ratio.This enables the design of new materials(e.g.superhard materials,optical materials,superconducting materials,topological materials,magnetic materials,etc.)to provide references for experimental work.In this article,we focus on two aspects:One aspect is to use the crystal structure prediction method to explore the high pressure phase diagram(Chapter three),the other aspect is to design new low-dimensional magnetic materials using crystal structure prediction methods(Chapter four and five).In Chapter three,we use the crystal structure prediction method to explore the structural phase of topological semimetals TaAs family under high pressure.Weyl semimetals have been widely concerned because of its unique energy band structure and surface state properties.However,the behavior of the Weyl semimetals under high pressure is still to be explored.We selected the most representative TaAs family to study the structural phase diagram under high pressure.In Chapter four and five,we design one dimensional VI3 single-chain nanowire and two dimensional Fe2SnSe monolayer using crystal structure prediction methods.In recent years,because of the physical significance of low-dimensional magnetic system and the value of potential application,low-dimensional magnetic materials have been widely concerned.By means of crystal structure prediction,we designed two kinds of low-dimensional ferromagnetic materials,hoping to provide potential materials for the experimental work.1.High-pressure phases of Weyl semimetals NbP,NbAs,Ta P,and TaAs.In this study,we used the crystal structure search method and first-principles calculations to systematically explore the high-pressure phase diagrams of the TaAs family(NbP,NbAs,Ta P,and TaAs).Our calculation results show that NbAs and TaAs have similar phase diagrams,the same structural phase transition sequence I41md?P-6m2?P21/c?Pm-3m,and slightly different transition pressures.The phase transition sequence of NbP and Ta P differs somewhat from that of NbAs and TaAs,in which new structures emerge,such as the Cmcm structure in NbP and the Pmmn structure in Ta P.Interestingly,we found that in the electronic structure of the high-pressure phase P-6m2-NbAs,there are coexisting Weyl points and triple degenerate points,similar to those found in high-pressure P-6m2-TaAs.2.Ferromagnetic Semiconducting VI₃ Single-Chain Nanowire.One-dimensional ferromagnetic semiconductor has important research value for fundamental research and nanoscale spintronic applications.In this article,we systematically studied the vanadium-trihalide VX3(X=F,Cl,Br,I)nanowires.Among the VX3 nanowires,we found that a finite-length single-chain VI3 nanowire is a ferromagnetic semiconductor with a blocking temperature of around 64K using Monte Carlo simulations.In addition,we constructed a 2D nanowire array and found that the interchain exchange interactions of VI3 nanowires favor ferromagnetic states.The VI3nanowire array has a Curie temperature of around 113K,which is higher than the liquidus temperature of nitrogen.Moreover,the ferromagnetic VI3 nanowire keeps stable during deformation and doping.The intrinsic semiconductivity found in ferromagnetic VI3 nanowire allows it to have potential applications in nanoscale spintronic.3.Room temperature ferromagnetism in Fe₂SnSe monolayer.Single-layer magnetic materials attracted extensive attention in recent years.Using machine learning accelerated crystal structure search method,we predicted a new two-dimensional Fe2SnSe monolayer.Density functional theory calculation using the GGA+U method shows that Fe2SnSe monolayer has a strong exchange interaction and a strong perpendicular magnetic anisotropy.Using the Monte Carlo simulation based on the classical Heisenberg model,we estimated that the critical temperature of ferromagnetic transition is up to 788K.The dynamic stability and thermodynamic stability of the Fe2SnSe monolayer are confirmed by phonon spectrum and ab initio molecular dynamics simulations at 800K.The high Curie temperature and thermodynamic stability make the Fe2SnSe monolayer a potential material for nanoscale spintronic devices.