First-principles Investigations On The Structures And Properties Of Materials Under High Pressure

Pressure as an effective tool has been used in many fields,such as physics,chemistry,biology,and geography.It also plays an important role in the evolution of planet.Due to the effects of pressure,the lattice structuresand electrons distribution change in matters.Meanwhile,many properties such as optical,elastic and chemical properties also change a lot.Some novel phenomena appear under pressure,which attract wide interests.On the other hand,phase transition,superconductivity of hydrogen-rich materials,magnetic transition,and metal-insulator transition are the hottest issues in high-pressure science.In this thesis,we focus on the pressure effects on spin-state crossover,superconductivity,and electronic structures.Besides,we also applied machine learning method to construct C potential for under high pressure studies.The main contents are as follows:1.The iron spin-state crossover pressure in Fe-bearing MgO.We investigated the spin-state crossover pressure of different iron concentration in Fe-bearing MgO.The differencies are obvious when using different functionals for spin-state crossover pressure calculations.For example,the spin-state crossover pressure of(Mg0.96875Fe0.03125)O is 22 GPa and 68 GPa for PBE and PBE +U,respectively.Meanwhile,the intermediate spin state is the ground state of Fe-bearing MgO at some iron concentrations in the framework of PBE,which contravenes the experimental measurements.We simulated the spin-state crossover pressures of(Mg1-xFex)O by using the hydrid functional with a uniform parameter.Our results indicate that the spin-state crossover pressure increases with increasing iron concentration generally.For example,the spin-state crossover pressure of(Mg0.03125Fe0.96875)O and FeO is 56 GPa and 127 GPa,respectively.The calculated crossover pressures agree well with the experimental observations.Therefore,the hybrid functional is an effective method for describing the pressure-induced spin-state crossover behaviors in transition metal oxides.2.Superconductivity of boron-doped graphene under high pressure.Based on first-principles calculations,we investigated the properties of B-doped graphane under high pressure up to 380 GPa.We found that B-doped graphene undergoes a phase transition from phase-? to phase-? at 6 GPa.Different from pristine graphene,phase-y of B-doped graphene is kinetically unstable.Phase-a and phase-? keep metallic in the pressure range we studied.Based on the BCS theory,we calculated the superconductivities of B-doped graphene under high pressure.The obtained superconducting transition temperature of B-doped graphene at ambient pressure is 45 K.The pressurization can increase the transition temperature notably,e.g.,77 K at 100 GPa.Both the electronic density of states at Fermi level and the electron-phonon coupling are mainly contributed by B-C characteristic,indicating that the B-doping plays a key role in the superconductivity.The differences between uniformed hole-doping and chemical dopant in B-doped graphene also were analyzed.The T,of chemical doping model we simulated at ambient pressure is 45 K,much lower than that(96 K)of uniformed hole-doping model.We have clarified that in the ideal uniformed electronic doping model by removing electrons from the investigated system,due to the nesting of the Fermi surface,large medium frequency phonon softening appears at the Gamma point,which significantly enhances the electron-phonon coupling of hole-doping graphane.3.Compressibility and photoluminescence of CeOCl.Our experimental parteners found that under high pressure CeOCl will undergo an isostructural phase transformation around 6 GPa in high-pressure mesurements.We therefore did the first-principles simulations to explore the mechanism of these phenomena.Our calculations show that the lattice constants and volume will exhibit a discontinuity under the pressure from 3 GPa to 6 GPa,which agrees well with experimental meaurements.The mechanism is that the transition of 4f electron in Ce3+induces the isostructural transformation proved by our Bader charge analysis.The band structures show that CeOCl is an indirect(direct)band gap semiconductor for the spin-up(spin-down)branch,which suggests that the CeOCl can be expected to be a semiconductor material.In addition,we have done least-squares fitting of a BM-EOS of our calculated data,which reveals that the bulk modulus of CeOCl is 47.65 GPa,which is consistent with that(52.88 GPa)of experiment revealed.4.Construction of C atomic potential by using machine learning artificial neural network methods.Machine learning interpolation of atomic potential energy surfaces enables the nearly automatic construction of highly accurate interaction potentials.Generally,these atomic potentials are more effective than first-principles methods,and more accurate than empirical potential molecular dynamic(MD)simulations.We use the free and open-sourceatomic energy network(aenet)package combining with MD method to investigate the evolution of materials under high pressure.Using first-principles methods we calculated the energies and forces of 15855 structures of carbon in different dense.After testing atomic fingerprints and training methods,we obtained an artificial neural network atomic potential.This atomic potential works well in calculating lattice parameters and energies of carbon,which indicates that this atomic potential is good enough for MD simulations under high pressure.