Theoretical Study On Pure Hydrogen And Transition Metal Hydrides Under High Pressure

Hydrogen is the lightest and simplest element,and it exhibits amazing properties under extreme pressure and temperature conditions.Hydrogen is composed of an electron and a proton.Due to the lighter mass of the proton in hydrogen,hydrogen has a strong quantum property.Therefore,strict quantum mechanics is needed to study hydrogen and hydrogen-containing systems to make accurate predictions.It is necessary to understand the effect of proton motion on the energy of the system when calculating the properties of hydrogen-containing substances.In 1930 s,Wigner and Huntington predicted that hydrogen might turn into metallic hydrogen at 25 GPa.Since then,the preparation of metallic hydrogen has become a huge challenge in condensed matter physics,attracting experimental and theoretical researchers for more than 80 years.Over the past few decades,numerous experimental and theoretical predictions have greatly supplemented the phase diagram of hydrogen.Recently,Dias and Silvera published their results in Science,claming that they observed atomic metallic hydrogen at 495 GPa.However,it need to be further verified.In addition,based on the chemical pre-compression concept proposed by Ashcroft,researchers attempt to study hydrogen-rich compounds under high pressure in the hope of finding high-temperature superconductors.In this work,we used the Monte Carlo method to study the properties of hydrogen molecular ions and hydrogen molecular systems under confinement conditions,and employed density functional theory theoretically predicted the crystal structure,electronic properties and superconducting properties of the Cu-H and Pa-H system under high pressure,and obtained the following innovative results:Firstly,considering the zero point motion of protons,the energy of ground state,pressure and equilibrium bond length of hydrogen molecular ions and hydrogen molecular systems under restricted conditions are all larger than the previous calculation results.We calculate the effect of the motion of the proton on that of the electron in detail,and find that the kinetic energy of the electron increases by about 5% considering the motion of the proton.According to the change of the ratio of proton kinetic energy and electron kinetic energy with pressure,the growth rate of electron kinetic energy is faster than that of proton kinetic energy at the high pressure.On the whole,the kinetic energy of protons is only a few thousandths of the kinetic energy of electrons,so it can be ignored when the calculation accuracy is not high.For hydrogen molecular ion system,the curve of the ratio of proton kinetic energy and electron kinetic energy and the pressure is approximately a straight line,so no phase transition occurred.In the hydrogen molecular system,the curve of kinetic energy ratio and pressure has three turning points: 65,220 and 440 GPa,which should correspond to the phase transition caused by the change of the distance between protons.Secondly,we systematically study the crystal structure,electronic properties,kinetics and superconductivity of Cu2 H and Cu Hn(n=1-3)at high pressure.The synthetic pressure of Cu2 H is theoretically predicted to be 16.8 GPa,which is in good agreement with the 18.6 GPa measured in the experiment.For Cu H,the synthetic pressure is theoretically predicted to be 30 GPa,but it is not thermodynamicly stable until 120 GPa.In the range of pressure studied,Cu H2 is thermodynamically unstable.Cu H3 is thermodynamically stable at 160 GPa,but it is dynamically unstable.Cu H is an insulator,and Cu2 H is a metal with an estimated superconducting transition temperature of 0.028 K at 40 GPa.Finally,we systematically studied the crystal structure,electronic properties,kinetics and superconductivity of Pa-H systems under high pressure for the first time.Stable stoichiometries are abundant in the range of pressure studied,including Pm-3m-Pa H,Pm-3n-Pa H3,P2/c-Pa H3,Fmmm-Pa H4,F-43m-Pa H5,Fm-3m-Pa H8,and F-43m-Pa H9.Pm-3n-Pa H3 is stable at atmospheric pressure,which is consistent with the known result.Both Fm-3m-Pa H8 and F-43m-Pa H9 have H8 cage,and the H8 cage in Fm-3m-Pa H8 is cubic.However,the H8 cage of the F-43m-Pa H9 is twisted,and the two H8 cages are connected by an H atom.By calculating the density of state,it is found that except that F-43m-Pa H5 is a semiconductor,the other stable stoichiometries are all metals.Finally,the superconducting properties of Fm-3m-Pa H8 and F-43m-Pa H9 are calculated.The superconducting transition temperature of Fm-3m-Pa H8 at 10 GPa is 71.4 – 79.4 K,and the superconducting transition temperature of F-43m-Pa H9 at 80 GPa is 27.8–36.1 K.This work provides a new perspective for the structure and properties of Pa-H systems and contributes to the study of the properties of other actinide hydrides.