First-principles Calculations Of The Crystal Structures Of Thorium-boron-carbon Compounds Under High Pressure

Economic development has accelerated the consumption of non-renewable resources and caused environmental pollution,which requires us to find a new energy source.Nuclear energy is environmentally friendly,clean,and consumes less fuel.It is an ideal new energy source.However,nuclear energy produces radioactive decay products,which requires us to find suitable nuclear fission materials.Among them,thorium-based compounds are considered to be one of the important nuclear fission materials in the fourth-generation reactor,and generally serve under high temperature and high pres sure.Thorium-Boron-Carbon compounds are also one of the possible candidates for nuclear fission materials,however,there are few studies on the physical properties of Thorium-Boron-Carbon compounds at ambient pressure and high pressure.This thesis uses first-principles calculations to study Th2B2C3,ThB2C,Th3B2C3 and ThBC at ambient pressure,and combines particle swarm optimization to calculate the physical properties of ThBC at high pressure.The main contents include:1.At ambient pressure,according to phonon spectra,we find that Th2B2C3,ThB2C,Th3B2C3 and ThBC are all dynamically stable.In addition,the electronic energy bands show that Th2B2C3 is semi-metallic,while ThB2C,Th3B2C3 and ThBC are metallic.We also calculate the elastic constants of the four materials,and the results show that all four materials are brittle materials.Given the anisotropy factor of the materials,the anisotropy is:ThBC>Th3B2C3>ThB2C>Th2B2C3.Besides,we calculate the ideal tensile and shear strengths of the four materials.Under the tensile stress,Th2B2C3,Th3B2C3 and ThBC are most easily broken along the[111]direction,but the tensile strength of ThB2C are along the[101]direction.According to shear stress calculations,ThBC and ThB2C slide the same in all directions in each plane,while Th2B2C3 slides most easily along the[110]direction within the plane perpendicular to the[111]direction,and Th3B2C3 slides most easily along the[110]direction within the plane perpendicular to the[001]direction.Finally,we calculated the Debye temperature:ThB2C>Th2B2C3>ThBC>Th3B2C3.2.At high press,we performed a crystal structure search for ThBC with a range of 0 to 100 GPa.Through thermodynamic stability,we identified three new phases:Cmcm,Cmce and Immm phases.To determine the phase transition order and pressure,we made the enthalpy-pressure curves of ThBC,and we determined the phase transition order and three phase transition pressures of the four phases of ThBC from 0 to 100 GPa:15 GPa 36 GPa 69 GPa P4122(?)Cmcm(?)Cmce(?)Immm.According to the volume-pressure curve,all three phase transitions belong to first-order phase transitions.According to Helmholtz free energy,the material does not undergo new phase transitions with changes in temperature.According to the phonon spectrum,the four phases are all dynamically stable at 0 GPa and the corresponding phase transition interval.Furthermore,based on the calculated elastic constants,we find that all four phases are mechanically stable and all are brittle materials.The anisotropy factor indicates that the P4122 phase has the largest degree of anisotropy and the Immm phase the smallest.We calculated the electronic energy bands of the four phases of ThBC,and the calculated results show that all four phases are metallic at different pressures.The calculation results of Debye temperature and Grüneisen parameter show that with the increase of pressure,the Debye temperature and sound velocity of ThBC become larger,but the Grüneisen parameter becomes smaller.