First-principles Study Of Structural Phase Transitions And Physical Properties Of Ternary Thorium-based Compounds Under High Pressure

Traditional fossil fuels are unable to satisfy the ever-increasing needs of mankind.Nuclear energy,on the other hand,distinguishes itself from other new energy sources due to its high energy density,lack of greenhouse gas emissions,dependability,and excellent dispatchability.However,the utilization of traditional uranium-based nuclear fuel is fraught with numerous challenges and global uranium resources are finite.As a result,it is imperative that we explore new generations of nuclear fuel.Thorium-based nuclear fuel is one of the promising candidates.Due to the harsh conditions within the reactor,it is necessary to examine the changes in crystal structure and physical properties of thorium-based nuclear fuel under extreme circumstances.Theoretical research into the phase transitions and physical properties of thorium-based compounds under high pressure provides a foundation and guidance for the selection of thoriumbased nuclear fuel and subsequent experimental synthesis.This study primarily employs the first-principles calculation methods based on density functional theory and a particle swarm optimization algorithm to investigate the structural phase transitions and physical properties of two ternary thorium-based compounds,Th2N2S and Th2B2C3,under high pressure.The specific contents are as follows:The study of structural phase transitions and physical properties of Th2N2S under high pressure.Through the structural search of Th2N2S,we reproduced the experimental phase P3m1 phase at ambient pressure and proposed two new highpressure phases:the I4/mmm phase and the Cmmm phase.The results of enthalpypressure curves determined the phase transition sequence of Th2N2S:P3m1(?)I4/mmm(?)Cmmm.The results of phonon dispersion curves and elastic constants prove that the ambient-pressure phase and high-pressure phases of Th2N2S under their respective specific pressures are dynamically and mechanically stable.The calculation results of mechanical properties show that the P3m1,14/mmm and Cmmm phases are all ductile materials.Among the three phases,the P3ml phase has the smallest anisotropy degree and the Cmmm phase has the largest anisotropy degree.The calculation results of electronic structure shows that the ambient-pressure phase P3m1 phase is a semiconductor,while the two high-pressure phases I4/mmm and Cmmm phases both show metallic properties.The calculation results of Debye temperature and Grüneisen parameter show that among the three phases,the Cmmm phase has the strongest chemical bond strength and also has the smallest Grüneisen constant.The study of structural phase transitions and physical properties of Th2B2C3 under high pressure.Through the structural search of Th2B2C3,we proposed two new highpressure phases:the P2/m phase and the Cm phase.The results of phonon dispersion curves and elastic constants prove that the ambient-pressure phase and high-pressure phases of Th2B2C3 under their respective specific pressures are dynamically and mechanically stable.The results of energy-volume curves and enthalpy-pressure curves revealed that Th2B2C3 undergoes two phase transitions with increasing pressure.The 17 GPa 47 GPa phase transition sequence:Pnnm(?)P2/m(?)Cm.Based on the elastic constants,we studied the mechanical properties of the three phases of Th2B2C3,and the results showed that the B/G values of all three phases are less than 1.75,indicating that they are all classified as brittle materials.In the discussion on anisotropy,the Pnnm and Cm phases are both close to isotropic,while the P2/m phase has the largest degree of anisotropy among the three phases,tending to be anisotropic.The calculation results of thermodynamic properties shows that as the pressure increases,the Debye temperature and sound velocity of each phase of Th2B2C3 increase,while the Grüneisen parameter decreases.The results of electronic density of states revealed that the Pnnm phase,P2/m phase and Cm phase have metallic properties.