First-principles Study Of Mechanical And Thermodynamic Properties Of Thorium Non-metal Compounds

Thorium compounds have been paid much attention in recent years because they are considered as potential nuclear fuels for Generation-IV reactors.A clear understanding of the physical and chemical properties of thorium compounds that are present at various stages of the fuel cycle is crucial to develop and exploit thorium fuel.In this thesis,the physical properties of ThBC and Th-P system were investigated by first-principles calculation based on the density functional theory.For the ThBC system,we focused on its mechanical and thermodynamic properties.For the Th-P system,we systematically studied its structural phase transition and physical properties under high pressure.The mechanical and thermodynamic properties of ThBC have been examined by first-principles calculations based on density functional theory.The equilibrium geometry,elastic stiffness constants,mechanical moduli,Poisson's ratio,elastic wave velocities,and Debye temperature of ThBC were systematically investigated.It is shown that ThBC exhibits ductile characteristics.The directional Young's modulus E and bulk modulus B and several mechanical anisotropic factors show the mechanical anisotropic properties of ThBC.Under tensile strain,ThBC is easy to cleave along the[111]direction with an ideal strength of 5.7 GPa.The fracture in ThBC under shear strain occurs along the[1-10]direction of the?111?plane.Phonon calculation reveals the thermodynamic stability of ThBC,which is consistent with the experimental observation.Helmholtz free energy,entropy,and heat capacity were obtained under the quasiharmonic approximation.We studied the Th-P system under pressure and established the phase diagram using the structural search combing with the first-principles calculation.Except for the experimental compounds ThP?Th₃P₄ and Th₁₂P,we have found ten new thorium phosphides in different given pressures,including seven phosphorus-rich compounds?ThP₉?ThP₅?Th₂P₇?ThP₃?ThP₂?Th₃P₅ and Th₂P₃?and three thorium-rich compounds(Th₃P₂?Th₉P₂ and Th₁₂P).It is shown that ThP transforms from Fm-3m phase into Pm-3m phase at about 22.43 GPa.The crystal structures of phosphorus-rich compounds present various forms of the P bonding.The complexities degree of these structures well correlate with the contents of P.At 25 GPa,we find that the new phase of Th₄P₃and the experimental phase of Th₃P₄ are mutual structures.However,Th contents decide the complexity of polyhedrons of thorium-rich compounds.In summary,the theoretical researches of ThBC and Th-P system established a solid theory foundation to our subsequent works in the physical properties of Th-P system and scientific study of other thorium non-metal compounds.