First-Principles Calculations Of Phase Transitions And Properties Of Thorium-Based Nitrides Under High Pressure

Compared with the traditional uranium,thorium-based material has more abundant reserves,better non-proliferation performance,higher energy density and less nuclear waste.Thorium-based material can replace uranium as nuclear fuel and is a technical solution to solve the challenge of long-term energy supply.Among thoriumbased materials,thorium-based nitrides have become one of the most promising nuclear fuels for the fourth generation nuclear reactor due to its high fissionable material density,high melting point and excellent radiation resistance.ThN,Th₃N₄ and Th₂N₃ are all important candidate fuels for the fourth generation nuclear reactor,among which scientists have conducted in-depth research on ThN,while there are few studies on Th₃N₄ and Th₂N₃.During the service of nuclear fuel,long-term exposure to extreme environment such as high temperature and high pressure may seriously affect the size,crystal system,mechanical properties,thermodynamic stability and electronic structure of materials,which may eventually lead to change of material performance.Therefore,it is of great significance to study the structures and physical properties of nuclear fuel in extreme conditions.In this paper,we used CALYPSO and first-principles calculations to systematically study Th₃N₄ and Th₂N₃ at ambient pressure and high pressure,the main results are as follows:1.By searching the structure of Th₃N₄,we not only found the R(?)m phase(experimental phase)under ambient pressure,but also successfully predicted three new structures under high pressure:the I43d,R(?) and C2/m phases.The phase transition sequence of Th₃N₄ is determined by relation of enthalpy and pressure:(?)(?).The phonon dispersion curves and Born-Huang criteria prove the dynamic and mechanical stability of the four phases of Th₃N₄.By analyzing the mechanical properties,we found that the R(?)m,143d and R(?) phases are ductile materials,while the C2/m phase is brittle material.Among these four phases,the I43d phase shows obvious anisotropic nature.The calculated electronic structures of Th₃N₄ reveal that the phase transition from the I43d phase to the R(?) phase at 71 GPa belongs to a pressure-induced semiconductor-metal phase transition.The R(?) phase exhibits strong electron-phonon coupling,further studies show that this is related to phonon softening in the Brillouin zone.2.For Th₂N₃,the experimental phase P(?)m1 was reproduced by the structure search under ambient pressure,and two new structures which were the P4/nmm and C2/m phases were predicted theoretically under high pressure.Interestingly,we found that the C2/m of our work and the C2/m phase found by Sahoo et al.are not the same structure.The relationship of enthalpy and pressure proves that our result is more reliable and the phase transition sequence is:(?).The phonon dispersion curves and Born-Huang criteria prove the dynamic and mechanical stability of the three phases of Th₂N₃.The mechanical properties of Th₂N₃ show that P(?)m1 and P4/nmm phases are ductile materials,and C2/m phase is a brittle material.In addition,the P(?)m1 phase is closest to the ideal isotropic material,while the C2/m phase shows obvious anisotropy.The calculated electronic structures show that the P(?)m1,P4/nmm and C2/m phases have metallic properties.The new structures which we predicted lay a theoretical foundation for the experimental synthesis and industrial application of Th₃N₄ and Th₂N₃.