First-Principles Study On The Binary Systems Of Thorium-Hydrogen And Thorium-Arsenic At High Temperature And Pressure

Thorium (Th) and thorium based materials as promising candidates for advanced nuclear reactors have attracted much attention with a high level of safety. Thorium has better thermal properties than uranium. The high pressure behaviors of Th4H15 and ThH2 are investigated. In addition, ThAs is calculated under the high pressure of crystal structure, electron structure and phonon spectrum and thermodynamic properties, providing theoretical data for the design and application of Th-based alloy in the nuclear industry. Main conclusions are as follows:1. From the energy-volume relation curves, the bct phase ThH2 is more stable than fcc phase at ambient condition. At high pressure, the bct ThH2 and bcc Th4H15 phases are more brittle than that at ambient pressure from the calculated elastic constants and the Poisson's ratios. The thermodynamic stability of the bdt phase ThH2 is determined from the calculated phonon dispersion relations. In the pressure domain of interest, the phonon dispersions of bcc Th4H15 and bct ThH2 are positive, indicating the dynamical stability of these two phases, while the fcc ThH2 is unstable. The thermodynamic properties are predicted for these stable phases. The vibrational free energy decreases with the increase monotonously of temperature clearly, while the entropy is proportional to temperature and heat capacity is in inverse proportional to pressure respectively. As pressure increases, the resistance to external pressure for Th4H15 and ThH2 is strengthened.2. In this work, the phase transition behavior, the mechanical properties and thermodynamic properties of ThAs at high pressures have been investigated. These properties are derived from the lattice dynamics in the quasi-harmonic approximation. The calculated equilibrium lattice parameters and bulk modulus agree well with experimental results. Structural phase transition from NaCl type B1 ThAs to CsCl type B2 ThAs is found at pressure of 17.1 GPa, in excellent agreement with experimental results. The mechanical properties of B2 structure in ThAs at high pressures has been investigated due to instability of B1 structure. With the pressure increasing, the elastic constants and elastic modulus increase gradually, while B/G and Possion's ratio decrease. It's shown that ThAs has more rigid under high pressure and the brittleness of ThAs in CsCl structure is significant. We further study the ThAs stability of the structure. The phonon spectrums of ThAs at different pressures presented in this article, to the best of our knowledge, have not been studied. ThAs is found to be stable in CsCl structure for pressures below 50 GPa. Finally, we discuss the thermodynamic properties of ThAs in CsCl structure at high pressures.