First-Principles Study Of Pressure-induced Phase Transition For Heavy Metals And Their Oxides

Phase transition upon high-pressure is one of the most focused research topics in material physics, and plays an important role in exploring the structural and electronic properties of real materials. The knowledge of the structure at high pressure is very important in the fields of chemistry, bioscience, and geosciences. Under hydrostatic pressure, the structural phase transition, isostructural phase transition, and transition path of some typical heavy metals (mainly the actinides) and their oxides are studied by first-principles density-functional theory (DFT). Pressure-induced electron transfer, pressure-induced changes of conductivity and magnetism are systematically investigated. Specifically, the main results consist of three parts:(Ⅰ) the high-pressure phase transition features and ground-state electronic, magnetic, mechanical, and thermodynamic properties of actinide dioxides (AO2,;A=Th, U, Np, or Pu) are presented in chapters 3 and 4; (Ⅱ) the phase transition, elasticity, superconductivity, and electron transfer under pressure for Group IV transition metal Zr and TiZr alloy are reported in chapter 5; (Ⅲ) the high-pressure phase transition, strongly correlated localization, equation of state, and magnetic structure for transition-metal oxide PbCrO3 are discussed in last chapter.Calculations of actinide dioxides (AO2, A=Th, U, Np, or Pu) and two kinds of thorium hydrides (ThH2 and Th4H15) illustrate that the 5f electrons in U, Np, and Pu have significant effects on their compounds'electronic structure, and need use the local-density approximation (LDA)+U approach, while for thorium hydrides without 5f electrons, GGA method can give proper results. The LDA+U approach with U=4 eV can prominently improve upon the conventional DFT and, thus, can provide a satisfactory qualitative electronic-structure description comparable with experiments for oxides of U, Np, or Pu. Based on the electronic-structure analysis, we demonstrate that the Th-O, U-O, Np-O, and Pu-O bonds can be interpreted as displaying a mixed ionic/covalent character. Pu-O, U-O, and Np-O bonds have stronger covalency than the Th-O bond. The ionicity of Th-O bond is the largest. Results of elastic constants and phonon dispersions indicate that the ambient phase of AO2 is stable. For ThO2 and PuO2, the Fm3m→Pnma transition occurs at 26.5 and 24.3 GPa, respectively. Isostrctural transitions in the pressure ranges of 80-130 GPa and 75-133 GPa are predicted for these two kinds of actinide dioxides, respectively. The Pnma phase of PuO2 will occur a metallic transition around 133 GPa. Through first-principles computational tensile test on PuO2, we obtain the mechanical features along different crystalline orientations. The theoretical tensile strengths are firstly calculated. We find that the strong ionic/covalent character of Pu-O bond is weakened by tensile stain and PuO2 will exhibit an insulator-to-metal transition. Results of elastic constants and phonon dispersions indicate that the body-centered tetragonal phase of ThH2 is stable, while the fluorite structure of ThH2 is mechanically and dynamically unstable.Using generalized gradient approximation (GGA) method, we study the high-pressure phase transition properties of Group IV transition metal Zr and the equiatomic TiZr alloy. Firstly, we calculate the fundamental parameters for Zr and TiZr alloy inαandωphases. Results show that the calculated structural parameters, elastic constants, elastic moduli, Poisson's ratio, ultrasonic velocities, Debye temperature, and equation of state coincide well with experiments. Theα→ωandω->βphase transition pressures for Zr (TiZr) at T=0 K are calculated to be -3.7 GPa (-13.8 GPa) and 32.4 GPa (33.9 GPa), respectively. Mechanical stability of a andωphases are predicted and theβphase is mechanically unstable at zero pressure. Theβphase of Zr and TiZr alloy will become mechanically stable under pressure up to 3.13 and 2.19 GPa, respectively. At low pressure inβphase, metal Zr can satisfy the low elastic modulus condition. Dynamic stability test illustrates thatβZr is unstable till being compressed over 25 GPa. For both metal Zr and TiZr alloy, the s-d electron transfer upon compression plays an important role in the pressure-induced enhancements of the mechanical properties. Based on the phonon dispersion soft modes ofβZr along [111] and [110] directions, we show the paths ofβ→ωandβ→αtransitions. Superconductivity of Zr is obtained by electron-phonon coupling calculations and our calculated Tc accords well with experiments. The main contribution to the pressure-dependent behavior of the superconductivity comes from the d orbital. The large Tc at around 30 GPa forβZr is mainly due to the TA1 soft mode. Under pressure, the increase or decrease of Tc for Zr in all three phases has tight relation with the corresponding behavior of the electron-phonon coupling constantλ.Using LDA+U and GGA+U formalisms, we investigate the high-pressure phase transition of transition-metal oxides PbCrO3. Only by choosing the Hubbard U parameter around 4 eV within the LDA+U approach, FM and/or AFM ground states can be achieved. This fact explicitly indicates that strongly correlated localization exists in this material. While the PbCrO3-CrPbO3 in R3 phase is consistent with the experimental low-pressure PhaseⅠ, both Pm3m and R3c phases of PbCrO3 coincide well with high-pressure PhaseⅡ. We predict that the R3 phase PbCrO3-CrPbO3 transforms to Pm3m phase PbCrO3 at 1.5 GPa, while R3 phase PbCrO3-CrPbO3 transits to R3c phase PbCrO3 at -6.7 GPa. Our electronic spectrums illustrate a clear hybridization of Cr 3d and O 2p orbitals in wide energy range. The abnormally large volume and compressibility of PhaseⅠis due to the contain of CrPbO3 in the experimental sample and the transition of PbO6/2 octahedron to CrO6/2 upon compression.