Vibrational Spectroscopy of Complex Materials: First-principles Calculations and Inelastic Neutron Scattering Study.

ZHANG, JIAYONG. Vibrational Spectroscopy of Complex Materials: First-principles Calculations and Inelastic Neutron Scattering Study. (Under the direction of Jerzy Bernholc.) The studies on vibrational spectroscopy are playing very important roles in multiple disciplines, and both theoretical frameworks and experimental techniques have been well developed. On the one hand, the progress made in experimental techniques such as neutron scattering with high resolutions can now enable people to obtain more detailed vibrational information in phonon spectra and phonon dispersions. On the other hand, with the advancement of first-principles molecular dynamics (MD) and total energy calculations within density functional theory (DFT), one can now simulate the vibrational properties of molecules and crystals and access the phonon information including phonon modes, phonon density of states, and phonon interactions with other particles. Combination of the high-resolution experimental spectra and the highly accurate theoretical simulation, we can gain deep a understanding of new materials. This work will make use of those techniques to provide a better understanding of vibrational spectroscopy and phonon behaviors in complex materials. In this dissertation, three projects have been addressed: (I) The methodology of large-scale phonon calculation with the real-space multigrid (RMG) method. The RMG package is used as the core kernel for ab initio molecular dynamics (AIMD) and total energy calculations, the phonon properties are investigated based on both lattice dynamics method (finite displacement method, FDM) and AIMD. By comparing with other widely used DFT packages and experimental data from inelastic neutron scattering (INS), we demonstrate that RMG is very accurate in calculating forces at small displacements from equilibrium positions. The calculated phonon band structures and vibrational spectra for a variety of different systems are in very good agreement with simulated results by other plane-waves-based DFT codes, Quantum ESPRESSO, CASTEP, and VASP, as well as inelastic neutron scattering experimental data. (II) The anharmonicity in zirconium hydrides. A detailed study on the anharmonicity of zirconium hydrides/deuterides is performed by exploring the 2D potential energy surface of hydrogen/deuterium atoms. The corresponding 2D single-particle Schrodinger equation is solved to get the phonon eigenfrequencies. We demonstrated that the obtained results can well describe the experimental INS spectra, typically the strong anharmonicity at high energies. (III) The high pressure induced phase change in alkali hydrides. We have measured the INS spectra of NaH at pressures 1 and 2 GPa for the first time and simulated INS from the first-principles calculations with either local density approximation (LDA) or the generalized gradient approximation (GGA). Thermodynamic properties such as lattice parameters, bulk modulus, and their derivatives are calculated using full lattice dynamics theory within the quasi-harmonic approximation (QHA) for all alkali hydrides. Anharmonic effects are investigated from the molecular dynamics trajectories and are found to be negligible at a given temperature and pressure. © Copyright 2020 by Jiayong Zhang