Type
Text
Type
Dissertation
Advisor
Weidner, Donald | Phillips, Brian | Oganov, Artem R. | Rasbury, Troy | Allen, Phillips.
Date
2015-05-01
Keywords
Geophysics | evolutionary algorithm, hydrogen hydrate, hydronitrogens, Phase transition, Structure Prediction, USPEX
Department
Department of Geosciences.
Language
en_US
Source
This work is sponsored by the Stony Brook University Graduate School in compliance with the requirements for completion of degree.
Identifier
http://hdl.handle.net/11401/77649
Publisher
The Graduate School, Stony Brook University: Stony Brook, NY.
Format
application/pdf
Abstract
A physics-based understanding of the behavior of materials under conditions of high-pressure and high-temperature is full of excitement and challenges. Here, we mainly cover two parts within the subject of high pressure science with USPEX – crystal structure prediction in materials containing hydrogen, and investigation of mechanisms of structural phase transitions. We applied the USPEX method to study hydrogen hydrate, and hydronitrogen materials. In hydrogen hydrate system, we confirmed that the H2O-H2 system undergoes a series of transformations with pressure, and adopts the known open-network clathrate structures (sII, C0), dense " filled ice" structures (C1, C2) and found two novel hydrogen hydrate phases. For hydronitrogen under pressure, numerous unreported and exotic phases are found at pressure up to 800 GPa, such as N4H, N3H, N2H and NH phases composed of nitrogen backbones, the N9H4 phase containing two-dimensional metallic nitrogen planes and novel N8H, NH2, N3H7, NH4 and NH5 molecular phases. Another surprise is that NH3 becomes thermodynamically unstable above ∼460 GPa. To reveal the phase transition mechanism and pathway for simple and small systems, the variable-cell NEB (VC-NEB) method was developed, which is very efficient for finding the phase transition path within a static mean-field picture. The method has been applied to the various reconstructive solid-solid B4→B1 and B3→B1 phase transitions of GaN, phase transition in MgF2 system and guide for experiments for materials synthesis. Combine with the evolutionary metadynamics (EV-metadynamics) and the transition path sampling (TPS) approach, a general procedure for structure phase transition investigation in USPEX is introduced. Different levels of investigation of the fcc→hcp transformation in argon with EV-metadynamics, VC-NEB and TPS methods are performed, helping us to understand comprehensively and deeply the phase transition pathway and mechanism. | A physics-based understanding of the behavior of materials under conditions of high-pressure and high-temperature is full of excitement and challenges. Here, we mainly cover two parts within the subject of high pressure science with USPEX – crystal structure prediction in materials containing hydrogen, and investigation of mechanisms of structural phase transitions. We applied the USPEX method to study hydrogen hydrate, and hydronitrogen materials. In hydrogen hydrate system, we confirmed that the H2O-H2 system undergoes a series of transformations with pressure, and adopts the known open-network clathrate structures (sII, C0), dense " filled ice" structures (C1, C2) and found two novel hydrogen hydrate phases. For hydronitrogen under pressure, numerous unreported and exotic phases are found at pressure up to 800 GPa, such as N4H, N3H, N2H and NH phases composed of nitrogen backbones, the N9H4 phase containing two-dimensional metallic nitrogen planes and novel N8H, NH2, N3H7, NH4 and NH5 molecular phases. Another surprise is that NH3 becomes thermodynamically unstable above ∼460 GPa. To reveal the phase transition mechanism and pathway for simple and small systems, the variable-cell NEB (VC-NEB) method was developed, which is very efficient for finding the phase transition path within a static mean-field picture. The method has been applied to the various reconstructive solid-solid B4→B1 and B3→B1 phase transitions of GaN, phase transition in MgF2 system and guide for experiments for materials synthesis. Combine with the evolutionary metadynamics (EV-metadynamics) and the transition path sampling (TPS) approach, a general procedure for structure phase transition investigation in USPEX is introduced. Different levels of investigation of the fcc→hcp transformation in argon with EV-metadynamics, VC-NEB and TPS methods are performed, helping us to understand comprehensively and deeply the phase transition pathway and mechanism. | 133 pages
Recommended Citation
Qian, Guangrui, "Materials Structure Prediction and Phase Transition Mechanism Investigation" (2015). Stony Brook Theses and Dissertations Collection, 2006-2020 (closed to submissions). 3442.
https://commons.library.stonybrook.edu/stony-brook-theses-and-dissertations-collection/3442