Dissertation
Phase Transitions, Crystal Structures, Composition Dependence and Isotope Effects of N2-H2 Mixtures Under High Pressure
Washington State University
Doctor of Philosophy (PhD), Washington State University
2022
DOI:
https://doi.org/10.7273/000004997
Abstract
Simple molecular compounds like nitrogen (N2) and hydrogen (H2) have fundamental significance, yet the phase diagram for the binary system of N2-H2 is poorly known within a limited pressure range, 0-20 GPa. In addition, the phase diagram of N2-H2 suffers with other unknowns regarding the effects of H2 on the phase boundaries/phase transitions, and the stability/miscibility of N2 and H2 . Previous studies on the N2-H2 phase diagram have presented evidences for two van der Waal solids, (N2)6(H2)7, and N2(H2)2, coexisting with N2-rich solid S1 and H2-rich solids S2, below 20 GPa. Using Raman spectroscopy and x-ray diffraction we have studied N2-H2 mixtures with seven different compositions, all up to 60 GPa. We have identified four phase regions each with characteristic polymorphs; Region I with a fluid mixture of nitrogen and hydrogen below 10GPa; Region II with δ-N2 like solid and (N2)6(H2)7 below 20 GPa; Region III with -N2-like phase and (N2)6(H2)7 below 50 GPa; Region IV with an amorphous (or disordered mixed) phase, signified by the loss of H2 vibron and the appearance of a broad peak ~2360 cm-1. The structural analysis has been performed for the 9:1, 4:1, 2:1, and 1:4 N2-H2 mixtures to 60GPa. The results signifies several findings: (1) the stability of (N2)6(H2)7 and N2(H2)2 to 60 GPa; (2) accurate determination of equations of state for (N2)6(H2)7 and N2(H2)2 for the first time; (3) the structural origins for S1 and S2 phases in terms of δ, δL and ε-N2-like structures; and (4) the presence of an inclusion compound (N2)12H2 in the 9:1 N2-H2 mixture in a small pressure region, 13-19 GPa. A similar inclusion compound, (N2)12D2, was previously found in the 9:1 N2-D2 mixture between 13-30GPa. Finally, we have found strong isotope effects on the phase behavior and phase diagram. The most notable is the strong composition dependence on pressure-induced Raman shifts in N2-H2 mixtures, while N2-D2 mixtures exhibit a little to no composition dependence. This result clearly indicates substantially stronger electron repulsion between N2 and H2 than between N2 and D2 and, thereby, the presence of stronger internal chemical pressure in N2-H2 mixtures than N2-D2.
Metrics
7 File views/ downloads
48 Record Views
Details
- Title
- Phase Transitions, Crystal Structures, Composition Dependence and Isotope Effects of N2-H2 Mixtures Under High Pressure
- Creators
- Brittany Kay Thiessen
- Contributors
- Choong-Shik Yoo (Advisor)Ursula Mazur (Committee Member)James A Brozik (Committee Member)Qiang Zhang (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- Graduate School
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Publisher
- Washington State University
- Number of pages
- 112
- Identifiers
- 99901019840301842
- Language
- English
- Resource Type
- Dissertation