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Dissertation
Thermodynamics and Structure of Lanthanide and Actinide Silicates and Phosphates Under Extreme Conditions
Washington State University
Doctor of Philosophy (PhD), Washington State University
2022
DOI:
https://doi.org/10.7273/000005027
Abstract
Lanthanide and actinide silicates and phosphates possessing the zircon structure were of primary interest to this dissertation because of their diverse chemical and physical properties, including compositional flexibility, low chemical reactivity, and high thermal stability. This lends zircon structure-type materials to be of interest to a wide variety of applications/fields, including ceramic nuclear waste forms, environmental barrier coating, and sourcing of rare earth elements. Lanthanide silicate materials possessing the apatite structure materials were also investigated in detail as they are found to be a common degradation product of zircon structured materials, indicating an energetic relationship. The materials investigated included CeSiO4, A-Ce2Si2O7, Ce4.67(SiO4)3O, USiO4, Ca2Pr8(SiO4)6O2, Ca2Tb8(SiO4)6O2, Ca2Ho8(SiO4)6O2, Ca2Tm8(SiO4)6O2, and Er(x)Yb(1-x)PO4. The methods used were oxide melt drop solution calorimetry, differential scanning calorimetry coupled with thermogravimetric analysis (TG-DSC), X-ray diffraction, and vibrational spectroscopic techniques. This allowed for the thermodynamics and structures of these materials at high temperature (T > 100°C) and pressure (P > 1 GPa) conditions to be assessed.
The major findings of this dissertation have been determining the standard enthalpies of formation, ΔHf (kJ/mol), of CeSiO4 (-1971.9 ± 3.6 kJ/mol), A-Ce2Si2O7 (-3825.1 ± 6.0 kJ/mol), Ce4.67(SiO4)3O (-7391.3 ± 9.5 kJ/mol), Ca2Pr8(SiO4)6O2 (-14739.1 ± 23.7 kJ/mol), Ca2Tb8(SiO4)6O2 (-14834.1± 23.9 kJ/mol), Ca2Ho8(SiO4)6O2 (-14909.7 ± 19.6 kJ/mol), Ca2Tm8(SiO4)6O2 (-14694.3 ± 12.8 kJ/mol). This information was valuable in assessing the energetic landscapes of zircon and apatite structured materials. The enthalpy of mixing, ΔHmix (kJ/mol), and interaction parameter, Wx = 20.3 ± 0.8 kJ/mol, for Er(x)Yb(1-x)PO4 was determined to be weakly endothermic. Such implies that entropy of mixing, ΔSmix (kJ/mol), is the dominating factor which controls the thermodynamic stability of ErPO4-YbPO4. Next, a combination of TG-DSC, X-ray diffraction, and vibrational spectroscopy found that the zircon structured (CeSiO4, USiO4, Er(x)Yb(1-x)PO4) materials can host occluded molecular water in it (001) channel. This occluded molecular water was found to have profound effects on the lattice at high temperatures. Lastly, zircon structured CeSiO4 at high-pressure was found to exhibit the high pressure low-symmetry (HPLS) phase transition before converting to the scheelite structure. This offers one of the first experimental examples of this phase transition occurring outside of ZrSiO4.
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Details
- Title
- Thermodynamics and Structure of Lanthanide and Actinide Silicates and Phosphates Under Extreme Conditions
- Creators
- Andrew Charles Strzelecki
- Contributors
- Xiaofeng Guo (Advisor)John S McCloy (Committee Member)Choong-Shik Yoo (Committee Member)Hongwu Xu (Committee Member)
- Awarding Institution
- Washington State University
- Academic Unit
- Materials Science and Engineering Program
- Theses and Dissertations
- Doctor of Philosophy (PhD), Washington State University
- Publisher
- Washington State University
- Number of pages
- 440
- Identifiers
- 99901019237401842
- Language
- English
- Resource Type
- Dissertation