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Probing pressure-induced structural dynamics in xenotime rare earth orthophosphates via in situ diffraction and spectroscopy

Sharma, Jai
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Advisor
Packard, Corinne E.
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Date
Date Issued
2023
Date Submitted
Keywords
ceramics
 diamond anvil cell
 high pressure
 photoluminescence spectroscopy
 rare earth orthophosphates
 x-ray diffraction
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2023 - Mines Theses & Dissertations
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Sharma_mines_0052E_12779.pdf
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Sharma_mines_0052E_316/ACS-Runowski.pdf
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Sharma_mines_0052E_316/J-ACerS-Hay.pdf
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Sharma_mines_0052E_316/J-ACerS-Musselman.pdf
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Sharma_mines_0052E_316/J-SSC-Heuser.pdf
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https://hdl.handle.net/11124/179131
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Abstract
Rare earth orthophosphate (REPO4) ceramics have attracted decades-long interest in research fields ranging from geoscience to structural composites to photonics. While these fields have historically been largely separate, their growing convergence brings added relevance to REPO4 phase transformations, the influence of stress state on transformation, and transformation detection methods. This dissertation employs in situ diamond anvil cell (DAC) synchrotron x-ray diffraction (XRD) to shed light on the activation conditions of the xenotime-monazite transformation in DyPO4 and TbPO4. First, the transformation onset pressure (Ponset) of DyPO4 (measured under hydrostatic conditions) shows Raman spectroscopy-based Ponset values are significant over-estimations, and REPO4 Ponset does not decrease linearly with RE ionic radius. Experiments also reveal the shear-sensitivity of this transformation as shear reduces Ponset significantly in TbPO4 and DyPO4 and widens the xenotime-monazite phase coexistence range in TbPO4. In addition, XRD indicates a high-pressure, post-monazite phase (likely of the scheelite structure) exists across a wider range of xenotime REPO4s than was previously known. These findings show REPO4 transformation can offer enhanced plasticity and toughening in ceramic matrix composites at lower pressures and over wider pressure ranges than expected. Long phase coexistence pressure ranges across all XRD experiments also point toward this transformation being diffusional rather than martensitic. This dissertation also shows direct-excitation photoluminescence (PL) spectroscopy can be utilized to detect the xenotime-monazite phase transformation. PL experiments yield TbPO4 transformation onset and end pressures consistent with synchrotron XRD results. In addition, PL spectra of recovered TbPO4 samples can offer insight into stress history, including history of transformation.
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