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Magnetic Resonance Characterization of Structure and Dynamics in Rare Earth Phosphate Materials

  • Author(s): Stettler, Joel Michael
  • Advisor(s): Reimer, Jeffrey A
  • et al.

This thesis develops magnetic resonance techniques to characterize the structure and proton dynamics of rare earth phosphates. Structural characterization is accomplished primarily through the use of 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR). Spectroscopic characterization is used to develop chemical shift references on calcium and lanthanum phosphate materials in order to better understand the phases and phosphate environments in a calcium lanthanum phosphate glass-ceramic material. Similar methods are used to characterize cerium orthopohsphate, a material that has often been called "NMR invisible" in the literature, and the phosphate structures that are formed when two differing synthetic approaches are performed.

Structural characterization is enhanced through the use of double resonance techniques, such as 1H -31P cross polarization (CP) MAS NMR and heteronuclear correlation (HETCOR) spectroscopy. Such techniques are a powerful way to associate protons present in the material with phosphate hosting sites in the material. In a strontium cerium metaphosphate glass-ceramic, 31P MAS NMR spectroscopic, relaxation, and variable temperature techniques are used to characterize and identify the structure of this partially crystalline material, while 1H -31P cross polarization (CP) MAS NMR allows the preferred proton hosting site in the material to be identified.

In order to measure proton dynamics in the rare earth phosphates, field gradient NMR

techniques are adapted to the particular challenges of rare earth phosphates. Relatively low proton conductivities in the materials being studied suggest that proton motion in these materials will be slow. The combination of slow proton motion, the known short 1H relaxation times of rare earth phosphates, and the required high temperatures at which proton diffusion takes place rule out standard field gradient techniques that have been developed for liquids. Small scale pulsed field gradient NMR probe designs and stray field steady gradient NMR have been investigated, and methods by which one can make such measurements more straightforward are discussed. It is demonstrated that high temperature stray field steady gradient NMR methods can measure proton self diffusion in a relatively proton-rich barium lanthanum metaphosphate glass.

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