Understanding the crystal chemistry of actinides in nuclear waste forms is critical for the evaluation of the material’s potential use and stability as solid state waste repositories. Because of the ability of apatite to incorporate lanthanides and actinides there is great interest in the phase as a solid nuclear waste form. However, the crystal chemistry of U and Th in the apatite structure is still poorly understood. This dissertation investigates the structural crystal chemical parameters in a variety of natural and synthetic apatites with substituent U and Th through the complimentary use of single crystal X-ray diffraction and X-ray absorption spectroscopy.
1) Site preference of U and Th in F, Cl, Sr apatites, investigated the site preference of U and Th and the structural response to these substituents in a series of synthetic fluor-, chlor-, and strontium-apatite crystals using single crystal X-ray diffraction.
2) Crystal chemistry of Th in fluorapatite, obtained quantitative information of the local structure of Th in both natural and synthetic fluorapatite by Extended X-ray absorption fine-structure spectroscopy (EXAFS). Understanding the mechanism of incorporation and the structural response of fluorapatite to Th is important in assessing the use of apatite as a possible host for tetravalent radionuclides and understanding the behavior of Th in geological systems where fluorapatite is present.
3) Orientation dependent polarized micro-XAS study of single crystal apatite, developed the technique/equipment to accommodate polarization effects of synchrotron radiation on single crystal apatites. A goniometer was designed for precise positioning of single crystals for microXAS data collection. Lattice orientation is determined from X-ray diffraction data and then can be applied to EXAFS data analysis. The outcome of this technique development will have application to many other studies with similar sample constraints.