Solid state synthesis, x-ray diffraction and scanning electron microscopy of rare-earth silicate apatite

• Main Author: Lim, Soo Min
• Other Authors: Dong Zhili
• Format: Final Year Project
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Materials
• Online Access: http://hdl.handle.net/10356/55715
• Institution: Nanyang Technological University

Apatite structures have great potential in the area of nuclear waste stabilization due to the steady columns of polyhedron within its crystal structure. Since during the process of nuclear waste disposal, apatite structures are required to remain stable at elevated temperatures, making britholite apatite a promising matrix for this project with the chemical formula of RE8Sr2(SiO4)6O2.This thesis illustrates the study of crystal structures regarding rare earth element (REE) britholite apatite via solid state using different characterization techniques. In order to obtain the product of interest, precursors of stoichiometric ratio were preheated to dry completely before grounded into evenly distributed powder form. Such powders were then calcined at 1200 ℃ before sintering at different optimal temperature ranging from 1300℃ to 1650℃. Synthesized products were then analyzed using X-ray diffraction with rietveld refinement to check for apatite phase before further characterization. In the end, the study concludes that Yb8Sr2(SiO4)6O2(s) is synthesized at sintering temperature of 1400℃, while Nd8Sr2(SiO4)6O2(s) is synthesized at 1600℃, both confirmed to adopt the P63/m symmetry through XRD characterization. AI position is shared by Nd and Sr, and AI O6 polyhedra are face-connected to be column structures which are linked to SiO4 tetrahedra forming continuous channels, proving both it feasible to incorporate Ytterbium or Neodymium with Sr into britholite apatite materials. Structure variation by twist angle was calculated using figures obtained from XRD results, showing ϕYb = 34.87º while ϕNd = 20.59º. This fits the crystallographic speculation whereby twist angle varies inversely with average crystal radii and unit-cell volume since Nd has a larger crystal radii compared to Yb, thus possessing smaller twist angle as found.