X-ray Absorption Fine Structure and X-ray Excited Optical Luminescence Studies of Calcium Compounds

Calcium-bearing materials have received considerable attention due to their importance in many fields of research ranging from biological to materials sciences; however, X-ray absorption of calcium compounds has not been studied as extensively due to lack of accessibility to instruments with capability of performing high-resolution X-ray absorption spectroscopy. Focus of this Ph.D. thesis is on analysis of various calcium compounds by X-ray absorption fine structure (XAFS), including the near-edge region, the X-ray absorption near-edge structure (XANES) and the extended region, the extended X-ray absorption fine structure (EXAFS) for a better understanding of local chemistry and structure around calcium using tunable X-ray from a synchrotron light source.;Keywords: X-ray absorption near-edge structure, Extended X-ray absorption fine structure, X-ray excited optical luminescence, colour centre, calcium oxide, calcium tungstate, barium calcium borate, lanthanum calcium borate, europium, cerium, rare-earth doping, calcium carbonate, calcite, amorphous calcium carbonate, biomineralization;From XANES studies of a series of calcium-bearing compounds, it has been shown that spectral features can be correlated to the chemical environment of calcium, such as local symmetry, coordination number, coordinating atoms, and crystal field splitting. XANES and X-ray excited optical luminescence (XEOL) have also been used to determine whether a small amount of dopant alters crystal structure of a host material, and to study energy transfer mechanism and efficiency to luminescence channels in europium-doped calcium tungstate, and cerium-doped barium calcium borate and lanthanum calcium borate upon X-ray excitation across various absorption edges. Also, XEOL and its related technique, time-resolved XEOL (TRXEOL) have been used successfully in tracking luminescence pathways and decay characteristics of colour centres in calcium oxide. In addition to the above, XANES and Extended X-ray absorption fine structure have been used to study the local symmetry and the nature of neighbouring atoms of calcite and amorphous calcium carbonate (ACCs) synthesized by biomineralization. Calcite, the most stable form of calcium carbonate, exhibited luminescence properties, and these have been studied by XEOL.