The incorporation of small cations in hydroxyapatite : synthesis, crystal chemistry and functionality

Abstract

Hydroxyapatite (HAp) is an important oxide in biotechnology, and also holds promise as a heterogeneous catalyst. Its ideal composition is [(Ca F 4)(Ca T 6)(PO 4) 6 (OH) 2 ] where an adaptable framework (F) structure creates one-dimensional tunnels (T) that provide multiple ionic acceptor sites (Ca F /Ca T /P/OH) which can be adjusted to deliver specific functionality. Modification of cation and anion type and proportions, together with selection of appropriate syntheses, can be used to regulate structural distortions and phase transformations. However, first-principles crystallographic adaptation to optimize performance is less explored, especially for tailoring excipient responses and catalytic activity, which are the primary concern of this study. Bioactive and catalytic HAp is nanocrystalline and usually nonstoichiometric, especially with respect to cation vacancies, carbonate and water incorporation, and oxygen displacement of hydroxyl ions. The hypothesis explored in this thesis is that controlled doping of HAp by small cations (Fe 3+ , Zn 2+ , Mg 2+) will alter the crystal chemistry in a manner that enhances cell proliferation (biological application) and product yield (catalytic application). The selection of small cations for investigation was deduced from their abundance and essential role in bone metabolism and the potential of these metals for enhancing chemical conversion of alcohol while fixed in an environmentally benign HAp matrix. To this end, methods to synthesize homogeneous ceramic powders of single phase Fe-, Zn-, and Mg-HAps to the solid solubility limits of these metals were developed from which structure-property correlations were established. All three series were characterized with X-ray and neutron diffraction, electron microscopy, and several spectroscopies, including FTIR and NMR techniques, to corroborate crystallographic incorporation and concomitant symmetry transformations, determine phase proportions (crystalline and amorphous), and metal partitioning preferences across the Ca F /Ca T sites. Crystal structure refinement was challenging due to the fine-grain size, although in this physical form, large reactive surfaces are produced that are preferable for the targeted applications. It was found that the solid solution limit of Fe-HAps {nominally [(Ca 10-x Fe x)(PO 4) 6 (OH 2-x O x)]} was x = 0.5, with Fe 3+ entering the Ca F site at low loadings (x = 0.1) and subsequently residing in both the Ca F and Ca T at x = 0.5.