a-tricalcium phosphate (a-TCP, Ca3(PO4)2) demonstrates both bioactive and resorbable characteristics. Substitution of SiO4 4- for PO4 3- in a-TCP (Si-a-TCP) is found to stabilize the structure at lower temperatures and improve mechanical (and possibly bioactive) properties. The mechanism of electroneutrality in the Si-a-TCP structure is not fully understood, though is thought to take place through the creation of O2- vacancies or through excess Ca2+. This study addresses some structural properties of a-TCP using 31P MAS NMR at intermediate B0 fields (11.7 T) and 43Ca DOR NMR at multiple fields (20.0 T, 14.1 T, 11.7 T), and via correlation of the measured 31P and 43Ca isotropic chemical shifts (Oiso) against calculated values obtained with GIPAW DFT methods using the CASTEP code. These results show that the structure has high short range order and clearly support the monoclinic P21/a (12 P site/18 Ca site) model. In contrast, solid state 31P MAS and 43Ca DOR NMR studies of Si-a-TCP demonstrate that significant disorder broadening is characteristic of these data, however the corresponding 29Si MAS NMR data affords reasonably resolved resonances, a low intensity Q2 resonance at 0iso {84.5 ppm, and multiple resonances in the range 0iso - {70 -{75 ppm, despite this shift range normally associated with Q0 speciation, 29Si refocused-INADEQUATE data shows that Q1 resonances can also exist in this region. 31P-29Si HETCOR data from these systems suggests that, despite the intrinsic disorder, explicit PO43- framework species can be associated with the different Q0/Q1 Si species, and while there is some dispersion of the silicon throughout the structure it is predominantly associated with a small number of P sites. DFT calculations for the Si-a-TCP system suggest that the more favourable mechanism for charge balance is Ca2+ excess, despite this, the 29Si NMR data is in greater agreement with the DFT calculations for the O2- mechanism, whereby two adjoining cation-anion-columns in the structure are bridged together by a Q1 unit , 29Si NMR data also suggests the existence of Ca2SiO4 and Ca3Si3O9 existing as solid solutions within the a-TCP structure.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:582303 |
| Date | January 2012 |
| Creators | MacDonald, James F. |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/56932/ |
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