The structure of divalent and trivalent cation substituted β-tricalcium phosphate

Abstract

Current methods of nuclear waste disposal are not suitable for the immobilisation of novel defence-based waste due to the high halogen content resulting from pyrochemical reprocessing. The objective of this thesis was to investigate β-tricalcium phosphate (β- TCP, Ca3(PO4)2) encapsulated in a sodium aluminoborophosphate glass (NABP) matrix as a potential host for this waste using a variety of structural probes. Samples were prepared to determine the structural changes in β-TCP as components of the simulated waste streams were substituted into the material. Zn, Mg, Al, and Ga incorporation was investigated. A combination of X-ray and neutron diffraction was used to determine the changes in long-range order as a function of Zn and Mg substitution up to 13% and 25% cation substitution respectively. Both Zn and Mg substitution caused a contraction of the unit cell up to complete substitution of the Ca(5) site, at which point the contraction ceased. Under further substitution on the Ca(4) site, the a lattice parameter continued to decrease, while the c lattice parameter increased, resulting in an unchanged unit cell volume. Evidence of tricalcium trimagnesium phosphate second phase was observed for the Mg-based compositions above Ca2:8Mg0:2(PO4)2, as has been previously documented, however single phase samples were observed for all Zn-based compositions, in contrast to previous studies. 31P NMR was used to confirm this Ca(5)-Ca(4) substitution model for the Zn-based β-TCP compositions by tightly constrained simulations as a function of composition. A combination of solid-state NMR techniques were used to identify the substitution mechanism of Al and Ga in β-TCP up to the composition Ca9M(PO4)7, where M is Al or Ga respectively. The 31P and 43Ca NMR spectra were simulated as with the divalent cations mentioned above to determine the origin of each resonance in the spectra. Subsequently, 27Al-f31Pg and 71Ga-f31Pg R3-HMQC experiments were performed to explicitly identify substitution on the Ca(5) site only. Studies were also performed to model the NABP:β-TCP interface formed as a result of the encapsulation process, for both pure β-TCP and Ga-substituted β-TCP. To simulate the range of compositions expected at this interface, calcium phosphate and NABP preparations were mixed in proportions from 10 wt:% to 80 wt:% (Ga-substituted) β- TCP. 31P NMR and Raman spectroscopy showed a progressive depolymerisation of the phosphorus network, consistent with the replacement of Al3+ and Na+ with Ca2+. The Al3+ was shown to exist primarily in a 4 coordinated state, showing a tendency to exist within the phosphorus network, whereas 11B NMR showed the B to move from a 4 coordinated site in NABP to a B-rich 3 coordinated environment. Differential thermal analysis showed an increase in the temperature of the two recrystallisation events as a function of both β-TCP and Ga-substituted β-TCP. Studies of the phases present after recrystallisation of the pure β-TCP-based samples showed calcium sodium phosphate and Na Al co-substituted β-TCP for the lower and higher crystallisation temperatures with β-TCP incorporation. For the Ga-containing samples, Na Al Ga co-substituted β-TCP was observed for both crystallisation temperatures. Critically, Ga was shown to displace Al in the β-TCP phase