Cation substitution in β-tricalcium phosphate investigated using multi-nuclear, solid-state NMR

• β-Ca3(PO4)2 has been prepared pure and also with Al3+, Ga3+ and Na+ substituents.
• Multi-nuclear 1D NMR and heteronuclear X{31P} recoupling have been used.
• Models for substitution correctly predict site preference and occupancy.
• Progressive changes in 31P spectra have been explained.
• Al3+ and Ga3+ substitute onto the Ca(5) site, and Na+ onto the Ca(4) site.

The substitution of aluminium, gallium and sodium cations into β-tricalcium phosphate (β-TCP; Ca3(PO4)2) has been investigated, and the Ca sites involved successfully determined, using a combination of 1D 31P, 27Al, 71Ga, 23Na and 43Ca (natural abundance) NMR and 2D 27Al{31P}, 71Ga{31P} and 23Na{31P} rotary-resonance-recoupled heteronuclear multiple-quantum correlation (R3-HMQC) NMR. Over the compositional range studied, substitution of Ca2+ by Al3+ or Ga3+ was observed only on the Ca(5) site, whilst substitution by Na+ was confined to the Ca(4) site. Some AlPO4 or GaPO4 second phase was observed at the highest doping levels in the Al3+ and Ga3+ substituted samples.

2D contour plots with skyline projections showing recoupling of 27Al, 71Ga and 23Na to different 31P sites.Figure optionsDownload as PowerPoint slide