Exploring zinc apatites through different synthesis routes

Inclusion of zinc in apatites is not completely understood due to complexes in solution preventing the incorporation or the inclusion of defects. Some work has shown zinc addition by alternative synthesis routes, but the level of incorporation has not been reassessed. This work uses a newly developed approach whereby zinc is included in an amorphous phase and subsequently crystallized. Different phosphate reactants (ammonium, sodium or potassium phosphates) were mixed together with calcium salts (calcium nitrate or calcium acetate) and the pH level adjusted (with ammonia or KOH) to investigate the ease of forming an amorphous phase and the inclusion of zinc. X-ray diffraction revealed that all reactants showed a tendency to form an amorphous phase, supporting the requirement of a high pH to prevent crystallization. X-ray fluorescence showed higher inclusion in the absence of ammonia. Potassium phosphate led to the highest zinc incorporation, accompanied by inclusion of potassium. Crystallization at 700 °C indicated that zinc levels close to 20% replacement of calcium were the highest possible before inclusion of a secondary phase. FTIR-DRIFT showed a change in bonding conditions at higher zinc concentrations. This work shows a) the importance of reactants for including zinc, b) more zinc inclusion with potassium reactants and c) zinc levels that maintain the apatite structure.