Thermally Induced Crystallization And Phase Evolution Of Amorphous Calcium Phosphate Substituted With Divalent Cations Having Different Sizes

The present work investigates the crystallization and phase evolution of amorphous calcium phosphate (ACP) partially substituted with smaller (Mg2+, Zn2+) and larger (Sr2+, Ba2+) divalent ions in the range from 1 to 10 mol % (M/P = 1.5:1). It has been demonstrated that the crystallization of ACP to tricalcium phosphate (TCP) and subsequent phase transformations between TCP polymorphs strongly depend on the nature and concentration of the substituents in the ACP matrix. Two different effects induced by the presence of foreign ions were observed. Substitution with smaller ions did not affect the crystallization temperature but promoted the formation of beta-TCP at a significantly lower temperature compared to the phase evolution of pristine ACP. Moreover, the effect of Mg was found to be more significant than that of Zn in the transformation of metastable alpha-TCP to beta-TCP. On the other hand, substitution with larger ions retarded the crystallization process, which was observed at elevated temperatures. While the substitution level of 10 mol % of Sr did not affect the thermal stability of metastable alpha-TCP, the presence of 10 mol % of Ba stabilized alpha-TCP crystal structure and totally prevented the phase transition to beta-TCP.