From Nano-Hydroxyapatite to Pakhomovskyite: A Particular Phase Transformation

Hydroxyapatite nanocrystals (nano-HAp) were dispersed in a highly concentrated solution of cobalt(II) ions. After 5 days the particles were completely transformed into a new phase, consistent with the one for the cobalt phosphate known as pakhomovskyite (PAk). The transformation process began with an amorphization, thus completely changing the thermodynamics involved in the crystallization process. The initial metastable amorphous material has a defined stoichiometry, Co2.5Ca6.5(PO4) (6)center dot nH(2)O, and it is capable of evolving to the PAk phase solely by remaining suspended in water over an extended period. Apparently, a non-classical crystallization has occurred, where there could be a process based on nano-cluster coalescence. Towards the end of the process the crystals, already transformed to the PAk phase, showed an accelerated growth accompanied by further enrichment in cobalt(II) ions.