Tracking the local environment of Eu3+ in Eu3+-doped calcium phosphate during hydroxyapatite crystallization and its phase transformation to tricalcium phosphate

Abstract

We present a study on the crystal structure and luminescence of Eu3+-doped calcium phosphate (Eu-CaP) nanoparticles. The sintering treatment on the pristine Eu-CaP leads to its crystallization into hydroxyapatite (HA) and further to biphasic structure that contains both HA and β-tricalcium phosphate (β-TCP). Photoluminescence spectroscopy has been a widely used technique to identify the site occupancy of Eu3+ and its thermal-induced migration behavior in pure phase CaP. However, for sintering process that involves the emergence of a second phase or the doped CaP is not luminescent, PL alone is insufficient to characterize how the dopant ions behave during the formation of a biphasic structure. In this work, we utilize X-ray absorption fine structure (XAFS) as an alternative structure probe, targeting the local structure around Eu3+, and track the change in the interatomic distance and coordination number of Eu with its neighboring O, P, and Ca atoms. A model describing formation process of a biphasic structure from HA with the presence of Eu3+ is proposed.