The incorporation of chromium (III) into hydroxyapatite crystals

• We analysed the formation and morphology of HAP crystals doped with chromium.
• We studied the Cr ion local symmetry into hydroxyapatite host structure.
• When the Cr concentration is increased to 18 mol% Cr2O3 and Cr 6+ is formed.
• It is not clear the Cr replace Calcium sites preference into hydroxyapatite.
• We report interstitial oxygen atoms as possible charge compensation mechanism.

Nanocrystals of hydroxyapatite (HAP) powder with different Cr3+ molar concentrations were synthesized using the co-precipitation method and a rod-like morphology was observed. Powder characterization results indicate the use of dopant influence in the HAP hexagonal phase formation, other Cr-containing phases and a charge compensation mechanism. By X-ray absorption spectroscopy, the local coordination structure of Cr incorporated into HAP was examined. X-ray absorption near-edge structure (XANES) spectroscopy results suggest Cr takes a different valence for the incorporated Cr concentrations. X-ray absorption fine structure (EXAFS) fitting confirms that Cr occurs in octahedral coordination as chromium local behaviour in Cr2O3 and can replace both Ca1 and Ca2 sites.