On the effect of temperature on the insertion of zinc into hydroxyapatite

Rietveld analysis of X-ray powder diffraction patterns recorded from 28 hydroxyapatite (HAp) samples containing various amounts of zinc (0, 1.6, 3.2 and 6.1 wt.% Zn) and heat treated at various temperatures (between 500 °C and 1100 °C) has enabled the Zn insertion mechanism into the HAp crystal structure to be finely characterized. The formation of Zn-doped HAp was achieved above 900 °C only. Zn-doped HAp has the Ca10Znx(PO4)6(OH)2−2xO2x (0 < x ⩽ 0.25) chemical composition with a constant Ca/P ratio of 1.67 due to the insertion mechanism into the hexagonal channel (partial occupancy of the 2b Wyckoff site with the formation of linear O–Zn–O entities). Samples heat treated at 500 °C were almost single phase, HAp did not incorporate Zn and about half of the Zn atoms incorporated during the synthesis are not observable by X-ray powder diffraction (contained in an amorphous compound or physisorbed at the HAp surface). The reversible formation of Zn-doped β-TCP phase was observed at 600 °C, reached its maximum content at 900 °C and had almost vanished at 1100 °C. The results presented here strengthen the recently described mechanism of Zn insertion in the interstitial 2b Wyckoff position of the HAp structure, and explain the origin of the contradictory reports in the corresponding literature.