Synthesis, structural features, and color of calcium-yttrium hydroxyapatite with copper ions in hexagonal channels

Abstract

Copper-containing calcium phosphates Ca5 − z Y z (PO4)3Cu x OH1 − x − z −δ, where x = 0, 0.1, 0.3 and z = 0, 0.5, with an apatite structure and a partial substitution of yttrium for calcium have been synthesized by a solid-phase method at a temperature of 1100–1150°C. The compounds have been characterized by X-ray powder diffraction and diffuse reflectance spectroscopy. It has been established that yttrium substitutes for calcium only in the Ca(2) position, and the occupancy of this position corresponds to the yttrium content z = 0.4–0.43. The substitution by yttrium slightly decreases the unit cell parameter of the compound (by ∼0.3%) and much more considerably (by 3%) reduces the diameter of a hexagonal channel formed by Ca(2) atoms. The predominant fraction of doping copper intrudes into hexagonal channels in both unsubstituted and yttrium-substituted calcium phosphates. Unsubstituted copper-containing phosphates are pink-crimson colored, whereas their yttrium-substituted analogues have much more pale colors from nearly white to greeny beidge. The absorption bands typical for a copper-containing calcium hydroxyapatite observed in diffuse reflectance spectra are strongly suppressed in yttrium-substituted samples probably due to a decreased average oxidation number of copper.