Chemical characterization of silicon-substituted hydroxyapatite

Bioceramic specimens have been prepared by incorporating a small amount of silicon (0.4 wt %) into the structure of hydroxyapatite [Ca-10(PO4)(6)(OH)( 2), HA] via an aqueous precipitation reaction to produce a silicon-substitu ted hydroxyapatite (Si-HA). The results of chemical analysis confirmed the proposed substitution of the silicon (or silicate) ion for the phosphorus ( or phosphate) ion in hydroxyapatite. The Si-HA was produced by first prepar ing a silicon-substituted apatite (Si-Ap) by a precipitation process. A sin gle-phase Si-HA was obtained by heating/calcining the as-prepared Si-Ap to temperatures above 700 degrees C; no secondary phases, such as tricalcium p hosphate (TCP), tetracalcium phosphate (TeCP), or calcium oxide (CaO), were observed by X-ray diffraction analysis. Although the X-ray diffraction pat terns of Si-HA and stoichiometric HA appeared to be identical, refinement o f the diffraction data revealed some small structural differences between t he two materials. The silicon substitution in the IIA lattice resulted in a small decrease in the a axis and an increase in the c axis of the unit cel l. This substitution also caused a decrease in the number of hydroxyl (OH) groups in the unit cell, which was expected from the proposed substitution mechanism. The incorporation of silicon in the HA lattice resulted in an in crease in the distortion of the PO, tetrahedra, indicated by an increase in the distortion index. Analysis of the Si-WA by Fourier transform infrared (FTIR) spectroscopy indicated that although the amount of silicon incorpora ted into the HA lattice was small, silicon substitution appeared to affect the FTIR spectra of HA, in particular the P-O vibrational bands. The result s demonstrate that phase-pure silicon-substituted hydroxyapatite may be pre pared using a simple precipitation technique. (C) 1999 John Wiley & Sons, I nc.