Preparation and Characterization of Na + , SiO 4 4− , and CO 3 2− Co‐Substituted Hydroxyapatite

The mechanism of Na + , SiO 4 4− , and CO 3 2− cosubstitution in hydroxyapatite (HAP) crystal structure was investigated. Several powders were prepared by wet chemical precipitation at 37°C in the presence of different concentrations of sodium silicate in open atmosphere. The powders were characterized using chemical analysis, scanning electron microscopy, X‐ray diffraction, and Fourier‐transform infrared spectrometry spectroscopy. Increasing of silicate ions substitutions were associated with increasing of carbonate ions substitutions for phosphates and vacancies of the OH − site. The crystallinity of HAP was reduced as the degree of substitution increased. Silicate substitution was dominated by two charge compensation mechanisms acting simultaneously. Firstly, SiO 4 4− and CO 3 2− groups were substituted for two PO 4 3− groups. Secondly, SiO 4 4− group was substituted for one PO 4 3− group and a vacancy formed in the OH − site. A phenomenological relationship between the shift on O–H vibration bands and the degree of silicate substitution was established. This relationship indicated the presence of bent hydrogen bonding between O–H groups and silicate ions. The imperfections introduced by silicon substitution affected the OH − position and motion in the channel, which may play a role in enhancing bioactivity.