Mechanochemical Synthesis of Hydroxyapatite with SiO4- Substitutions

Using the mechanochemical method during 30 min in the planetary mill, we synthesized the samples of hydroxyapatite (HAP) modified with silicate ions in the amount of 0.1, 0.2, 0.4 and 0.8 molar fractions of the ion SiO44- in HAP at the given molar ratio Ca /(P + Si) = 1.67. As a result of activation in the air medium, hydroxyapatite with the average crystallite size of similar to 20 nm was formed. The mechanochemically sy6nthesized samples contain carbon ate ion substituting the phosphate ion, with the formation of carbonatehydroxyapatite of B type; its amount decreases with an increase in the amount of silicate introduced. Modification of hydroxyapatite with silicate ions is a competing reaction for phosphate substitution with carbonate. Depending on the ratio of silicate to carbonate amounts in hydroxyapatite structure, the mechanism of substitution changes. In the case of predominance of the carbonate ion in apatite, the composition corresponds to the general formula Ca-10 -(x - y)/(2)(PO4)(6) (- (y + x))(SiO4)(y)(CO3) (x)(OH)(2) (x > y). In the case of the equal number of substituents (x = y) silicate-substituted hydroxyapatite has the stoichiometric composition with the general formula Ca-10(PO4)(6) (-) ((y + x)) (SiO4)(y)(CO3)(x)(OH)(2). In the case of predominance of silicate ion (x < y), charge compensation occurs due to the formation of vacancies in the positions of (OH)(-) groups: Ca-10(PO4)(6) (-) ((y + x))(SiO4)(y)(CO3)(y)(OH)(2) (-) (y). After annealing the samples, the silicate group occupies the positions of phosphate ion in hydroxyapatite structure with the formation of vacancies in the positions of (OH)(-) group; the composition is described by the general formula Ca-10(PO4)(6) (-) (x)(SiO4)(x)(OH)(2 -) (x) (x).