The effect of phosphate group replacement by sulfate groups on the phase formation in the synthesis of hydroxyapatite

Abstract

Sulfate-substituted hydroxyapatite materials with a degree of substitution of up to 20 mol % (Ca₁₀(PO₄)_{(6 – 0.06x)}(SO₄)_0.09x(OH)₂, x = 0, 0.1, 0.5, 1, 5, 10, and 20) were synthesized. For substitutions of 0, 0.1, 0.5, and 1 mol %, a single-phase material with the apatite structure is formed. On further increase in the concentration of SO₄²⁻ groups up to 20 mol %, a second phase, CaSO₄, is formed; the amount of this phase increases for higher degrees of substitution. The unit cell parameters of hydroxyapatite-based materials change slightly upon the replacement of phosphate groups by sulfate groups: the parameter a tends to increase, while c tends to decrease. The introduction of sulfate groups results in decreasing particle size.