Silicon Location in Silicate-Substituted Calcium Phosphate Ceramics Determined by Neutron Diffraction

The silicate incorporation in calcium phosphate bioceramics was studied by X-ray and neutron powder diffraction (joint Rietveld refinement) and micro-Raman spectroscopy. For a phosphate substitution level of IS mol % multiphase ceramic is obtained passing from biphasic calcium phosphate (BCP) ceramic to ceramic composed of hydroxyapatite (HAp), silicocarnotite, and tricalcium phosphate (both alpha- and beta-TCP polyrnorphs). The quantity of silicate incorporation increases when passing from beta-TCP (Ca-3(PO4)(2), silicate level not measurable) to alpha-TCP (Ca-3(PO4)(1.875)(SiO4)(0.125) with 6.25 mol % of phosphate substitution or 1.13 wt % of incorporated silicon), to HAp (Ca-10(PO4)(4.9(2))(SiO4)(1.1(2))(OH)(1.0)(1)O-0.66(7) with 18.3 mol % of phosphate substitution or 3.11 wt % of silicon), and to silicocarnotite (Ca-5(PO4)(1.9(2))(SiO4)(1.1(2)) with 36.7 mol % of phosphate substitution or 6.40 wt % of silicon). The mechanism of silicate incorporation into HAp structure is multiple and involves the creation of two kinds of vacancies: hydroxyl vacancies and phosphate/silicate vacancies due to a carbonate-involving mechanism leading to the composition Ca-10(PO4)(6-x-2y)(Sia(4))(x+y)(square(T))(y)(OH)(2-x)O-y square(x-y). The formation of important hydroxyl vacancies in the hexagonal channel of the HAp structure leads to the stabilization of silicocarnotite (that can be considered as a polymorph of the highly Si-substituted HAp phase). The Ca-10(PO4)(4.9(2))(SiO4)(1.1(2))(OH)(1.0(1))O-0.66(7) composition appears then to be the end-member of the hexagonal Si-HAp solid solution. Silicocarnotite is formed above this silicate insertion level.