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 15 mol % multiphase ceramic is obtained passing from biphasic calcium phosphate (BCP) ceramic to ceramic composed of hydroxyapatite (HAp), silicocarnotite, and tricalcium phosphate (both α- and β-TCP polymorphs). The quantity of silicate incorporation increases when passing from β-TCP (Ca 3 (PO 4 ) 2, silicate level not measurable) to α-TCP (Ca 3 (PO 4 ) 1.875 (SiO 4 ) 0.125 with 6.25 mol % of phosphate substitution or 1.13 wt % of incorporated silicon), to HAp (Ca 10 (PO 4 ) 4.9(2) (SiO 4 ) 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 (PO 4 ) 1.9(2) (SiO 4 ) 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 (PO 4 ) 6– x −2 y (SiO 4 ) x+y (◻ T ) y (OH) 2– x O y ◻ 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 (PO 4 ) 4.9(2) (SiO 4 ) 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.