Structure and composition of silicon-stabilized tricalcium phosphate

Silicon stabilized tricalcium phosphate [Si-TCP] is formed within the calcium hydroxyapatite (HA)—tricalcium phosphate (TCP) system when a stoichiometric precipitate of hydroxyapatite is fired at 1000° in the presence of SiO2. This paper proposes a composition range and crystallographic structure for Si-TCP. Reitveld XRD powder diffraction, transmission electron microscopy, infrared and proton nuclear magnetic resonance measurements show that crystalline Si-TCP is associated with the displacement of OH from an initial hydroxyapatite structure. The resulting calcium phosphate is modified by the incorporation of silicon into its structure with excess silica contributing to an amorphous component. Si-TCP has a monoclinic structure with a space group P21/a akin to α-TCP with estimated lattice constants of a=12.863±0.004 Å, b=9.119 ±0.003 Å, c=15.232±0.004 Å, β=126.3±0.1°. It is proposed that Si4+ substitutes for P5+in the TCP lattice with the average chemical composition of Si-TCP set primarily by the mechanisms available for charge compensation. While the formation of OH vacancies in HA initiates the transformation to Si-TCP, two mechanisms of charge compensation in the Si-TCP structure are plausible. If O2− vacancies provide charge compensation, the composition of Si-TCP is Ca3(P0.9Si0.1O3.95)2 derived for the addition of 0.33 mol SiO2:mol HA. If excess Ca2+ compensates, the composition is Ca3.08(P0.92Si0.08O4)2 derived for the addition of 0.25 mol SiO2:mol HA. The reaction occurs most effectively when SiO2 is added as a colloidal suspension rather than by the in-situ thermal decomposition of a silicon metallorganic compound. The material is a bioceramic of major biological interest because of its osteoconductivity and unique influence on skeletal tissue repair and remodeling.