Joel W. Reid

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 1,000 degrees in the presence of SiO(2). 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 P2(1)/a akin to alpha-TCP with estimated lattice constants of a=12.863+/-0.004 A, b=9.119 +/-0.003 A, c=15.232+/-0.004 A, beta=126.3+/-0.1 degrees. It is proposed that Si(4+) substitutes for P(5+)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 O(2-) vacancies provide charge compensation, the composition of Si-TCP is Ca(3)(P(0.9)Si(0.1)O(3.95))(2) derived for the addition of 0.33 mol SiO(2):mol HA. If excess Ca(2+) compensates, the composition is Ca(3.08)(P(0.92)Si(0.08)O(4))(2) derived for the addition of 0.25 mol SiO(2):mol HA. The reaction occurs most effectively when SiO(2) 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.

Rapid, accurate phase quantification of multiphase calcium phosphate materials using Rietveld refinement

Rietveld refinement has been employed to estimate the crystalline phase compositions of multiphase calcium phosphate mixtures containing calcium hydroxyapatite [Ca 5 (PO 4 ) 3 OH], and alpha and beta tricalcium phosphate [Ca 3 (PO 4 ) 2 ]. Two methods were employed using fixed structural models for all three phases and refining the zero offset, scale factors, lattice parameters and one peak breadth parameter using either a constant background (method A) or a background with two refined parameters (method B). Analysis of a matrix of quantitative standards across a broad spectrum of phase compositions indicates that method A results in small systematic deviations of the Rietveld phase compositions (< 1 wt%) from the nominal values, but the systematic deviations are eliminated by refining the background (method B). The methods require approximately 10 min to complete, and are suitable for quality control of calcium phosphate production (and potentially other multiphase systems) when accuracy, precision and time are all significant considerations.