Ions Traces Substitution in Stoichiometric Calcium-Hydroxyapatite: Structural Analysis by Rietveld Refinement

Abstract

Impact of trace ions as Mg²⁺, Na⁺, Sr²⁺, SO₄^2-, and CO₃^2- on stoichiometric hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) crystal structure is investigated. The first four ions are brought by the raw material while the carbonate group is due to contamination by air. The powders were prepared, under industrial conditions, at 22°C from aqueous solutions by reaction between the calcium hydroxide and orthophosphoric acid at pH from 7 to 7.5. Two samples of hydroxyapatite powder, which differ by the content of ions trace, calcined at 1100°C have been investigated by X-ray powder patterns fitting methods. Results showed that in the absence of ion traces, the partial replacement of PO₄^3- by CO₃^2- groups leads to the - relaxation of the lattice by creation of vacant calcium (Ca1 and Ca2) sites, - reduction of the cell parameters, and new lattice rearrangement with lower PO₄ distortion index, (D_ind = 0.488) compared to the calculated theoretical value (D_ind=3.079). For trace ions incorporated hydroxyapatite sample, the presence of traces cation favors the incorporation of carbonate ions. Coupled substitution on Ca2 and P position affect neither theoretical structure nor thermal stability of the hydroxyapatite but causes vacancies creation in calcium (Ca1) and oxygen (O1 and O3) sites. This structural change leads to a slight lattice distortion (D_ind=3.204). Composition and microstructure of analyzed materials appear very promising for biological applications.

Keywords: Biomaterials, Calcium-hydroxyapatite, Traces ions, Crystalline structure.