Structure refinements by the Rietveld method of partially substituted hydroxyapatite:: Ca9Na0.5(PO4)4.5(CO3)1.5(OH)2

The structural location and effect due to partially substitution of Na+ and CO32- to Ca+ + and PO43-, respectively, in hydroxyapatite, Ca-10(PO4)(6)OH2 (HAP), synthesised in aqueous solution have been investigated by X-ray powder pattern fitting methods. The compound crystallises in the hexagonal system (space group P63/m) with the following cell: a = b = 9.3892(4) Angstrom, c = 6.9019(3) Angstrom. Using the Rietveld method, the refinement of occupancy factors shows that the sodium is located preferentially in the calcium site II of the apatite structure. Vacancies in both calcium sites are found and the OH site is fully occupied. No carbonate ion is found in the apatite channel, leading to a B type carbonate apatite. The carbonate ions are located an two faces of the PO4 tetrahedron. An extra shift of the OH- ion outside of the metal II triangle is attributed to the replacement of calcium by sodium and a vacancy. The corresponding new substitution mechanism is compared to already known fundamental mechanisms. (C) 1999 Elsevier Science S.A. All rights reserved.