H. El Feki et al., Structure refinements by the Rietveld method of partially substituted hydroxyapatite: Ca9Na0.5(PO4)(4.5)(CO3)(1.5)(OH)(2), J ALLOY COM, 287(1-2), 1999, pp. 114-120
The structural location and effect due to partially substitution of Na+ and
CO32- to Ca+ + and PO43-, respectively, in hydroxyapatite, Ca-10(PO4)(6)OH
2 (HAP), synthesised in aqueous solution have been investigated by X-ray po
wder pattern fitting methods. The compound crystallises in the hexagonal sy
stem (space group P63/m) with the following cell: a = b = 9.3892(4) Angstro
m, c = 6.9019(3) Angstrom. Using the Rietveld method, the refinement of occ
upancy factors shows that the sodium is located preferentially in the calci
um site II of the apatite structure. Vacancies in both calcium sites are fo
und and the OH site is fully occupied. No carbonate ion is found in the apa
tite 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 calci
um by sodium and a vacancy. The corresponding new substitution mechanism is
compared to already known fundamental mechanisms. (C) 1999 Elsevier Scienc
e S.A. All rights reserved.