Abstract:
:Seven nominally identical samples of Na-free carbonate apatite (CO(3)Ap) were prepared by reaction of CaHPO(4) with ammonium carbonate solution at 70 degrees C over 3 days. They were studied by chemical analysis, Rietveld analysis of powder X-ray diffraction (XRD) data, Ca/P ratio determinations (quantitative phase analysis of CaO, Ca(OH)(2) and hydroxyapatite formed after heating to 900 degrees C from Rietveld analysis of XRD data), He pycknometry, (1)H, (13)C and (31)P MAS NMR spectrometry and Fourier transform infrared and Raman spectroscopy. Spectroscopy showed the apatite products were B-type CO(3)Aps (CO(3)(2-) replacing PO(4)(3-)) and XRD that one sample contained 1.6 wt% calcite with a trace in another. Mean results of the six essentially calcite-free samples were: a=9.405(5)A, c=6.896(2)A; 11.2 wt% CO(3); unit cell contents, Ca(8.241)(PO(4))(4.344)(CO(3))(1.656)(OH)(0.139) x 2.29H(2)O; mole Ca/P ratio from chemical analyses, 1.897(22) and from powder XRD phase analysis of samples decarbonated at 900 degrees C, 1.892(25). Density determinations indicated that the 2.29mol of H(2)O were in the unit cell. Rietveld refinements were undertaken without and with explicit modelling of the CO(3)(2-) ion. The latter used constraints to maintain the CO(3)(2-) ion in its known geometry and the total of PO(4)(3-) and CO(3)(2-) ions per unit cell at six. Without the CO(3)(2-) ion in the model, PO(4) volume, P-O bond lengths and P occupancy were apparently reduced, consistent with CO(3)(2-) replacing PO(4)(3-) ions. With the CO(3)(2-) ion modelled, the reductions were less and the CO(3)(2-) ion occupied the "sloping" face of the replaced PO(4)(3-) ion in two-fold disorder about the mirror plane. The angle between the normal to the plane of the ion and the c-axis was 34 degrees , close to 35.3 degrees , the equivalent angle for the PO(4)(3-) ion. When modelled, the CO(3)(2-) ion occupancy was 1.81 ions per unit cell, in reasonable agreement with unit cell contents determined chemically (1.66). The OH(-) ion occupancy was elevated (2.33 ions per unit cell versus 0.14 inferred from the charge balance), which we ascribe to H(2)O molecules occupying sites in c-axis channels. The Ca/P ratio from occupancies (2.31) was also elevated over that determined chemically (1.90). We attribute this to loss of Ca from Ca sites increasing the apparent anisotropic displacement parameters of remaining Ca atoms, leading to an apparently increased occupancy.
journal_name
Biomaterialsjournal_title
Biomaterialsauthors
Wilson RM,Dowker SE,Elliott JCdoi
10.1016/j.biomaterials.2006.04.033subject
Has Abstractpub_date
2006-09-01 00:00:00pages
4682-92issue
27eissn
0142-9612issn
1878-5905pii
S0142-9612(06)00398-Xjournal_volume
27pub_type
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