8316
J. Phys. Chem. B 2003, 107, 8316-8320
Structural and Chemical Analysis of Well-Crystallized Hydroxyfluorapatites Luis M. Rodrı´guez-Lorenzo, Judy N. Hart, and Kajrlis A. Gross* School of Physics and Materials Engineering, Building 69, Monash UniVersity, VIC 3800, Australia ReceiVed: NoVember 25, 2002; In Final Form: April 22, 2003
X-ray diffraction (XRD) patterns of hydroxyfluorapatites with a nominal composition of Ca10(PO4)6(OH)2-xFx, where x ) 0.0, 0.4, 0.8, 1.2, 1.6, and 2.0, were collected and analyzed using the Rietveld method. A multiphase Rietveld analysis performed on calcined samples reveals a range of Ca/P ratios from 1.56 for the sample with x ) 0.4 to 1.67 for the sample with x ) 2.0 on the synthesized powders. Calcined apatites exhibit a decrease in the lattice parameter a, a greater crystallite size, lower distortion in the phosphate tetrahedra, and greater stability of the apatite structure for higher fluoride contents.
Introduction Calcium hydroxyapatite (OHAp) has received intense focus over the past 25 years1-5 for applications in implant materials. Interest is turning to modified calcium OHAp that involves chemical species found naturally in the body. Such substitutions can modify the mechanical properties, solubility, and bonebonding capability of the materials. Clinical trials have shown that fluoride salts produce an increase of bone mass in spinal osteoporosis; however, this increase in mass was not associated with a reduction of vertebral fractures. The explanation given is related to a reduction in bone quality, because of the appearance of mineralization defects when fluoride is accumulated.6 Therefore, fluorapatite (FAp) and solid solutions of hydroxyfluorapatite that can provide similar effects have gained increased interest; however, more papers have focused on the synthesis7-9 and dissolution properties10,11 than in the structure leading to those properties with analysis limited to interpretation of infrared spectra.12 Stoichiometric calcium phosphate hydroxyapatites crystallize in the monoclinic P21/b group, although a very small amount of impurities or crystal imperfections leads to the hexagonal P63/m. A detailed description of the structure can be found in the literature.1,13 It is a structure that readily accommodates substitutional elements, and this fact can be used to optimize its properties for any intended application; however, the effect of chemical substitutions on the structure is not satisfactorily solved with characterization using routine techniques. Hydroxyl groups are found at (0,0,0.198) along the hexagonal screw axes for OHAp, whereas F- ions lie on a mirror plane at (0,0,0.25) in FAp. When F- ions substitute for OH- ions in OHAp, it has a tendency to occupy sites on the hexad axes, as in FAp. Such positioning places the F- ions close to the adjacent unsubstituted OH- ions that consequently become hydrogen-bonded. The consequences of the partial substitution of OH- ions by F- ions on the X-ray patterns, and, therefore, on the apatite structure, will be discussed in this work. Experimental Section Materials. Hydroxyfluorapatites with various compositions were synthesized through a wet chemical reaction, as described in detail elsewhere.14 Apatites were synthesized by adding a 0.6 M diammonium hydrogen phosphate solution at a rate of * Author to whom correspondence should be addressed. E-mail: karlis.
[email protected].
10 mL/min into a 1 M calcium nitrate solution at room temperature. The pH in the calcium solution was adjusted to 9.4 with a pH stat (Titrino 736, Metrohm, Switzerland) after which the addition of the phosphate solution commenced. During the reaction, the pH was continuously adjusted to 9.4 where to ensure constant pH conditions during the synthesis. The fluoride was added to the ammonium phosphate in varying quantities, to produce powders with a chemical composition of Ca10(PO4)6(OH)2-xFx where x ) 0.0, 0.4, 0.8, 1.2, 1.6, and 2.0. This observation was confirmed by determining the fluoride contents with a F- ion-selective electrode. Synthesized powders were calcined by heating at a rate of 10 °C/min to 900 °C and holding for 120 min before cooling in the furnace. Sample codes are provided in Table 1. The F--ion content was determined following the method that was outlined by Singer and Armstrong.15 A solid-state Fion-selective electrode, which was coupled to a PHM ion analyzer (Radiometer; Copenhagen, Denmark), was used. Fourier Transform infrared (FTIR) spectra were recorded using the KBr pellet technique in a Perkin-Elmer 1600 Series FTIR spectrometer over a range of 400-4000 cm-1 at a resolution of 4 cm-1. X-ray diffraction (XRD) patterns of synthesized and calcined powders were collected from 10° 2θ to 90° 2θ, using a Rigaku Geigerflex diffractometer with Bragg-Brentano geometry in a step mode with a step size of 0.02° and 10 s per step using Cu KR radiation at 40 kV/22.5 mA that was passed through a 0.5° divergence slit and a 0.3° receiver slit. XRD Analysis. Crystal size and lattice strain were calculated for the [100] and [001] directions, using the Scherrer equation.16 The formulas used for the calculations are as follows:
Cs ≡ Ls ≡
Kλ Wsize cos θ
Wstrain 4 tan θ
(where Wsize ) Wb - Ws)
(where Wstrain ) (Wb2 - Ws2)1/2)
where Cs is the crystallite size and Ls is the lattice strain. K is the form factor (assumed to be 0.9), λ the wavelength, Wsize the broadening caused by small crystals, Wb the broadened profile width, Ws the standard profile width, and Wstrain the broadening caused by lattice distortion. Rietveld Method. The X’pert Plus Program17 was used for the whole-pattern-fitting structure refinement. A pseudo-Voigt profile function was used in the modeling of the diffraction peaks. The background was refined simultaneously with a
10.1021/jp027556o CCC: $25.00 © 2003 American Chemical Society Published on Web 07/15/2003
Structural Analysis of Hydroxyfluorapatites
J. Phys. Chem. B, Vol. 107, No. 33, 2003 8317
TABLE 1: Sample Codes Used for Synthesized and Calcined Powders with Various Amounts of Fluoride Substitutiona Sample Code powder
0%
20%
40%
60%
80%
100%
synthesized 0.0 0.4 0.8 1.2 1.6 2.0 calcined 0.0-900 0.4-900 0.8-900 1.2-900 1.6-900 2.0-900 a Sample codes are presented in bold italic typeface throughout the paper.
polynomial of six refinable parameters. Preferred orientation was modeled using the March-Dollase model. Initial atomic parameters, including anisotropic displacement parameters, were taken from a single-crystal data refinement in P63/m of Holly Springs OHAp, which is known to contain impurities.18 The background, zero shift, scale factor, full width at half-maximum (fwhm) sample displacement, cell parameters, preferential orientation, asymmetry, peak shape, and atom positions were allowed to vary. Displacement parameters and occupancies were kept constant through the refinements. Atomic positions were set back to the original values when the shift produced nonsensible structural values. Some of the calcined samples contain tricalcium phosphate (TCP), either in the β form, the R form, or both, and these phases were consequently introduced in the refinement. Initial atomic parameters for β-TCP and R-TCP were taken from references.19,20 A quantitative phase analysis was performed on multiphase samples, by21
% phase ≡
SZMV
∑i (SZMV)i
where S is the scale factor, Z the number of formula units per unit cell, M the molecular weight of the formula unit, V the volume of the unit cell, and i an index applied to all phases. The Ca/P ratio of the raw material can then be calculated according to the decomposition products when heated:
Ca10-x(HPO4)x(PO4)6-x(OH)2-x f (1 - x)Ca10(PO4)6(OH)2 + 3xCa3(PO4)2 + xH2O and with the following formula:22
Ca/P )
1.5(% TCP) + 1.667(% OHAp) 100
From the calculated data, a numerical index (dimensionless) of the overall distortion of the structure may be calculated from the PO4 tetrahedra. It is a numerical index of the deviation of the PO4 tetrahedra from an ideal configuration. The distortion index of the phosphate tetrahedral was calculated using the relation23 6
Dind ≡
∑1 (θi - 109.17°)2 6
Results Ca/P Analysis. XRD patterns of the calcined samples reveal multiple calcium phosphate phases, with the exception of the 2.0-900 sample. The patterns shown in Figure 1 depict the three situations where calcium phosphates are present in different
Figure 1. XRD patterns of significant calcined samples ((2) main maximum of β-TCP and (4) main maxima of R-TCP).
TABLE 2: Weight Percentage of Phases Contained in the Calcined Powders, Ca/P Ratio Calculated from Them for the Synthesized Powders, and Weight Percentage of Fluoride in the Raw Powders 0.0-900
0.4-900
0.8-900 1.2-900 1.6-900 2.0-900
OHAp 88.38 (7) 65.3 (8) 88.3 (7) 96 (1) (wt %) R-TCP 5.1 (2) 12.527 (4) 9.942 (2) 3.7 (2) (wt %) β-TCP 6.5 (3) 22 (1) 1.8 (3) (wt %)
94.5 (4)
0.0 Ca/P 1.62 F (wt %) 0
1.6 1.64 2.75
0.4 1.56 0.69
0.8 1.61 1.36
1.2 1.65 2.04
100
5.5 (2)
2.0 1.67 3.43
combinations. Other patterns have been omitted to simplify the figure. TCP, either in the R-form, the β-form, or both, in addition to a well-crystallized hexagonal apatite phase, are shown. Quantification of the phases, as described previously, enables the calculation of the Ca/P ratio. Results are shown in Table 2, as well as the Ca/P ratio calculated for the synthesized powders. The fluoride content of the synthesized powders is also included in Table 2. Each of the analyzed synthesized powders, except the x ) 2.0 sample, are calcium-deficient hydroxyfluorapatites. Those samples in which >50% of the OH- ions are substituted by F- ions have a Ca/P ratio closer to 1.67 than those with a fluoride occupancy of
