Comparison of crystal structure parameters of natural and synthetic apatites from neutron powder diffraction
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B.C. Chakoumakos Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6393
N. Papanearchou Physics Department, Florida Atlantic University, Boca Raton, Florida 33431
V. Perdikatsis Institute of Geology & Mineral Exploration, Messogion 70, 11527 Athens, Greece (Received 31 August 2000; accepted 25 June 2001)
A systematic behavior in the crystal structure parameters of natural, synthetic, carbonate, and non-carbonate apatite is revealed from Rietveld refinements of neutron powder diffraction experiments. The results of this work on synthetic carbonate hydroxyapatites (CHAps) are consistent with the mechanism of carbonate substitution on the mirror plane of the phosphate tetrahedron, as it was introduced for the natural carbonate fluorapatite (CFAp). The present comparison shows that the tetrahedral bond lengths P–O1 and P–O2 decrease by 3–4% in all carbonate apatites. The atomic displacement parameters (ADPs) of the tetrahedral (T) and the O3 sites are greater in the carbonate than in the non-carbonate apatites. The atomic positional disorder of the T site (P/C site) is greater in the CFAp than in the CHAps, while the opposite happens at the O3 sites. Finally, the room-temperature ADPs of all of the atoms in the CFAp and CHAps show the same behavior as in the corresponding non-carbonate materials.
I. INTRODUCTION
Calcium phosphate apatites of the formula Ca5(PO4)3X with X a F − ion (fluorapatite), OH− ion (hydroxyapatite), or a Cl− ion (chlorapatite) are materials of multidisciplinary research interest and applications. Coupled substitutions frequently occur in these compounds, where one ion is replaced by another of the same sign, but different charge, and neutrality is maintained by substitutions of ions with dissimilar charges or vacancies elsewhere. Among those, the carbonate hydroxyapatites (CHAps) containing 4% to 6% carbonate by weight are an important, complex system, which has been the subject of much controversy related to the mechanism of the CO3 substitution.1–3 Carbonate ions can substitute in the apatite structure either at the (OH) site (A-type, when prepared at high temperatures), or at the (PO4) site (B-type). Type B carbonate apatites most closely resemble biological apatites. They are generally obtained by precipitation in solutions at the temperature range 50–100 °C (CO3 for PO4 and Na for Ca).4 The inherent biocompatibility of apatite makes it a prominent biomaterial, and the physical and physiological behaviors of apatite ceramics for prosthetic implants and thin film coatings are an active area of research.5–7
The strong correlation between the structure of the apatitic material and physical properties such as solubility, strain, thermal stability, and birefringence has attracted a great deal of research interest in the carbonate apatite structure.1,8–13 It is natural to expect that systematic, detailed structural studies of the various types of mineral and synthetic apatite will lead to a better understanding of the effect of the structural variations on their physical properties an
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