Accommodation of the carbonate ion in apatite: An FTIR and X-ray structure study of crystals synthesized at 2–4 GPa

Carbonated hydroxylapatite (C-OHAp) and carbonate apatite (CAp; x 0.5) in the composition series Ca10(PO4)6–y[(CO3)x+(3/2)y(OH)2–2x], x = 0.0–0.7, y = 0.0–0.6, have been synthesized at 2–4 GPa, and studied by FTIR spectroscopy and single-crystal X-ray diffraction. Three structural locations for the carbonate ion have been identifi ed: (1) apatite channel, oriented with two oxygen atoms close to the c-axis (type A1); (2) close to a sloping face of the PO4 tetrahedron (type B); and, (3) stuffed channel position (type A2). Type A1 and B carbonate are equivalent to type A and B CAp of bone and enamel, whereas type A2 is a high-pressure feature. In type A CAp, ordering of type A1 carbonate within the apatite channel results in space group P3–; all other apatites studied have average structures with P63/m symmetry. Results for three new structures are: type A C-OHAp, x = 0.14, y = 0.0, a = 9.4468(4), c = 6.8806(4) Å, and R (residual index of structure refi nement) = 0.025; type B C-OHAp, x = 0.0, y = 0.17, a = 9.4234(2), c = 6.8801(3) Å, and R = 0.025; and type A-B CAp, x = 0.7, y = 0.5, a = 9.4817(6), c = 6.8843(3) Å, and R = 0.025. A fourth structure analysis suggests that the type A-B CAp exchanges some of its channel carbonate with OH during room-temperature storage in nujol oil, with x and y reduced to 0.6 and 0.4, respectively. Local structural adjustments to accommodate the carbonate ion in the c-axis channel of OHAp include dilation of the channel, contraction of the Ca1On polyhedron, and rotation of the PO4 tetrahedron about the P-O1 bond. The progressive increase in the a unit-cell edge length with increase in carbonate content of type A CAp is readily attributed to the dilation of the apatite channel. Carbonate-for-phosphate substitution in OHAp (type B CAp) requires displacement of O3 along ±[001] and, thus, results in expansion of c (and contraction of a). FLEET ET AL.: CARBONATE APATITE 1423 [Ca10(PO4)6CO3] appeared to be pseudo-hexagonal with the monoclinic space group Pb (Elliott et al. 1980). Alternatively, Suetsugu et al. (1998, 2000) reported that the space group of type A CAp grown from a carbonate fl ux at 0.55 kbar was P6–. The disordered average X-ray structure in Suetsugu et al. (2000) was interpreted and refi ned to show that the equilateral triangular cluster of the channel carbonate ion was bisected by the c-axis (see Fig. 1a). More recently, study of CAp synthesized at high pressure, but with an IR signature similar to that of type A CAp in bone and enamel, revealed the space group P3– (Fleet and Liu 2003). The type A carbonate ion was ordered along the apatite channel at z = 0.5, and oriented with two of its oxygen atoms close to the c-axis. Fleet and Liu (2003) referred to this orientation (with a bisector of the CO3 triangle normal to the c-axis) as the “closed” confi guration (e.g., Fig. 1b), and contrasted it with the “open” vertical (a bisector parallel to c-axis) confi guration of the channel carbonate ion in type A CAp studied by Suetsugu et al. (2000). Powder XRD Rietveld structure refi nement of a synthetic Ca-defi cient type B CAp (space group P63/m) suggested that the type B carbonate ion occupied a vertical (parallel to the c-axis) face of the substituted phosphate ion (Ivanova et al. 2001). Very recently, crystals of type A-B CAp (space group P63/m) were grown at high pressure in the presence of excess CaCO3 (experiment PC55; Fleet and Liu 2004). Single-crystal X-ray structural analysis revealed three locations for the carbonate ion: (1) the channel position of the type A carbonate of Fleet and Liu (2003; re-labeled type A1 carbonate); (2) close to a sloping face of the PO4 tetrahedron and consistent with type B carbonate as characterized by FTIR (Fig. 1c); and (3) a second (stuffed) channel position characterized by IR bands at 1563 and 1506 cm, which was present in subordinate amounts and appeared to charge-compensate the substitution of PO4 by CO3 (type A2 carbonate; Fig. 1c). We presently report on the X-ray structural analysisof type B C-OHAp, type A C-OHAp, and a second preparation of the complex type A-B CAp (high-pressure experiment PC18), interpret the Fourier transform infrared (FTIR) spectra for a wide range in composition of CAp and C-OHAp synthesized at high pressure, and discuss the accommodation of the carbonate ion by the apatite structure. Study of two crystals from PC18 provides independent confi rmation of the three structurally distinct locations for carbonate in type A-B CAp and indicates incipient hydroxyl-for-carbonate exchange at room-temperature. EXPERIMENTAL PROCEDURES Single crystals of carbonate apatite and carbonated hydroxylapatite were prepared by direct reaction of stoichiometric amounts of Ca2P2O7 (Alfa Aesar; 98%), CaO (Alfa Aesar; 99.95%), and CaCO3 (Alfa Aesar; 99.99%) at high pressure and temperature in an end-loaded piston-cylinder apparatus. Calcium fl uoride (CaF2) was added to a single experiment (PC58) for the preparation of carbonated FAp (CFAp). Starting materials were mixed in the proportions indicated in Table 1. Calcium pyrophosphate, CaO, and CaF2 were dried at 1000 °C, 12 hours, and CaCO3 at 200 °C, 12 hours. In addition, furnace parts were previously fi red at 1000 °C in air. The starting mixture was encapsulated in a sealed platinum tube with a diameter of 5 mm and a height of 12 mm, which was separated by crushable MgO tubing from a graphite tube. The pressure was calibrated from the melting of dry NaCl at 1050 °C (Bohlen 1984) and the quartz coesite transformation at 500 °C (Bohlen and Boettcher 1982). Temperature was measured by inserting a Pt-Pt90%Rh10% thermocouple into the high-pressure cell; it was allowed to fl uctuate by about ±5 °C to aid crystal growth. The experiments were initially over-pressurized by about (a)