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; × ≥ 0.5) in the composition series Ca₁₀(PO₄)_6-y[(CO₃) _×+(3/2)y(OH)_2-2×], 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 identified: (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, × = 0.14, y = 0.0, a = 9.4468(4), c = 6.8806(4) Å, and R (residual index of structure refinement) = 0.025; type B C-OHAp, × = 0.0, y = 0.17, α = 9.4234(2), c = 6.8801(3) Å, and R = 0.025; and type A-B CAp, × = 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 × 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 Ca1O_n polyhedron, and rotation of the PO₄ tetrahedron about the P-O1 bond. The progressive increase in the α 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).