Effect of Carbonate Contents on Crystal Structure of A-Type Carbonate Apatites

A-type carbonate apatites with different CO sub 3 contents were obtained by heating hydroxyapatite powder in dry carbon dioxide atmosphere at 1173 K for 1 to 64 hours. The replacement of CO sub 3 ions lowered the crystal symmetry to hexagonal (P6 sub 3 /m) - > hexagonal (P-6) - > monoclinic (Pb) in that order. The a-axis parameters were linearly increased with the increase of CO sub 3 ions, while the b- and c-axis parameters were varied little. The lattice parameters of the A-type carbonate apatite completely replaced by CO sub 3 ions were refined to be a = 0.9572(1), b = 1.9084(1) and c = 0.6877(1). The splitting of IR absorption bands attributed to a PO sub 4 ion also supported the lowering of crystal symmetries.