Crystal Structure of Calcium-Deficient Carbonated Hydroxyapatite. Thermal Decomposition

Full Rietveld refinement of the crystal structure of the synthetic calcium-deficient carbonated apatite Ca13.40[Ca25.90 (NH4)0.10] [(PO4)4.95(CO3)1.05(H2O)0.30][(OH)1.65(H2O)0.45] (space group P63/m; a=9.437(1), c=6.888(1) ?; Z=1; Rwp=5.23%) was carried out using X-ray powder diffraction data. The use of the model with the split position of O3 atom made it possible to find two orientations of CO3 triangles sharing one of their edges. They occupy randomly the adjacent faces of a PO4 tetrahedron that are parallel to the c axis. O3c atoms coordinating carbon atoms are shifted by 0.37 ? from O3p atoms belonging to PO4 tetrahedra. The charge unbalance occurring when [CO3]2- ions replace [PO4]3- groups is primarily compensated by vacancies in Ca1 sites. The studies of the sample thermal decomposition performed by simultaneous thermal analysis and by X-ray diffraction helped to analyze the localization and the amount of lattice water that enhanced the reliability of the structural model.