Crystal structure refinement of A-type carbonate apatite by X-ray powder diffraction

Complete carbonate substitution at A-sites (OH) of low-crystallinity hydroxyapatite with near stoichiometric composition (i.e. Ca/P ratio of 1.65) was achieved by heating in a dry carbon dioxide flow at 1173 K for 64 h. The carbonate content was analysed by thermogravimetry and infrared absorption spectrometry; the chemical composition was determined to be Ca_9.9±0.1(PO₄)_6.00±0.1(CO₃)_0.9±0.1. The crystal structure and atomic configuration of the carbonate ion were determined by Rietveld refinement using X-ray powder diffraction data. This analysis revealed that the space group was monoclinic Pb with a = 0.9571(1), b = 1.9085(2), c = 0.68755(3) nm and γ = 119.847(7)°. The triangular planes of the carbonate ions were oriented parallel to the c- and a-axes, though there were two independent carbonate sites with occupancy factors of 0.56(1) and 0.34(1), where the triangles were oppositely rotated about the corresponding carbon atoms by 23° and −18°, respectively. The arrangement of the ions was disordered, which explains the lack of a thermal phase transition below 623 K.