Crystal structure analysis of beta-tricalcium phosphate Ca-3(PO4)(2) by neutron powder diffraction

The crystal structure of sintered beta-tricalcium phosphate, Ca-3(PO4)(2), was refined using a high-resolution neutron powder diffraction data and the Rietveld method. This material was confirmed to have a rhombohedral structure (space group R3c, Z = 21). Unit-cell parameters with higher precision (a = b = 10.4352(2) Angstrom, c = 37.4029(5) Angstrom, alpha = beta = 90degrees, and gamma = 120degrees in the hexagonal setting) and positional parameters for oxygen with equal precision were obtained by the neutron powder diffraction technique, compared with the single-crystal X-ray diffraction data by Dickens et al. (J. Solid State Chem. 10 (1974) 232). The site Ca(4) with atomic coordinates [0.0, 0.0, -0.0851(6)] was confirmed to be very different from the other four Ca sites: The position Ca(4) is threefold coordinated with oxygen atoms, and has lower occupancy factor of 0.43(4), and a higher isotropic thermal parameter. On the contrary, each of the Ca(1), Ca(2), Ca(3), and Ca(5) is fully occupied by one Ca atom and these positions are coordinated with seven, eight, eight, and six oxygen atoms, respectively. The bond valence sums of Ca(4) and Ca(5) are lower (0.7) and higher (2.7), respectively, than the others (1.8-2.1). (C) 2003 Elsevier Inc. All rights reserved.