Neutron and XRD Single-Crystal Diffraction Study and Vibrational Properties of Whitlockite, the Natural Counterpart of Synthetic Tricalcium Phosphate

A crystal chemical investigation of a natural specimen of whitlockite, ideally Ca9Mg(PO4)(6)[PO3(OH)], from Palermo Mine (USA), was achieved by means of a combination of electron microprobe analysis (EMPA) in WDS mode, single-crystal neutron diffraction probe (NDP) and single-crystal X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. The crystal-chemical characterization resulted in the empirical formula (Ca8.682Na0.274Sr0.045)(sigma 9.000)(Ca-0.034(0.996))(sigma 1.000)(Mg0.533Fe0.3422+Mn0.0622+Al0.046)(sigma 0.983)(P1.006O4)(6)[PO3(OH0.968F0.032)(sigma 1.000)]. Crystal-structure refinement, in the space group R3c, converged to R1 = 7.12% using 3273 unique reflections from NDP data and to R1 = 2.43% using 2687 unique reflections from XRD data. Unit cell parameters from NDP are a = 10.357(3) angstrom, c = 37.095(15) angstrom and V = 3446(2) angstrom(3), and from XRD, the parameters are a = 10.3685(4) angstrom, c = 37.1444(13) angstrom and V = 3458.2(3) angstrom(3). NDP results allowed a deeper definition of the hydrogen-bond system and its relation with the structural unit [PO3(OH)]. The FTIR spectrum is very similar to that of synthetic tricalcium phosphate Ca-3(PO4)(2) and displays minor band shifts due to slightly different P-O bond lengths and to the presence of additional elements in the structure. A comparison between whitlockite, isotypic phases from the largest merrillite group, and its synthetic counterpart Ca-3(PO4)(2) is provided, based on the XRD/NDP and FTIR results.