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

• Main Authors: Francesco Capitelli, Ferdinando Bosi, Silvia C. Capelli, Francesco Radica, Giancarlo Della Ventura
• Format: Article
• Language: English
• Published: MDPI AG 2021-02-01
• Series: Crystals
• Subjects: whitlockite; calcium phosphates; WDS; NDP; XRD; FTIR
• Online Access: https://www.mdpi.com/2073-4352/11/3/225

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 (Ca_8.682Na_0.274Sr_0.045)_Σ9.000(Ca_0.034□_0.996)_Σ1.000(Mg_0.533Fe²⁺_0.342Mn²⁺_0.062Al_0.046)_Σ0.983(P_1.006O₄)₆[PO₃(OH_0.968F_0.032)_Σ1.000]. Crystal-structure refinement, in the space group <i>R</i>3<i>c</i>, converged to <i>R</i>1 = 7.12% using 3273 unique reflections from NDP data and to <i>R</i>1 = 2.43% using 2687 unique reflections from XRD data. Unit cell parameters from NDP are <i>a</i> = 10.357(3) Å, <i>c </i>= 37.095(15) Å and <i>V</i> = 3446(2) ų, and from XRD, the parameters are<i> a</i> = 10.3685(4) Å, <i>c </i>= 37.1444(13) Å and <i>V</i> = 3458.2(3) ų. 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 Ca₃(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 Ca₃(PO₄)₂ is provided, based on the XRD/NDP and FTIR results.