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

Abstract

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)Σ9.000(Ca0.034□0.996)Σ1.000(Mg0.533Fe2+0.342Mn2+0.062Al0.046)Σ0.983(P1.006O4)6[PO3(OH0.968F0.032)Σ1.000]. 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) Å, c = 37.095(15) Å and V = 3446(2) Å3, and from XRD, the parameters are a = 10.3685(4) Å, c = 37.1444(13) Å and V = 3458.2(3) Å3. 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.

Peculiarities of the formation, structural and morphological properties of zinc whitlockite (Ca18Zn2(HPO4)2(PO4)12) synthesized via a phase transformation process under hydrothermal conditions

“…In the HETCOR spectrum measured using 2 ms contact time the correlation from the impurity phase is masked, though the correlation between the P(2) phosphorus and HPO 4 proton is now better seen. According to the neutron diffraction data published very recently by Capitelli et al 4 for natural WH, the HP(1)O 4 tetrahedron may flip 180° along the c -axis. This results in the displacement of P(1) along the c axis and two different positions of protons – downward ( −c ) and upward ( +c ).…”

“…Besides being a biomineral existing in the human body, Mg-WH can also be found in nature. The first studies on the structural properties of natural and synthetic Mg-WH were performed by Gopal et al 2,3 in the early 1970s; however, just recently a neutron diffraction study of natural WH was reported by Capitelli et al 4 It is noteworthy that due to the hardly distinguishable X-ray diffraction patterns of β-TCP and Mg-WH, these two names are often used in the literature interchangeably and synonymously. Nevertheless, the crystal structures of β-TCP and Mg-WH differ significantly.…”

“…Nevertheless, the crystal structures of β-TCP and Mg-WH differ significantly. 2 Unlike WH, pristine β-TCP contains only Ca 2+ cations and neither β-TCP nor its Mgsubstituted version contains HPO 4 2− such a powerful technique for structural analysis as solidstate NMR. The rare exceptions of the use of solid-state NMR are two recent studies on Mg-WH.…”

Spectroscopic and Structural Properties of β-Tricalcium Phosphates Ca9RE(PO4)7 (RE = Nd, Gd, Dy)

“…β-TCP is rhombohedral with space group R3c (a = b = 10.4352(2) Å; c = 37.4029(5) Å; V = 3527.26 Å 3 [20]) and, at T > 1125 • C, changes into the polymorph phase α-TCP, monoclinic in P2 1 /a (a = 12.887(2) Å; b = 27.280(4); c = 15.219(2) Å; β = 126.20(1) • ; V = 4317.52 Å 3 [21]); the β → α transition in TCP may occur later at a higher temperature in the presence of RE 2 O 3 reactants [6], as in the case of the present RE-TCP samples calcined at 1200 • (see Synthesis paragraph) but displaying a β-TCP structure as described later. β-TCP has its natural counterpart in the isotypic mineral whitlockite, ideally Ca 9 Mg(PO 4 ) 6 [PO 3 (OH)], rhombohedral in R3c (a = b = 10.357(3) Å, c = 37.095 (15) Å and V = 3446(2) Å 3 ) [22]; because of isotypicity among natural whitlockite and β-TCP, the latter is often indicated as synthetic whitlockite, and its structure is indicated as whitlockite-type. TCP presents a third polymorph, the high-pressure form known as γ-TCP, rhombohedral in R-3m, usually coming from transformation both at high temperature and pressure of β-TCP [23]; it is also found with mineral name tuite in chondritic meteorites, as a result of shock impact of other calcium phosphate minerals [24].…”

“…In the isotypic mineral whitlockite, despite the presence of Mg 2+ replacing Ca 2+ at the M5 octahedral site, there are no substantial modifications in number and coordination of sites [22]. Eventually, M4 coordination number possibly increases up to six, due to a second group of three oxygen atoms related for symmetry found over 2.81 Å [22], providing very weak contributions in terms of valence bonds [33].…”