Research Article: Hydrogen-substituted β-tricalcium phosphate synthesized in organic media

Date Published: December 01, 2016

Publisher: International Union of Crystallography

Author(s): Christoph Stähli, Jürg Thüring, Laëtitia Galea, Solène Tadier, Marc Bohner, Nicola Döbelin.


A hydrogen substitution mechanism, previously unknown in pure β-tricalcium phosphate, was discovered in crystals precipitated from ethylene glycol solutions. The structure was described by means of Rietveld refinement of powder X-ray diffraction data and corroborated by chemical analysis and IR spectroscopy.

Partial Text

Calcium phosphates (CaPs) have been widely used as synthetic bone graft substitutes and exhibit excellent biocompatibility, osteoconductivity and a chemical composition similar to bone mineral (LeGeros, 2002 ▸). Sintered hydroxyapatite [HA, Ca5(PO4)3OH], β-tricalcium phosphate [β-TCP, Ca3(PO4)2], or biphasic blends of the two constitute the most common commercially available CaP materials. In particular, β-TCP is of interest owing to its cell-mediated resorbability in vivo (Bohner, 2010 ▸).

This study examined the particular features distinguishing the crystal structure of β-TCP platelets synthesized in ethylene glycol from high-temperature sintered, stoichiometric β-TCP. For this purpose, the structure was analyzed by means of Rietveld refinement of XRD patterns, along with IR spectroscopy and chemical analysis.

This study elucidated the crystal structure of β-TCP platelets precipitated in ethylene glycol. Rietveld refinement of XRD patterns indicated sub-occupied Ca4 and O2 atomic sites compared with the published β-TCP crystal model. In contrast, a model adopted from the whitlockite structure, where Ca is partly substituted by H along with the inversion of P1O4 tetrahedra, resulted in precise and reproducible refinements with stable convergence towards a Ca/P ratio of 1.443 ± 0.003 (n = 36) and was corroborated by the presence of HPO42− absorptions in FTIR spectra. Calcination of the platelets led to phase separation into H-free β-TCP (Ca/P = 1.5) and β-CPP (Ca/P = 1.0) which served as a quantitative verification of the initially refined β-TCP Ca/P ratio. Specifically, the global Ca/P ratio closely matched the β-TCP Ca/P ratio before calcination (ΔCa/P = 0.003) and the chemical composition measured by ICP-MS (ΔCa/P = 0.003). The Ca/P ratio was independent of synthesis parameters including temperature, time, pH, precursor Ca/P ratio and concentration, which indicates a thermodynamically stable phase. These findings describe for the first time a hydrogen-substituted β-TCP structure or, in other words, an Mg-free whitlockite and thus raise the question of the role of hydrogen substitution in β-TCP solubility.

References cited in the supporting information include: Cerruti et al. (2014 ▸), Nuevo et al. (2006 ▸), Ping et al. (2001 ▸) and Tortet (1997 ▸).




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