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.
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 ▸).