Hydrogen-substituted β-tricalcium phosphate synthesized in organic media

• Published in: Acta crystallographica Section B, Structural science, crystal engineering and materials Structural science, crystal engineering and materials, 2016-12, Vol.72 (6), p.875-884
• Main Authors: Stähli, Christoph, Thüring, Jürg, Galea, Laëtitia, Tadier, Solène, Bohner, Marc, Döbelin, Nicola
• Format: Article
• Language: English
• Subjects: bone substitute; calcium deficiency; Crystal structure; Crystallography; Engineering Sciences; Ethylene glycol; Infrared spectroscopy; Materials; Mathematical models; Phosphates; Platelets; Research Papers; Rietveld refinement; Synthesis; whitlockite; X-ray diffraction; β-tricalcium phosphate
• ISSN: 2052-5206; 2052-5192; 2052-5206
• DOI: 10.1107/S2052520616015675

β‐Tricalcium phosphate (β‐TCP) platelets synthesized in ethylene glycol offer interesting geometries for nano‐structured composite bone substitutes but were never crystallographically analyzed. In this study, powder X‐ray diffraction and Rietveld refinement revealed a discrepancy between the platelet structure and the known β‐TCP crystal model. In contrast, a model featuring partial H for Ca substitution and the inversion of P1O4 tetrahedra, adopted from the whitlockite structure, allowed for a refinement with minimal misfits and was corroborated by HPO42− absorptions in Fourier‐transform IR spectra. The Ca/P ratio converged to 1.443 ± 0.003 (n = 36), independently of synthesis conditions. As a quantitative verification, the platelets were thermally decomposed into hydrogen‐free β‐TCP and β‐calcium pyrophosphate which resulted in a global Ca/P ratio in close agreement with the initial β‐TCP Ca/P ratio (ΔCa/P = 0.003) and with the chemical composition measured by inductively coupled plasma (ΔCa/P = 0.003). These findings thus describe for the first time a hydrogen‐substituted β‐TCP structure, i.e. a Mg‐free whitlockite, represented by the formula Ca21 − x(HPO4)2x(PO4)14 − 2x, where x = 0.80 ± 0.04, and may have implications for resorption properties of bone regenerative materials. 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.