Hydrogen-substituted β-tricalcium phosphate synthesized in organic media

Abstract

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 HPO₄^2- 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 Ca_{21 - x}(HPO₄)_2x(PO₄)_{14 - 2x}, where x = 0.80 ± 0.04, and may have implications for resorption properties of bone regenerative materials.