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Hydrogen-substituted β -tricalcium phosphate synthesized in organic media

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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 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.

No MeSH data available.


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XRD patterns of β-TCP platelets before, during and after calcination up to 1273 K (RT: room temperature). Note that peak shifts are due to thermal expansion of the crystal lattice. In addition to the predominant β-TCP phase (non-labelled peaks), γ-CPP was observed between 823 and 1123 K whereas β-CPP appeared at 1123 K and also remained stable up to 1273 K after cooling to room temperature.
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fig3: XRD patterns of β-TCP platelets before, during and after calcination up to 1273 K (RT: room temperature). Note that peak shifts are due to thermal expansion of the crystal lattice. In addition to the predominant β-TCP phase (non-labelled peaks), γ-CPP was observed between 823 and 1123 K whereas β-CPP appeared at 1123 K and also remained stable up to 1273 K after cooling to room temperature.

Mentions: In order to examine the thermal stability of the Ca-deficient β-TCP phase, XRD patterns were acquired during and after calcination, which revealed the presence of γ-CPP (PDF# 00-017-0499) above 773 K and β-CPP (PDF# 04-009-3876) between 1073 and 1273 K as well as after returning to room temperature (Fig. 3 ▸). Note that the patterns obtained in situ during stepwise heating could not be refined due to the unknown crystal structure of the γ-CPP phase. Extensive peak shifts due to thermal expansion of the unit cells and the sample holder, the latter resulting in a sample height displacement error, were also observed. The Ca/P ratios as well as phase fractions determined by Rietveld refinement before and after calcination at 1273 K are given in Table 3 ▸. Before calcination, the overall Ca/P ratio (1.437 ± 0.003), calculated based on the weight fraction and molecular mass of each phase, was slightly lower than the refined β-TCP Ca/P ratio (1.445 ± 0.001) due to the presence of monetite (Ca/P = 1.0). After calcination, the refinement determined a β-TCP Ca/P ratio equal to the stoichiometric value of 1.5. This increase in the β-TCP Ca/P ratio was compensated for by the appearance of approximately 10 wt% β-CPP (Ca/P = 1.0, Table 3 ▸), where the resulting overall Ca/P ratio was in close agreement with the overall Ca/P ratio determined before calcination (difference: 0.2%). In summary, the thermal treatment induced a phase separation of Ca-deficient hydrogen-substituted β-TCP, along with the small quantities of monetite, into stoichiometric β-TCP and β-CPP, while maintaining the bulk Ca/P ratio. Good agreement of the Ca/P ratios determined from stoichiometric phase quantities after thermal treatment and from the structure refinement of hydrogen-substituted β-TCP prior to calcination thus corroborates the accuracy of the hydrogen-substituted refinement model.


Hydrogen-substituted β -tricalcium phosphate synthesized in organic media
XRD patterns of β-TCP platelets before, during and after calcination up to 1273 K (RT: room temperature). Note that peak shifts are due to thermal expansion of the crystal lattice. In addition to the predominant β-TCP phase (non-labelled peaks), γ-CPP was observed between 823 and 1123 K whereas β-CPP appeared at 1123 K and also remained stable up to 1273 K after cooling to room temperature.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5134762&req=5

fig3: XRD patterns of β-TCP platelets before, during and after calcination up to 1273 K (RT: room temperature). Note that peak shifts are due to thermal expansion of the crystal lattice. In addition to the predominant β-TCP phase (non-labelled peaks), γ-CPP was observed between 823 and 1123 K whereas β-CPP appeared at 1123 K and also remained stable up to 1273 K after cooling to room temperature.
Mentions: In order to examine the thermal stability of the Ca-deficient β-TCP phase, XRD patterns were acquired during and after calcination, which revealed the presence of γ-CPP (PDF# 00-017-0499) above 773 K and β-CPP (PDF# 04-009-3876) between 1073 and 1273 K as well as after returning to room temperature (Fig. 3 ▸). Note that the patterns obtained in situ during stepwise heating could not be refined due to the unknown crystal structure of the γ-CPP phase. Extensive peak shifts due to thermal expansion of the unit cells and the sample holder, the latter resulting in a sample height displacement error, were also observed. The Ca/P ratios as well as phase fractions determined by Rietveld refinement before and after calcination at 1273 K are given in Table 3 ▸. Before calcination, the overall Ca/P ratio (1.437 ± 0.003), calculated based on the weight fraction and molecular mass of each phase, was slightly lower than the refined β-TCP Ca/P ratio (1.445 ± 0.001) due to the presence of monetite (Ca/P = 1.0). After calcination, the refinement determined a β-TCP Ca/P ratio equal to the stoichiometric value of 1.5. This increase in the β-TCP Ca/P ratio was compensated for by the appearance of approximately 10 wt% β-CPP (Ca/P = 1.0, Table 3 ▸), where the resulting overall Ca/P ratio was in close agreement with the overall Ca/P ratio determined before calcination (difference: 0.2%). In summary, the thermal treatment induced a phase separation of Ca-deficient hydrogen-substituted β-TCP, along with the small quantities of monetite, into stoichiometric β-TCP and β-CPP, while maintaining the bulk Ca/P ratio. Good agreement of the Ca/P ratios determined from stoichiometric phase quantities after thermal treatment and from the structure refinement of hydrogen-substituted β-TCP prior to calcination thus corroborates the accuracy of the hydrogen-substituted refinement model.

View Article: PubMed Central - HTML - PubMed

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 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.

No MeSH data available.


Related in: MedlinePlus