Probing the limit of magnesium uptake by β-tricalcium phosphate in biphasic mixtures formed from calcium deficient apatites

•Aqueous co-precipitation of calcium deficient apatites with excess magnesium (Mg2+) additions.•Heat treatments beyond 800 °C results in the formation of biphasic apatite mixtures.•Mg2+ gets accommodated at the β-Ca3(PO4)2 lattice of biphasic mixtures.•Mg2+ additions exceeding stoichiometric value (Ca/P>1.67) results in its formation as MgO.•Mg2+ occupancy at β-Ca3(PO4)2 lattice delays its allotropic conversion α-Ca3(PO4)2 till 1350 °C.

A series of magnesium doped non-stoichiometric calcium deficient apatites were synthesized through an aqueous precipitation route. The resultant structural changes during heat treatment were investigated by X-ray diffraction, Raman and FT-IR spectroscopy and Rietveld refinement. The results confirmed the formation of biphasic mixtures comprising Ca10(PO4)6(OH)2 and β-Ca3(PO4)2 after heat treatment at 1000 °C with the preferential occupancy of Mg2+ at the crystal lattice of β-Ca3(PO4)2. The concentration of Mg2+ uptake in β-Ca3(PO4)2 is limited till reaching the stoichiometric ratio of (Ca+Mg)/P=1.67 and beyond this stoichiometric value [(Ca+Mg)/P>1.67], Mg2+ precipitates as Mg(OH)2 and thereafter gets converted to MgO during heat treatment. Any kind of Mg2+ uptake in the crystal lattice of Ca10(PO4)6(OH)2 is discarded from the investigation.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideSaturation limit of Mg2+ occupancy at the Ca2+(5) site of β-TCP occurs when (Ca+Mg)/P ratio exceeds 1.67 and thereafter no change in the lattice parameters is noticed.Figure optionsDownload full-size imageDownload as PowerPoint slide