The present study was aimed at investigating the co-substitution of sodium and carbonate in strontium hydroxyfluorapatites because these compounds have many similarities to the mineral phase of bones. Herein, three series of sodium- and carbonate-doped strontium hydroxyfluorapatites were prepared via a co-precipitation method. Chemical analysis via inductively coupled plasma-atomic emission spectrometry (ICP-AES), X-ray diffraction (XRD), Rietveld refinement, Fourier transform infrared spectroscopy (FTIR), ³¹P nuclear magnetic resonance (NMR), and differential and gravimetric thermal analysis (DTA–GTA) characterizations were employed for the identification of the obtained products. Sodium- and fluoride-doped strontium hydroxyapatite was found to be a pure apatitic phase containing ionic vacancies. Rietveld analysis showed a significant decrease in the a lattice parameter as sodium and fluoride levels increased, whereas in carbonate- and fluoride-doped strontium hydroxyapatite, both the a and the c lattice parameters decreased as the amounts of carbonate that were added increased. The obtained powders were a B-type carbonate apatite with vacancies. The co-substitution of sodium and carbonate in the hydroxyfluorapatite structure typically gave rise to B-type carbonate apatite with vacancies. Thermal investigation in the range of 500–1200 °C revealed that decomposition occurred at a temperature of about 800 °C, and Sr₃(PO₄)₂ was formed.

L'étude structurale d'une série de solutions solides Ca _{10– x} Mg _x (PO ₄ ) ₆ F ₂ , préparée par précipitation en présence de 2,5 ; 5,0 ; 10,0 ; 15,0 ; 20,0 et 30,0 atom% de magnésium a été réalisée par affinement Rietveld. Les analyses chimiques montrent que les taux de magnésium contenus dans les échantillons sont respectivement de 2,3 ; 4,7 ; 9,6 ; 14,4 et 18,5 atom%. Ceux déterminés par affinement structural à partir des spectres RX indiquent une limite de substitution de l'ordre de 16,8 atom%. Pour des faibles teneurs, Mg occupe préférentiellement le site S(II), alors que, pour des teneurs supérieurs à 10 atom%, la répartition devient quasi statistique. La limite de substitution ainsi que la répartition cationique entre les deux sites cristallographiques ont été corrélées aux propriétés physico-chimiques des ions. .

Fluorapatite and magnesium-substituted fluorapatite powders synthesized by the precipitation method were pressureless sintered in the range 900–1300 °C. The results showed that both materials exhibited a good sinterability. Concerning fluorapatite, a relative density of about 97% was attained at 1050 °C for 1 h. Although the incorporation of Mg into the apatite framework induced a slight decrease in the density, the substituted samples presented slightly higher mechanical properties. The maximum values of flexural strength, fracture toughness, hardness and Young's modulus of these latter samples were about 50.8±4.0 MPa, 1.36±0.10 MPam1/2, 121.9±2.4 MPa and 650±8 Hv, respectively.

The present study describes the influence of potassium and hydroxyl substitutions on the structural, thermal and mechanical properties of fluorapatite bioceramics. A set of non-stoichiometric ion-substituted compounds, with a chemical formula of Ca_10−xK_x(PO₄)₆F_(2−2x)(OH)_x with 0 ≤ x ≤ 1 synthesized by the wet precipitation method, were found to be single-phase apatites crystallizing in the hexagonal P6₃/m space group. The structural parameters, as well as the crystallite sizes, increased accordingly to the amount of added dopant-ions. The thermal behavior of these compounds, studied within the temperature range 500–1200 °C, indicated a partial decomposition of the apatitic phase and its transformation to tricalcium phosphate β-Ca₃(PO₄)₂ at temperatures exceeding 750 °C. A relative density of the sintered samples achieved the highest value with x = 0.25 and reached about 95% after sintering at 1050 °C for 1 h. The microstructures of the sintered samples were of a trans-granular aspect and experienced an increase in the radius of their pores as x increased. The prepared bioceramic materials were mechanically characterized by means of Young’s modulus, flexural strength and fracture toughness measurements. The overall trend of these parameters evolved comparably to the relative density, and the maximum values obtained for x = 0.25 were measured to be 96 MPa, 47 MPa and 1.14 MPa·m^1/2, respectively.