Silver-substituted hydroxyapatite: Mechanochemical synthesis and thermal stability

Silver-substituted hydroxyapatites have been prepared by the mechanochemical method in a planetary ball mill. In order to prepare a series of samples of the Ca10−xAgx (PO4)6(OH)2−x composition, the concentration of silver was varied in the range x = 0–2. The resultant samples were characterized by means of X-ray diffraction, FTIR spectroscopy, and simultaneous thermal analysis. It was shown that the single-phase substituted hydroxyapatites were achieved with silver concentrations of up to х = 1.5. An increase in the concentration of silver results in an increase in the volume and the unit-cell parameters of crystals. At a concentration x of 2.0, the sample contains an impurity phase of silver phosphate. The thermal stability of the as-synthesized silver-substituted samples was shown to directly depend on the degree of substitution of calcium cations by silver cations. The decomposition temperature decreases as the degree of substitution increases. Upon heating of the substituted samples, silver is gradually released from hydroxyapatite with the formation of a metallic nanosized silver phase, followed by the structural transformation of the hydroxyapatite lattice into silver-substituted tricalcium phosphate at a higher temperature. In this case, the introduction of silver cations results in a decrease in the values of the lattice parameters of tricalcium phosphate, showing a non-linear dependence on the silver concentration.