Effect of strontium substitution on the composition and microstructure of sol–gel derived calcium phosphates

Sr-doped calcium phosphates have been prepared by sol–gel chemistry. All samples exhibit two phases: hydroxyapatite (HAp) and tricalcium phosphate (b-TCP). With respect to undoped sample, the Sr-doped samples exhibit higher proportion of b-TCP phase but the quantity appears to be quite independent of the doping level. To explain the mismatch with the nominal stoichiometry, the presence of amorphous CaO and SrO compounds have been postulated and their proportions evaluated. The insertion of Sr2+ ions in the two crystalline phases HAp and b-TCP is almost total for low doping levels but quite incomplete for the highest doping level. The majority of the inserted Sr2+ ions are in the b-TCP phase. Considering the acknowledged beneficial effect of Sr2+ on the bone regeneration process, the effective partial substitution of Sr in biphasic calcium phosphate makes these materials very interesting for clinical applications. The Sr-substituted HAp and b-TCP cell parameters agree fairly well with the Vegard's law and Sr2+ ions substitute preferentially for Ca2+ in the Ca2 site for hydroxyapatite and in the Ca4 site for b-TCP. The microstructural parameters confirm the previous observation and give a new evidence of clear stabilizing effect of Sr2+ ions towards the b-TCP structure.