Strontium-Substituted α-TCP: Structure, Stability, and Reactivity in Solution

& alpha;-Tricalcium phosphate (& alpha;-TCP) is widelyused as acomponent of bone cements, and many efforts have been made to dopeit with strontium ion (Sr), which is known for its beneficial rolein bone tissue. However, the range of possible substitution of strontiumfor calcium (Ca) into & alpha;-TCP, as well as its effect on the & alpha;-TCPstructure, has not been clarified yet. Herein, we investigate thissubstitution through the examination of & alpha;-TCP synthesized athigh temperatures in the presence of increasing amounts of strontiumaccording to two different routes: 1-step and 2-steps. The results show that Sr can enter into an & alpha;-TCPstructure up to about 10 atom % and substitutes for calcium mostlyat specific cation sites, namely, M(5), M(11), and M(17), characterizedby relatively low bond valence sums and long mean Ca-O distance.Strontium presence stabilizes & alpha;-TCP delaying its transformationinto octacalcium phosphate and hydroxyapatite in H3PO4, as well as in physiological solution. Although the two methodsof synthesis provide similar structural results, the products of 1-step synthesis display a slightly smaller crystallitesize and greater solubility and, as a consequence, a faster hydrolysisreaction.& alpha;-Tricalcium phosphatepreparation through the solid-statereaction is described following two different procedures. The investigationof the range of possible substitution of strontium for calcium into & alpha;-TCP as well as its effect on the & alpha;-TCP structure ispresented. Furthermore, it is shown that strontium presence stabilizes & alpha;-TCP and delays its hydrolysis reaction into octacalcium phosphateand hydroxyapatite.