Influences of strontium on the phase composition and lattice structure of biphasic calcium phosphate

Hydroxyapatite (HAp) and B-tricalcium phosphate (B-TCP) are widely used in bone graft materials, because they have excellent biocompatibility. However, the degradation rates of HAp and B-TCP do not match the rate of osteogenesis, which limits their clinical applications. Therefore, forming biphasic calcium phosphate (BCP) is a promising method to control the biodegradation rate of the material. In addition, strontium (Sr) is an important element that promotes osteogenesis and inhibits bone absorption. Therefore, this study focused on the effects of Sr in the as-synthesized BCP with varied Sr concentrations of 1, 5, and 15 mol%, which are synthesized by chemical precipitation and a high-temperature calcination method. In this study, we assume that Sr is doped into Ca(5) of B-TCP and Ca(1) & Ca(2) of HAp during the Rietveld refinement. On the basis of the Rietveld refinement results obtained from X-ray diffraction (XRD) patterns, with increasing Sr concentration, the mass percentage of HAp decreases first, which is due to the distortion of the HAp structure, resulting in a decrease in HAp stability and phase variation from HAp to B-TCP. However, when the concentration of Sr reaches 15 mol%, the mass percentage of HAp increases, which may be because of the phase transformation from B-TCP to calcium-deficient apatite by the diffusion of Ca. In addition, the increase in the lattice constants and volumes indicates that Sr has been successfully doped into the crystal lattices of both B-TCP and HAp, since the ion radius of Sr2+ (0.113 nm) is larger than the ion radius of Ca2+ (0.099 nm). As shown in the X-ray photoelectron spectroscopy (XPS) results, the intensities of Sr3d and Sr3p increase with increasing Sr concentrations, which indicates that Sr is not only on the surface of the sample but also in the crystalline structures of both B-TCP and HAp.

Automated summary

This study focused on the effects of Sr in BCP, with results showing that increasing Sr concentrations can affect HAp stability and phase transformation. Sr was successfully doped into the crystal lattices of B-TCP and HAp, with a larger ion radius than Ca2+, leading to lattice constant and volume increases. The intensities of Sr3d and Sr3p in XPS results indicated Sr presence in both B-TCP and HAp crystalline structures.