To increase the biocompatibility of hydroxyapatite (HA), Ca 10 (PO 4 ) 6 (OH) 2 , the Sr substitution of Ca into the HA structure was effected to yield Ca 10−x Sr x (PO 4 ) 6 (OH) 2 (Sr-HA). For medical and dental applications, it is important that Sr-HA is prepared as a thin film so that the Sr fully substitutes the Ca sites in the HA structure and does not form segregated impurities consisting of Sr compounds. If the segregated Sr forms different amounts of different impurities, the

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... n of the Sr into the living body will not be reproducible across different samples. To confirm the Sr substitution into the Ca site in the HA structure, the systematic variation in the lattice constants of the Sr-HA with Sr content was evaluated as the first step. The aand c-axis lengths were found to exhibit a linear relationship with the Sr content for six samples with different Sr contents, indicating that the prepared Sr-HA thin films likely possessed partial Sr substitution into the Ca sites of the HA structure. This result is an important first step in the accurate evaluation of the biological effects of Sr-HA thin films.