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new route, based on the use of ionic calcium phosphate cements, has been developed to synthesize hydroxyapatite. Cements were prepared from tetracalcium phosphate, ␣–tricalcium phosphate, phosphoric acid, strontium nitrate, and water. They were then mixed and shaped. Cements with various amounts of strontium and different (Ca + Sr)/P atomic ratios were prepared. All the materials obtained were coherent and solid. When the amount of strontium was low, mixed hydroxyapatite was obtained at low temperatures. Mixed calcium strontium apatite was obtained after heating. This original method can be of interest in various fields such as biomaterials or the storage of radioactive waste.

I. INTRODUCTION

The term apatite refers to a family of mineral compounds that all crystallize in the hexagonal system in the space group P63/m. The chemical formula of these compounds is Me10(XO4)6(Y)2. The apatitic structure accepts various partial or total substitutions. Me can be a cation: monovalent (Na+, . . .), bivalent (Ca2+, Sr2+, Pb2+, Mg2+, Ba2+, etc.), trivalent (La3+, Eu3+, etc.) or a vacancy; XO 4 can be an anionic group: monovalent (CO3F−, etc.), bivalent (HPO42−, CO32−, etc.), trivalent (AsO43−, PO43−, etc.), or tetravalent (SiO44−, etc.); Y can be an anion: monovalent (O2−, OH−, F−, Cl−, Br−, I−, etc.), bivalent (CO32−, S22−, etc.) or a vacancy.1 No apatites exist that contain vacancies in XO4 sites. A nonstoichiometric apatite is an apatite that contains vacancies in the cationic sites and whose atomic ratio Me/XO4 is below 1.67. The apatite Ca10(PO4)6(OH)2 is called stoichiometric calcium phosphate hydroxyapatite because its atomic ratio Ca/P is 1.67. Nonstoichiometric hydroxyapatite exists with the general formula Me 10−x 䊐 x (PO 4 ) 6−y (HPO 4 ) y (OH) 2−2x 䊐 2x . The Me/P atomic ratio can vary widely over the range 1.50 to 1.67. When heated to above 600 °C, a nonstoichiometric apatite is unstable and decomposes into a stoichiometric apatite and trimetal phosphate, according to the following reaction: Me10−x䊐x(PO4)6−y(HPO4)y(OH)2−2x䊐2x → a (1) Me10(PO4)6(OH)2 + b Me3(PO4)2 .

a)

Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 1, Jan 2001

http://journals.cambridge.org

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If the initial nonstoichiometric apatite is a mixed cationic apatite, the stoichiometric apatite and the trimetal phosphate are also mixed cationic compounds. Calcium strontium apatites can be used as stationary phases in liquid chromatography.3 Strontium is used as a marker for calcium4 and can significantly reduce demineralization of bone.5–7 But its exact role in the mineralization of calcified tissues is not yet totally elucidated.8–10 Some biomaterial implants contain strontium. Geochemical data on apatites indicate that they can incorporate various fissiogenic elements such as uranium, caesium, and strontium.11 Therefore, apatites can be used as storage materials, especially for radioactive strontium waste. The substitution of strontium ions for calcium ions in the apatitic struct