Single crystals of Sr-substituted hydroxyapatite, (Ca9.42Sr0.18 H0.8(PO4)6(OH)2, up to 0.2 × 0.2 × 5 mm3 maximum size were grown hydrothermally from calcium hydrogenphosphate (CaHPO4) and strontium hydrogenphosphate (SrHPO4) in a long tube-type autoclave using a temperature gradient technique. The single crystals, containing 1.92 mole% Sr to (Ca + Sr) and having a (Ca + Sr)/P atomic ratio of 1.60, had a hexagonal-prismatic shape with density of 3.207 g/cm3. The crystal had a hexagonal system, space group P63/m with a = 9.3920(7) Å, c = 6.890(1) Å, and Z = 1. A hexagonal-prismatic crystal, 165 × 165 × 231 μ3 in size, was used for crystal structure refinement. The atomic parameters in P63/m were refined to R = 5.9% and RwÅ = 6.3% with 2018 independent reflections. The preferential substitution site of Sr for Ca was determined as the Ca(1) site.
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