A series of sulfate-phosphate apatite samples of formal composition NaxCa10-x(SO4)x(PO4)6-xF2 (x = 1,2,…,6) were prepared by solid state reactions and hydrothermal techniques. Rietveld analyses based on X-ray diffraction data clearly indicate that the samples synthesized by solid state reaction of the reactants Na2SO4, CaSO4, CaF2, and Ca3(PO4)2 consist of a mixture of two apatite end-members Na6Ca4(SO4)6F2 and Ca10(PO4)6F2 which contradicts previous work assuming a solid solution series between the end-members. The X-ray diffraction powder patterns of the samples synthesized by hydrothermal methods show a splitting of reflections which can be interpreted as a monoclinic reduction of the hexagonal symmetry, thus representing an apatite-like phase of the solid solution series with sulfate and phosphate groups. The hydrothermally synthesized compounds exhibit physical properties that are required for dental applications where apatite-like phases are used as fillers in composite materials.

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