An Unprecedented Process of Peroxide Ion Formation and its Localization in the Crystal Structure of Strontium Peroxy-HydroxyapatiteSr10(PO4)6(O2)x(OH)2–2x.

Formation of strontium peroxy-hydroxyapatite solid solution Sr10(PO4)6(O2)x(OH)2–2x was studied on annealing the hydroxyapatite in the temperature range 900–1350 °C in oxygen, air, and argon atmosphere. The redox process was found to display unprecedented features: (i) the peroxide content increased with raising temperature, (ii) the peroxide content remained substantial even at a low oxygen pressure of 1.013 Pa, (iii) the peroxide content was extremely persistent, and even at a temperature of 1350 °C in oxygen atmosphere the peroxide groups substituted more than two-third of the original hydroxide groups. Chemical processes consistent with these features are suggested. In the UV/Vis spectrum, an absorption peak at 300 nm was recorded, which was attributed to an electronic transition in the peroxide ion, since its intensity depended linearly on the peroxide content. In the Raman spectra, a weak band at 765–770 cm–1 was ascribed to symmetric stretching vibrations of O22–. The structures of the compounds were refined from powder X-ray diffraction data using the Rietveld method supported by a maximum entropy method (MEM) electron density calculation. The peroxide ions are localized in the center of the hexagonal channel. At high concentration they tend to order with the associated vacancies along the channels. As a consequence, the width of the channel sections varies, the PO4 tetrahedra tilt, and the remote strontium atoms are displaced leading to changes in their coordination. Generally, the peroxide for hydroxide substitution manifests itself in the reduction of the overall channel diameter (and its volume), while the volume occupied by out-of-channel ions remains the same.