Radiation-induced amorphization and recrystallization of hydroxyapatite nanoparticles

Hydroxyapatite, Ca10(PO4)6(OH)2, is considered as an important apatite-type material for the incorporation and disposal of actinides and fission products. Hydroxyapatite nanoparticles with different size ranging from 20 nm to 280 nm are synthesized via calcining the bovine bones under different temperatures and durations. The samples are irradiated with 1 MeV Kr2+ ions and 200 keV electrons to study the displacive and ionizing effects on the irradiation behaviors. In situ transmission electron microscopy (TEM) observation shows hydroxyapatite nanoparticles can be amorphized by 1 MeV Kr2+ ions and the previously amorphized samples experience a rapid ionizing-radiation-induced recrystallization upon 200 keV electrons irradiations. A strong size effect on the displacive irradiation-induced amorphization and ionizing irradiation-induced recrystallization are observed. Under ion irradiation, a lower critical temperature (Tc) is observed for a larger sized sample, indicating enhanced amorphization tolerance. This result indicates excess interface energy resulting from the smaller sized hydroxyapatite nanoparticle, may lower the energy difference between the crystalline phase and amorphous state, degrading the radiation tolerance. Radiation-induced amorphization is attributed to the accumulation of oxygen vacancies upon ion irradiation and the glass-like structure formed by the rearrangement of displaced oxygen and phosphorus atoms. Whereas under electron irradiation, the recrystallization fluences decrease with the reduction of particle size since a higher density of dangling bonds exist in a smaller sized hydroxyapatite. These unstable bonds are relatively easy to break and reforming under ionizing electron irradiation, which drives the recrystallization process.