Electron-irradiation-induced phase segregation in crystalline and amorphous apatite; a TEM study

University of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM, United States

Single crystals of natural F-rich apatite and 800 keV Kr (super 2+) ion-beam-amorphized apatite were irradiated by an electron beam in a transmission electron microscope over a range of beam energies and beam currents. Irradiation of crystalline apatite using a high current density (16 A/cm ² ), caused the precipitation of cubic CaO from the crystalline apatite matrix. Using a lower beam current (1.6 A/cm ² ), the formation of nanometer-sized voids was observed, but CaO did not crystallize even after prolonged irradiation. Amorphous apatite crystallized to a coarse-grained polycrystalline assemblage of apatite crystallites at 85-200 keV. Increasing the beam current through the sample caused the formation of finegrained cubic CaO, and the crystallization of apatite was not observed, even at high doses. In each case, many beam-induced bubbles formed and were typically larger at the edge of the beam. Thermal annealing at 450 degrees C resulted in epitaxial crystallization from the thick portions of the TEM foil and resulted in a single crystal with a high defect density. Electron-beam irradiations at 300 degrees C confirmed that the difference in microstructural evolution as a function of current density is driven by dose-rate effects. In fact, temperature and dose rate are competing effects in the precipitation of CaO.

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