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MATERIALS RESEARCH

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Electron-irradiation-induced nucleation and growth in amorphous LaPO4 , ScPO4 , and zircon A. Meldrum Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131-1116

L. A. Boatner Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6056

R. C. Ewing Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131-1116 (Received 3 October 1996; accepted 21 March 1997)

Synthetic LaPO4 , ScPO4 , and crystalline natural zircon (ZrSiO4 ) from Mud Tanks, Australia were irradiated by 1.5 MeV Kr1 ions until complete amorphization occurred as indicated by the absence of electron-diffraction maxima. The resulting amorphous materials were subsequently irradiated by an 80 to 300 keV electron beam in the transmission electron microscope at temperatures between 130 and 800 K, and the resulting microstructural changes were monitored in situ. Thermal anneals in the range of 500 to 600 K were also conducted to compare the thermally induced microstructural development with that produced by the electron irradiations. Amorphous LaPO4 and ScPO4 annealed to form a randomly oriented polycrystalline assemblage of the same composition as the original material, but zircon recrystallized to ZrO2 (zirconia) 1 amorphous SiO2 for all beam energies and temperatures investigated. The rate of crystallization increased in the order: zircon, ScPO4 , LaPO4 . Submicrometer tracks of crystallites having a width equal to that of the electron beam could be “drawn” on the amorphous substrate. In contrast, thermal annealing resulted in epitaxial recrystallization from the thick edges of the TEM samples. Electron-irradiation-induced nucleation and growth in these materials can be explained by a combination of radiation-enhanced diffusion as a result of ionization processes and a strong thermodynamic driving force for crystallization. The structure of the amorphous orthophosphates may be less rigid than that of their silicate analogues because of the lower coordination across the PO4 tetrahedron, and thus a lower energy is required for reorientation and recrystallization. The more highly constrained monazite structure-type recovers at a lower electron dose than the zircon structure-type, consistent with recent models used to predict the crystalline-to-amorphous transition as a result of ion irradiation.

I. INTRODUCTION

Electron-irradiation-induced annealing phenomena have been well documented in the case of amorphous metals and alloys1–3 and have been shown to occur under a variety of conditions in semiconducting materials.4–12 Most of the work on semiconductors has involved the epitaxial recrystallization of buried amorphous layers due to the potential importance of this process in the semiconductor industry, specifically in very large scale integration (VLSI) technology. Many of these studies have therefore focused specifically on buried amorphous layers in silicon.4–9 The crystallization of isolated amorphous regions in other