The radiation-induced crystalline-to-amorphous transition in zircon
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Rodney C. Ewing and Lu-Min Wang Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 (Received 2 August 1993; accepted 1 November 1993)
A comprehensive understanding of radiation effects in zircon, ZrSiO 4 , over a broad range of time scales (0.5 h to 570 million years) has been obtained by a study of natural zircon, Pu-doped zircon, and ion-beam irradiated zircon. Radiation damage in zircon results in the simultaneous accumulation of both point defects and amorphous regions. The amorphization process is consistent with models based on the multiple overlap of particle tracks, suggesting that amorphization occurs as a result of a critical defect concentration. The amorphization dose increases with temperature in two stages (below 300 K and above 473 K) and is nearly independent of the damage source (a-decay events or heavy-ion beams) at 300 K. Recrystallization of completely amorphous zircon occurs above 1300 K and is a two-step process that involves the initial formation of pseudo-cubic ZrO 2 .
I. INTRODUCTION This past year marked the centennial of Br0gger's 1893 definition of the "metamikte" state1 and a century of effort by mineralogists who have studied the effect of a-decay events on the structure and properties of minerals.2 Zircon, ZrSiO 4 , which has a tetragonal structure (I4i/amd,Z — 4), is the most extensively studied metamict mineral2 and undergoes a radiation-induced transition from the crystalline to an amorphous state that is complete after the accumulation of ~10 1 6 a-decay events/mg.3"6 The zircon structure type has also been proposed as an actinide-bearing phase for nuclear waste forms that will be subject to a-decay damage.7 In addition, ion-beam modified zircon has been studied as a potential waveguide material,8 and it has been shown9 that zircon undergoes the crystalline-to-amorphous transition at room temperature under Kr + and Xe + ion-beam irradiations, with the completely amorphous state reached after an ion fluence on the order of 1014 ions/cm 2 . Nevertheless, the processes of radiation damage in zircon are poorly understood. Geologic specimens are not always available over the range of critical dose (damage) levels and always have suspect or undetermined thermal histories; furthermore, prior to this study, there has been no systematic investigation of the crystalline-toamorphous transition in zircon as a function of damage rate, particle mass, energy, and irradiation temperature. This paper synthesizes the results of detailed investigations of radiation effects in natural zircons (irradiation time: 570 million years), Pu-doped zircon (irradiation time: up to 6.5 years), and ion-beam irradiated zircon 688
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J. Mater. Res., Vol. 9, No. 3, Mar 1994
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(irradiation time: ~0.5 h), and it provides a model of the radiation damage process, defect accumulation, and annealing behavior in zircon as a function of cumulative dose, temperature, and recoil-energy spectra. Such an understanding is ess
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